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G has a "swell" time kayaking

G has a "swell" time kayaking
G has a "swell" time on Lake Michigan in an inflatable canoe

Dawn on the Gulf of Mexico

Dawn on the Gulf of Mexico
Dawn on the Gulf of Mexico

Warren Dunes Sunset

Warren Dunes Sunset
Warren Dunes Sunset
Showing posts with label Batteries for RV. Show all posts
Showing posts with label Batteries for RV. Show all posts

Wednesday, May 18, 2022

Solar and Potential Savings in a Class B RV

 


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Financial benefits of solar on a 210P
Is there a financial incentive? At the AZ resort, electricity is $13.00 basic service charge monthly + $0.07612 for the first 500kWh, then $0.09337 for each kWh in the range 501-1000. To this are added Arizona assessments, environmental compliance charges, a utility ‘power supply’ charge, a PPFAC charge and taxes.  Typical energy charges adding all of these is about $0.1495 per kWh.

I’ve monitored the AC power used to simply power the coach, keep the AGM batteries charged and a vent fan running. The cost for that amount of electricity is about $51.67 each month.  Using solar and a good battery may reduce my 120VAC power bill by about $310 each season in AZ. If the batteries run well for 7 years, that’s a possible $2,170 in electric bills I won’t pay. 

The other six months of the year, at our “lily pad” in MI the cost per kWh is currently $0.14. That’s a cost of $290.30 to charge batteries, power the 12VDC coach, etc. for six months.  

Add the possible AZ and MI savings, and I may save $600.30 each year, or $4,202 over the projected life span of the coach battery. 

These are approximate numbers taken with intermittent readings prior to replacing the AGM batteries with LiFePO4.

Of course, to this add any 120VAC consumed.  I don't have a "full-solar" installation. At present the Tripp-lite charger/inverter is "Off" and I use AC for the Cool-Cat air conditioner/heat pump, the refrigerator 120V heater and microwave/convection oven. 

If more solar were available, I could probably run the refrigerator off 12VDC during daylight hours, increasing the savings from solar. However, "full solar" existence isn't practical in my opinion.  The Cool-Cat heat pump requires 1,227 watts when the compressor is running and cooling.  To that add electricity for the refrigerator, etc. as well as battery charging current to be stored for overnight use.  I'd need a lot of batteries and a lot of solar panels.  The roof of the 210P has space for at most 200W of panels.  I can add portable panels to the capacity of the controller and more batteries but living off the grid is not my goal.

Why? I won't park a Roadtrek in full sun in Arizona when the ambient reaches 100F in the shade. That's a recipe for a human BBQ.  

This season a couple moved into the resort after a trial at boondocking in nearby Quartzite.  Their several months-long experiment occurred with peak temperatures only about 80F.  They related their experienced and told us that they made the decision to get a site in the resort. They decided the resort fees were worth it as this provided unlimited running water, sufficient power for air conditioning, easy tank dump, etc.  Oh, and a bar & grill, swimming pools and internet access, too. 

Measuring AC Power consumed
I have two methods. One is the Power Protection Device at the 30A connector.  It includes an ammeter display. It is useful for roughly monitoring the amperes being consumed @ 120VAC.

AC Amperes with Cool-Cat AC running, Tripp-lite charging, refrigerator on AC

The other device I use is a Kill-a-Watt meter.  This can be used to determine the amperes and watts consumed on 120VAC circuits, or for individual appliances.  It is accurate and precise. It allows a variety of measurements and price calculations at any entered cost per kW. It will calculate the power consumed at any entered price per KWh and it will totalize this cost.  Here's an example of an instantaneous ammeter reading:

Kill-a-Watt indicates Coach 120VAC at 10.23A. Measurement 
with Cool-Cat air conditioner running, Tripp-lite not charging and
minimal appliances powered up

Measuring the Solar Power Available
The MPPT controller has calculations and historical data available. Data is accumulated and stored monthly. Some of the statistics available:
  • Power generated
  • Charge Ah same day
  • Max charge power
  • Max battery volt
  • Running days total
  • Battery full charge times
  • Battery charge Ah total
  • Generation amount kWh
Initial Observation
How is the LiFePO4 battery and charger doing?  So far, very well. I keep the coach powered up on solar with the vent fan running. The Tripp-Lite charger/inverter not being used. This is an experiment.  Every day the coach battery is fully recharged after a few hours in the morning sun.  Of course, this is AZ with a lot of sun.  On the other hand, this is a small 50W solar panel which is showing its age. Power output has decreased and peaks at about 85% of rated. 

It is premature to call this a success, or the LiFePO4 battery superior to the AGM batteries. For one thing, this trial has been a little over a month in duration and we are stationary. For another, I did install the coach battery in the outside compartment. It is a well-known fact that the battery management system (BMS) of a LiFePO4 battery will not allow it to be charged if the battery temperature falls below 32F. G and I have trekked and camped overnight in temperatures as low as 5F. As a consequence I did install low-wattage 12VDC and 120VAC compartment heaters.  We’ll see how that works. 

There are some LiFePO4 batteries which incorporate 12VDC heaters in the battery.  I could have chosen this type.  However, I am concerned about how much energy those heaters may use when off the grid or while the Roadtrek is stored in cold weather and not plugged into shore power. I decided two 12VDC and a 120VAC heater and solar power were a better solution, in my circumstances.  I also installed an automatic low-voltage disconnect for the battery.  The BMS will disconnect and prevent the battery from completely discharging, but LiFePO4 batteries are best disconnected at a higher voltage threshold if maximum cycles and life are to be achieved. Ergo the automatic disconnect. 

Increasing the solar Available
My plans will increase the amount of solar for the Roadtrek.  There are limitations because of the available roof space.  I have no such limitations in AZ under the shelter, so I can add even more kW on my shelter roof. 

Background Information
It was time to replace the coach batteries in my 2013 210P.  I attended the FMCA Convention in Tucson in March, and I used that opportunity to visit with battery suppliers and manufacturers in the exhibit hall. There was a battery seminar scheduled, but an issue prevented the presenter from being there so that seminar was cancelled.

I had to choose:  AGMs or LiFePO4. To assist in making that choice I wanted to review the latest technology and I didn’t want to overspend on Lithium-ion if I went that route.

My Roadtrek is relatively new, but the first set of AGM batteries were ruined when the Roadtrek control panel “Inverter OFF-ON” switch failed to open when I put it in the “OFF” position.  Sitting in storage in that condition for a couple of weeks completely depleted the AGM batteries, ruining them. After that experience I turned off the inverter function when I store the Roadtrek.  I did this by changing the Tripp-lite mode switch to “Charge Only”.

When the first set of AGM batteries failed I replaced them with similar batteries and installed a 50W solar panel and 180W desulfating solar controller to keep them charged.

LiFePO4 Today
LiFePO4 technology continues to evolve. I’m currently aware of two different cell construction techniques. I investigated different constructions and manufacturers.

Prices for a 100Ah battery range from about $350 to $950 each, plus shipping and tax.  Why is that?  There are differing cell qualities.  There may also be large mark-ups by some sellers.  I determined that there is no standard for determining cell quality in China, where many or most of the “internals” in these batteries are manufactured.  As a consequence, it is important to purchase from a reputable manufacturer. It is possible to overpay, if one assumes the higher the price the better the quality.  On the other hand, there is a lower price threshold below which it would be better to avoid.

A Decision in favor of LiFePO4
I continued my research and decided upon a  LiFePO4 battery from a quality manufacturer.  Delivered and with state sales tax the price was   $624.93.  It arrived on April 1, 2022.

I'll run some of the solar numbers later in this post. However, I concluded the LiFePO4 would be more reliable and therefore more likely to achieve the solar performance I wanted.  Solar panels are only a  portion of the system.  Energy storage is also very important.

The was shipped 30% charged, which is normal for this manufacturer.  I promptly charged it using a 10A charger which was compatible with the specifications for this Lithium-ion battery.

A new MPPT solar controller
I had determined that my solar controller with a de-sulfating mode was not ideal for the Lithium-Ion battery. It was ideal for AGM batteries.  I replaced the solar controller with an MPPT type; it had a user mode with custom settings which were ideal per the battery manufacturer’s specification.  I installed that, entered the proper settings and have since allowed it to charge the coach battery and run basic appliances off the grid (lights, fan, etc.).  I’m satisfied with the initial performance.

I’m currently in southwest Arizona with the Roadtrek on my winter site. It is parked under a shelter, which is why I used a portable solar panel. The panel is currently on the shelter roof. The ambient may be 60F at night but can reach 90F during the day. We do get some morning sun loading in the front of the Roadtrek. I run the vent fan most of the day unless I am using the Cool-Cat heat pump/AC.



(c) N. Retzke 2022


Monday, May 2, 2022

Charging a LiFePO4 Battery in the Roadtrek Class B RV


Tripp-Lite Charger-Inverter DIP switches and LED indicators

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This precedes my recent post about the LiFePO4 installation.  However, it does include some of the groundwork.

The LiFePO4 battery arrived (April 1, 2022) , and I measured the terminal voltage.  It was 13.0V DC.  That's 30% State of Charge (SOC) for this battery.  The manufacturer's documentation states that the batteries "ship at 30% SOC". It's encouraging when the documentation matches reality.

My experience is, many RVer's overspend for what they "might" need.  That's true for RV battery and solar capacity, too.  Of course, if money is no object, then spend, spend away and when depleted, toss the junk into a landfill.

I think it is a good idea to read each manufacturer's specifications because there are variations and specifications should never be assumed from generalities. Matching actual charging parameters to the specifications is one thing that can be done to achieve maximum battery performance.  Ask the question: "Why is the battery rated between 4,000-8,000 cycles?"  Why that significant range?

Here's one example. In general, the recommended charge/discharge current is 0.5C for a LiFePO4 battery but "follow the manufacturer's guidance".  Higher discharge rates decrease battery life, which is to say, number of cycles of good performance.  For a 100Ah battery, 1C is 100A and so the maximum charge/discharge rate may be 50A. More on C rating later in this post.

If one wants to get maximum life from a coach battery, any coach battery, then adhere to the manufacturer's specifications for that battery.

Optionally, we can treat the coach batteries as throw-away AAA Duracell or Energizer batteries and deplete them and throw them away.  When we have money to burn, some RVers will do that, while voting to end "Climate Change".  LOL.

Battery Specifications

The LiFePO4 battery I purchased has these basic specifications.  (I'll post later about grades of LiFePO4 battery cells):

  • 100Ah.
  • Nominal voltage 12.8V. 
  • 4000-8000 cycles life span.
  • Removeable cover, replaceable BMS and cells.
  • Recommended charge voltage 14.6V.
  • Charge current 20A recommended, 50A maximum.
  • Discharge current 100A maximum, continuous. Peak 200A for 3 seconds.
  • Weight 28.2 lbs.
  • 7 year warranty.

Nominal Voltage 12.8 VDC
The LiFePO4 battery has a "nominal voltage" of 12.8 VDC.  In fact, the voltage at the terminals can range from 10.0V to 13.8V.  

What's the value of knowing the "nominal" battery terminal voltage? The nominal voltage is used to calculate the watts or watt-hour capacity of the battery. Watts = Volts x Amperes, at the "nominal" voltage: 
  • 1,280Wh = 12.8V x 100Ah
State of Charge - SOC
I provide a typical chart later in this post.  State of Charge (SOC) is another measurement. It states the present, actual capacity of the battery relative to its total capacity. 

State of Charge is in percentage: 
  • 100% SoC means the battery is fully charged.
  •  0% means fully discharged. 
 SOC is calculated this way: 
  • State of charge (%) = Remaining Capacity (Ah) / Total Capacity (Ah).
Recommended Settings for the Battery Charger:
The battery manufacturer of the LiFePO4 battery I am installing in my Roadtrek has these charging recommendations:

  • Charge (Absorption) Voltage 14.6V (14.4V minimum).
  • Float Voltage: 13.8V preferred (13.6 Minimum).
  • Charge Current 20A.
  • Equalization: Off.

The battery manufacturer states:

  • For optimal life don't discharge below 10% (About 12.1V).  To be more conservative, then don't discharge below 20% (12.9V).
  • Recommended external Low-voltage disconnect 11.2V (About 6% capacity).
  • Exceeding maximum current (50A) for charging can reduce the cycle life of the battery. 20A charging current is recommended.
Other Settings - Low Voltage Disconnect:
To protect the battery an external low voltage disconnect is recommended.  The internal BMS has a Low-Voltage disconnect >10.4v and BMS Low-Voltage recover >11.6v.
  • Recommended external Low Voltage Disconnect: 11.2V (about 6% capacity remaining).
Why Follow Manufacturer's Recommendations?
Following the manufacturer's recommendations is how one may achieve 8000 cycles from these batteries.  I have concluded that it is possible to get 10 years of good performance from a LiFePO4 battery constructed of Class A cells. If one doesn't follow recommendations the battery performance will degrade and 4000 or fewer cycles may be the maximum achieved.  It is also possible to damage the battery.

Three methods of Charging

I have three methods for charging the batteries in my Roadtrek. Ideally, each method would provide the appropriate charging voltage and current:

  1. Tripp-Lite charger inverter using 120VAC power (provides voltage and current control).
  2. Solar using solar panels and a controller (provides voltage and current control).   
  3. Engine alternator (simple voltage regulation, no control).

The first two methods can be adjusted to adhere to the battery charging specifications.  The alternator has no such adjustments.  To avoid over-charging the coach batteries I can use the battery separator to disconnect from the alternator and rely upon solar charging.  Alternately, a DC-to-DC charging system could be installed.  But there are practical limits to how much I'm willing to spend on this.

Battery C Rating and Charging

The Tripp-Lite is a 3-stage charger with a 4th equalize setting.  This is typical for lead-acid battery chargers.  

The LiFePO4 battery requires only two stages:  Bulk/Absorption are combined and there is a Float stage.  Using the equalize setting will damage the LiFePO4 battery, i.e. diminish it's capacity. So, the equalize setting of  the Tripp-lite must not be used.

It is very important that charger settings be matched to the manufacturer's recommended settings for charging the battery.

In the first stage the LiFePO4 battery is brought up to 14.4-14.6V at which point it is fully charged.  The charger should then reduce the current and hold the voltage constant.  The charge voltage will then be reduced.  In the final stage (float) the voltage is reduced to 13.6V.  



Typical 3-stage battery charging - Voltage versus current
Bulk, absorption (topping) and float stages
Bulk - voltage increases while current is held constant
Absorption - Charge voltage held constant; current varies as battery reaches full charge
Float - Battery resting - charging voltage is held constant and current reduced

To understand the diagram, charging current is expressed as a factor of C, which is the battery C rating.  The C rating is the measurement of current that a battery is charged and discharged at. The capacity of a battery is generally rated and labelled at the 1C Rate (1C current).   A 1C rate means that the discharge current would discharge the entire battery in 1 hour.  A fully charged battery with a capacity of 100Ah should be able to provide 100 Amperes for one hour: that's 1C.  For charging purposes, the C/10 rating is 100Ah/10A charge current which is 10 hours to charge.  The C/2 rating is 100Ah/50A charge current which is 2 hours to charge.  Charging times are approximate.  

The LiFePO4 battery manufacturer recommends charging in the range of 0 to 20A, with 40A permissible and a maximum of 50A.  Charging at 10A would be expressed as 100Ah/10 or C/10.  Charging at 20A would be expressed as 100Ah/20 or C/5 and at 50A it would be C/2. 

Setting up the Tripp-Lite 750-watt charger-inverter

Before installing the batteries, I checked the settings of the DIP switches on the Tripp-Lite. The Tripp-Lite is a 3-stage charger with a 4th equalize setting.  This is typical for lead-acid battery chargers.  

Charging Times, Approximate
One needs to decide how far to discharge the batteries prior to re-charging. If a minimum floor (minimum capacity before charging) voltage of the battery is 20% (12.9V) then the maximum battery life in cycles may be achieved. In other words, the amount of capacity used is 80%, or 80Ah of a 100Ah battery.  To replace that lost capacity at a charging at a rate of 20A would require about 4 hours to return the battery to full charge:
  • 80Ah/20A = 4 hours to fully charge from 20%
The Tripp-Lite has two charging current settings.
1. Using the same 20% cutoff (12.9V) charging at a rate of 11A, which is one of two possible settings on the Tripp-Lite would require about 7.3 hours to return the battery to full charge:
  • 80Ah/11A = 7.3 hours to fully charge from 20%
2. Using the same 20% cutoff (12.9V) but charging at a rate of 45A, which is the second setting on the Tripp-Lite.  That would increase the 120VAC draw on the 30A shore power, reducing power available for air conditioning, the microwave, etc. It is also higher than the battery manufacturer preferred 0-20A.  However, if this setting were used, it would require about 1.8 hours to return the battery to full charge:
  • 80Ah/45A = 1.8 hours to fully charge from 20%
Possible Tripp-Lite Charger Settings:

1. The Tripp-lite was designed for lead-acid batteries but it can be used with LiFePO4 batteries which have a BMS. The Tripp-lite has two charging settings:

  • "Wet Cell (vented)" (DIP Switch A1 in the DOWN position) and
  • "Gel Cell (sealed)" or AGM (DIP Switch A1 in the UP position).  

2: The Tripp-lite has three stages for charging: BULK, ABSORPTION and FLOAT, but only allows us to select two of the charging voltage settings.  The "Absorption" and "Float" stage voltages are adjusted by selecting the battery type.  The charger doesn't allow us to set the "Bulk" voltage:

  • Wet Cell: 14.4 VDC "Absorption", 13.5 V "Float".
  • AGM: 14.1 VDC "Absorption", 13.6 V "Float". 

3. The BULK stage brings a battery to about 80% capacity using a constant charge current. See the diagram above. The charge voltage can vary in this stage.  

4. The Tripp-lite has two charging current rates:

  1. 11 Amperes (DIP Switch B4 in the UP position).
  2. 45 Amperes (DIP Switch B4 in the DOWN position).

These charging current rates would require this approximate time to fully charge a discharged battery:

  • 100Ah/11A = 9.1 hours
  • 100Ah/45A = 2.22 hours

Selecting the appropriate Tripp-lite settings:

These are the settings I used, which are closest to the battery specifications:

  • DIP Switch A1 "Down"- 14.4V Absorption setting, which is the closest to the recommended 14.6V.
  • DIP Switch A2 "Down" - Charger Enabled.
  • DIP Switch B4 "UP"- 11 Amperes Charging Current.

Other Settings:

  • DIP Switch B3 "UP" - Equalize reset (equalize off).
  • DIP Switches B1 & B2 "Down" - No Load Sharing (Charge @ B4 ampere setting). 

Tripp-lite 750W DIP Switch location diagram
From the Tripp-Lite 932768 manual

Solar Charger

My 180-Watt de-sulfating solar charger has these presets. They are not adjustable:

  • Charge: 14.2V (at 70F, voltage varies with temperature).
  • Float: 13.4V (at 70F, voltage varies with temperature).

The solar charger is optimized for AGM lead-acid batteries and has a de-sulfating function.  That's not desirable for charging a LiFePO4 battery. LiFePO4 batteries do not suffer from sulfation. But they are sensitive to under voltage or over voltage. The solar charger uses a sensor to determine the charge voltage at various battery temperatures.  Charge voltage range 14.8V at 40F which decreases to 13.9V at 110F. The float voltage is also temperature regulated. Float voltage range 13.9V at 40F which decreases to 13.0V at 110F.

I'm inclined to replace this solar charger with a MPPT unit that can be adjusted to match the battery specifications. I'll be monitoring this to see if the charge voltage is close to 14.6V and meets the battery manufacturer's recommendations:

  • Charge Current: 0 to 20A (not an issue with the size of my solar panel).
  • Absorption Voltage: 14.6v preferred (14.4 Minimum).
  • Float Voltage: 13.8v preferred (13.6 Minimum).

NOCO 3500 Smart Battery Charger

I do have a NOCO battery charger which is suitable for charging a variety of batteries.  6V, 12V and Lithium up to 120Ah.  It is a 3.5A charger, so it can take a while to fully charge a discharged battery. I have used it on a variety of lead-acid batteries, but never on a LiFePO4 battery.  It is an older "genius 3500" so it isn't designed to use with a BMS equipped battery. It has selectable modes for different batteries.  These include:

  • Lithium Charging 14.2V, 3.5A. (29-hour charge time for a 100 Ah battery).
  • 12V Normal Charging: 14.5V, 3.5A.
The battery manufacturer recommends a charge (Absorption) Voltage of 14.6V (14.4V minimum).  It seems the NOCO could get the battery to above 14.5V if I use the 12V normal setting. 

Note:  The newer NOCO genius5 and genius10 replace the 3500. These can charge at 5A or 10A respectively. The genius10 has this specification for LiFePO4 batteries:
  •  "Lithium Charging 14.6V, 10A. For use on batteries with Battery Management System (BMS) only."

Initial Charge - Tripp-Lite and NOCO

I'll charge from 110VAC power using the Tripp-Lite before connecting the battery to the solar system or charging it with the alternator.

According to the charge status LEDs the Tripp-Lite the battery reached 91-100% before entering the float mode.  After disconnecting and a few minutes of rest the voltage at the battery terminals was 13.44VDC.  That's about 99% charged.

I decided to connect a NOCO 10A Lithium battery charger (14.6V) directly to the battery terminals with the coach disconnected. I wanted to see if it agreed and if it would continue the charging of the battery.  After allowing the charger about 15 minutes to settle I checked the status indicator on the charger. It was pulsing green:

  • Pulsing Green LED - "Bulk charge complete, optimizing battery for extended life." 

The battery terminal voltage was 14.45VDC.  Apparently the Tripp-Lite did an acceptable job.

It was getting late. I decided to leave the charger connected overnight, to see if the battery could achieve a 100% charged condition:

  • Solid Green LED - "When the battery is 100% charged, the Charge LED will be solid."

In the morning, I checked the charger. The LED was solid Green and the battery voltage was 13.83 VDC. The battery had apparently reached full charge at the specified float voltage.

Conclusion

  1. I wrote this while evaluating the transition to a LiFePO4 coach battery.
  2. I did decide to replace my older solar controller with an MPPT unit. 
  3. This has been an educational experience. Since writing this post I have posted more about the installation.
  4. The nominal battery voltage is 12.8, or 3.2V per cell.  However, a fully charged battery is about 13.8 V (3.45V per cell). A completely discharged battery is 10.0V (2.5V per cell).
  5. Earlier modifications, including the volt-ammeter will be helpful.  I used the digital voltmeter to monitor the status of the charging and I intend to use it to monitor the battery capacity.
  6. The Tripp-Lite charger seems to be capable of working with the LiFePO4 battery. It seemed to get the battery to about 99% charged condition before entering the "float" mode. That's okay with me.  I'll monitor the battery to get additional experience.  I'm inclined to rely upon an MPPT solar controller with settings to match the recommended battery settings.
  7. My existing coach battery disconnect switch doesn't have the proper terminals to fit the new battery. I'll be purchasing a new disconnect.
  8. A low-voltage automatic cut-off for the battery is desireable.  A low voltage alarm is also possible. Such a device would alert me to start the vehicle engine, or the generator to charge the coach battery. 
    • I'd prefer to keep the battery discharge low limit above 11.2V.
    • I do want to avoid discharging down to the 10.4 BMS cutoff. 
    • I'd like to get that 4000+ cycles out of the battery.  
    • I want this to be maintenance free (or reduced). With the voltmeter I installed in the Roadtrek I can easily determine the remaining capacity of a rested battery. 
    • The discharge of the batteries overnight if parking/boondocking and with no AC power is the cause of my concern.
  1. My $20 volt-ampere-wattmeter display works find.  I'm not inclined to replace it with a $90 gadget. 
  2. I'll be monitoring the battery to see how it does when connected to the alternator and running the engine of the Chevy chassis. I seldom see a voltage higher than 14.4V.  I can use the separator to prevent the alternator from charging or over-charging the battery. 
  3. Is there a better approach? One method to improve this arrangement would be to purchase a battery with blue-tooth(r) communications.  That could end any guess-work about the condition of the battery. Such a battery improvement isn't available from all manufacturers and could increase the battery cost by about $50 additional.  Of course, if one builds a battery a BMS can be selected which includes this communications feature. 

(c) N. Retzke 2022


Saturday, April 23, 2022

Adding LiFePO4 cold weather heaters, solar, etc.

 

3- Stage battery compartment heating

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The installation of the LiFePO4 battery in the exterior compartment of the Roadtrek was straightforward.  This is a progress report.

Second step was to upgrade the solar power system.  This is nearly complete.  Integrating it into the existing Roadtrek 12VDC did require some effort, because I wanted to retain the Tripp-lite charger-inverter and space is at a premium.

Third step was to install battery compartment heaters.  We do trek when the outside temperature is below 32F.  At that low temperature the internal battery management system (BMS) will not allow charging of the battery. This restraint was one of the reasons I resisted replacing the AGM batteries with a LiFePO4 upgrade.  

I decided that to have a useful LiFePO4 battery install I'd need supplemental heat in the exterior battery compartment. Otherwise, I would be restricted to warm weather trekking.  The fresh water system on the Roadtrek 210P is rated down to 14F, if specific steps are followed.  Cool weather camping will be determined by the weakest link in the chain.  I decided the batteries would not be the impediment. 

Approaches to keeping LiFePO4 batteries above 32F
Keep in mind that the batteries do generate some heat when discharging.  However, in an unheated compartment that is insufficient as temperatures fall, and the amount of heat is determined by the discharge rate.  In other words, batteries connected to a robust solar system do little self heating unless they are discharging, and the heat generated may be insufficient to keep the battery internal temperature above 32F.

There are two low temperature charging conditions to be dealt with:

  1. While in motion, temperatures fall below 32F.
  2. While stationary, temperatures fall below 32F.

It is possible to insulate the batteries, but that can create complications in hot weather conditions. So, while some compartment insulation is desirable, it must be used carefully in summer heat, which can reach 110F in the Southwest U.S. 

Today, one can purchase 12V LiFePO4 batteries with internal heaters.  The challenge with these is it is possible for the heaters to fully deplete the batteries if they aren't recharged on a frequent basis.  Of course, if one has sufficient solar panels, the battery heaters can run for extended periods while solar provides the necessary DC energy.

I decided that the most flexible approach was heaters external to the batteries:

  1. One set would be powered via 12VDC when on solar or travelling with the alternator providing DC.
  2. The second set would be 120VAC and would be powered via shore power or the generator.
  3. In really cold conditions, all three heaters could be used if 120VAC is available.
Choosing the heaters
One thing I wanted to avoid was "hot spots" on the battery. Cooking the batteries is undesirable and dangerous.

I decided to use three heating pads, each independently controlled, with heat distributed.  I sized the wattage of the heaters using the SWAG method because I don't know the thermal contribution of the batteries as they are discharging, nor do I know the actual heat loss of the battery compartment.  I decided upon smaller wattages, realizing the inherent limitations. 

I can always increase the wattage of the heaters based upon experience. 

I realize that as ambient temperatures fall, there will come a situation that with the heaters on, the battery temperature will decrease below 32F and charging will be impossible.

The system
I did add some insulation.  Passive systems are preferred to active ones.

I'm using three independently controlled heating pads. Two are 12VDC and are intended to be used while the Roadtrek is in motion, or not on 120VAC shore power.  These can be switched off from within the coach, if it is desired to conserve battery resources, or if they aren't needed due to ambient conditions.

One heater is 120VAC and is intended to be used when the Roadtrek is stationary and on shore power.

Each heater is independently controlled, and On-Off settings are independently adjustable.  

I ran a system test today and the heaters worked as intended. Of course, the outside temperature is currently 68F.  The test was a functional test. 

The solar charging MPPT controller includes a temperature sensor. I've installed that in the battery compartment.  Of course, each temperature controller also has a sensor.  So, I'll have four sensors monitoring the battery compartment temperature.  LOL.  As I write this the compartment temperature is 68.1F.

The Challenges
Space in a Class B is very limited, as we all know.  I gave up my wine storage location for the temperature controllers.  LOL. 

Next..................
I may post a few photos once this is completed.

(c) N. Retzke 2022

Friday, April 15, 2022

Transitioning to LiFePO batteries

LiFePO4 Battery on shelf, during installation
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After doing my most recent research, I decided to replace the faltering AGM coach batteries with a LiFePO4 battery.  The decision was the easy part. Next came selecting a battery, installing it and integrating it into the existing Roadtrek 12VDC system.  I also have a solar system and de-sulfating controller.  Such a controller is not recommended with the LiFePO4.

The battery can be mounted in any position, and the space available provided two options.  I decided to go with terminals up. I did elevate the battery slightly by resting it on plywood.  It has a metal case and I don't want it sitting in water.  I've never had an issue with water in the battery pan but I don't ford streams or drive through flooded underpasses. There is always a first time.

The new battery will use the existing shunt and power wiring in the compartment.  The integration of other components did require some additional work on my part. I didn't want to make alterations to the existing wiring of the coach: no shortening or removal.  I wanted the wiring to be recognizable to a technician familiar with the Roadtrek or a future owner.  I also wanted to provide the ability to transition back to AGM batteries at some future time, although I don't expect to do that.

Maintenance Free LiFePO4 batteries?

There are limitations with any coach battery and LiFePO4 batteries have their own.  I decided to address these in my installation. 

It is true the LiFePO4 batteries are "maintenance free".  One significant advantage over other battery types is the internal, electronic battery management system (BMS) which is there to protect the battery.  (See Note 1).

However, to achieve optimal battery life (4000+ cycles) it must be operated within the design parameters.  That may require some external hardware.  I concluded that in my Roadtrek, replacing AGM batteries with LiFePO4 is not quite "plug and play" or "drop in and forget". 

Integrating the various 12VDC Components - a List

The LiFePO4 batteries aren't simply a "drop in" proposition. Here are the things I considered:

  1. Tripp-Lite charger/inverter is to remain in place.
  2. Retain Tripp-Lite 750W inverter function (~59A at 12.8V).
  3. Make the provision for a future LiFePO4 charger. 
  4. Solar charging system - update to compatible LiFePO4 controller.
  5. Upgrade the battery manual disconnect.
  6. Retain existing volt-ammeter and shunt.
  7. Add a low voltage automatic disconnect to enhance the BMS cutoff (to preserve the battery).
  8. Add battery compartment supplemental AC/DC heat to extend charging time.

Low Temperature Battery Considerations

One of the issues with the LiFePO4 batteries is their intolerance to cold.  The BMS will not allow charging if it determines that the battery internal temperature is at 32F or below.  Power output will vary with ambient temperature, but not as much as with the AGMs I'm replacing.   

High ambient temperature may accelerate the aging of the battery while low temperature may reduce output power capability. In general LiFePO4 batteries perform better at low temperatures than do the AGMs I'm replacing. 

I decided to install the battery in the unheated compartment where the AGM batteries were installed.  Minimal 12VDC power wiring changes would be required.  In that location the battery will be exposed to freezing temperatures at any time the outside ambient is below freezing.  Of course, I do have the option of relocating it to the interior of the coach at some time in the future.

Keep in mind that if stored outdoors all Class B's will have their interior temperature decrease to below 32F in fall and winter if the vehicle is north the freeze line. In cold weather the coach interior will be at cold until it is warmed by running the engine.  So too will any batteries stored within.  If the coach is parked in the sun the interior may be warmer than outside. The Roadtrek coach battery can be discharged when cold, but the LiFePO4 battery can't be charged until the battery internal temperature rises above 32F.

I concluded that to store the Roadtrek in winter, if that is necessary below freezing conditions that I'll simply remove the batteries.  If I use it at below freezing temperatures some supplemental heat to warm the batteries would be desireable. That will extend the charging of the LiFePO4 battery as weather cools.  

I do realize that at some point the ambient temperature will be so low as to nullify the supplemental heat and the BMS will prevent charging the battery. I don't expect to encounter that situation. LOL.

Some batteries are available with internal 12V heaters but that draws down the battery in cold weather if the vehicle is not running and not connected to shore power.  Furthermore, if this is an internal battery function and can't be controlled by the user it simply runs the battery down faster if off the grid.  In my opinion that's undesirable in a RV.

I'll be installing both 120VAC and 12VDC supplemental heaters, which I can control.  I'll be monitoring the compartment temperature to get some data about the effectiveness. 

Avoid Over Charging and Over-discharging

Another issue is the possibility the internal battery cells can be damaged if they're discharged below a certain threshold. That low point is approximately 5 percent of total capacity. If the cells are discharged below this threshold their capacity can be permanently reduced.  The BMS will protect the battery, but at too low a threshold to preserve optimum life. 

The solution is an external, automatic cutoff or a low voltage alarm (or both).  

I have installed a low-voltage automatic cutoff. 

Three methods of Charging

I have three methods for charging the batteries in my Roadtrek. Ideally, each method would provide the appropriate charging voltage and current for LiFePO4 batteries, but they don't:

  1. Tripp-Lite charger inverter using 120VAC power (3-stage voltage and current control).
  2. Solar using solar panels and a controller (voltage and current control).   
  3. Engine alternator (simple voltage regulation, no control).

The first two methods can be adjusted to adhere to the battery charging specifications.  The alternator has no such adjustments.  To avoid over-charging the coach batteries I can use the battery separator to disconnect from the alternator and rely upon solar charging.  Alternately, a DC-to-DC charging system could be installed.  There are practical limits to how much I'm willing to spend on this.

External low voltage disconnect - Details

The battery manufacturer recommends that an external low voltage disconnect be used.  The manufacturer suggests 11.2V as the disconnect point.  

  • Battery low voltage disconnect < 11.2V

Such a voltage represents about 5% battery capacity remaining.

There are a couple of methods to achieve this:

  1. Manual switch.
  2. Automatic switch or relay.

It is true that the battery management system will protect the battery from complete discharge. However, 10.4V is about 2% battery capacity.

  • BMS Low-Voltage disconnect <10.4V

In general, a battery constructed of Grade A cells can probably achieve the specified cycles if the battery is operated within the manufacturer's parameters.  Discharge to very low voltage is to be avoided. 

A battery low voltage alarm is a possibility, too.  For example:

  • Battery low voltage alarm < 11.8V

Such an alarm would occur when the battery capacity has decreased to about 8%. 

Adjusting to a new reality.  As I use the LiFePO4 battery I'll have to adjust.  For one thing, the output voltage versus State of Charge is different than the AGMs.  The new battery is more stable than the AGMs.  After charging, and intermittent use for a week, the battery voltage has decreased by 0.06V.  This battery seems to be much "stiffer" than the AGMs.  This may change as the battery ages and capacity decreases to about 80%. 

State of Charge - SoC - Details

I provide a typical chart later in this post.  State of Charge (SoC) is a very useful battery measurement. It states the present, actual capacity of the battery compared to its total capacity. 

State of Charge is a percentage: 
  • 100% SoC means the battery is fully charged, new and undamaged.
  •  0% means fully discharged. 
 SoC is calculated this way: 
  • State of charge (%) = Remaining Capacity (Ah) / Total Capacity (Ah).

LiFePO4 batteries are chemical devices and so they operate similarly to the AGM lead acid batteries we are familiar with. There are several SoC values to keep in mind.  These are the absolute minimum SoC, the preferred SoC and the actual SoC.  

Absolute minimum SoC is the most discharged state with the lowest possible terminal voltage that doesn't destroy the battery.  For my battery this is 10.4V.  However, discharging to such a low voltage will diminish the capacity and useful cycles of the battery. 

For maximum battery life, only discharge it down to the Preferred level, but no lower. Hmmm, that seems reminiscent of the recommendations for AGM lead-acid batteries, doesn't it?  Here are the three values:

  • Alarm and recharge at <11.8V (the Preferred minimum state of charge, 8-10%). To maintain reasonable battery longevity and performance do not discharge below this value.
  • External battery cutoff <11.2V (Actual minimum state of charge, 5%). This is the realistic minimum.  The battery can be discharged to this state, but some battery degradation and performance will occur is the battery is operated this way.  This discharge level is a trade-off between available power and battery life.
  • Battery BMS low voltage cutoff <10.4V (Absolute minimum state of charge, 0%). This is approaching the point of battery destruction.  The BMS will stop battery discharge when this state is reached. 

LiFePO4 Terminal Voltage and SoC

A typical 12V LiFePO4 battery is constructed of four cells called a 4S battery pack. The battery output voltage decreases as the batteries discharge and remaining capacity decreases.  A typical 12.8V battery output will vary from 13.8V to as low as 10.0V (completely discharged).  

The nominal output voltage is 12.8V.  There are differences among manufacturers, but here are typical voltages:

  • Absorption Voltage (charging) 14.4-14.6V  (3.6-3.65V per cell).
  • Float (fully charged) 13.6-13.8V (3.4-3.45V per cell).
  • Full discharge 10.0V (2.50V per cell).
  • Nominal voltage 12.8V (3V per cell)

The terminal voltage will decrease as the battery discharges and the capacity diminishes.  To get an accurate reading of SoC the battery must be rested.   Here's a typical chart. You will note that the battery output voltage is reasonably flat throughout the useable discharge cycle, from about 100% down to 10%:

  • 13.6 V = 100% SoC (Fully charged)
  • 12.1V  = 10% SoC (Preferred minimum)
  • 1.5 V output decrease from Fully charged to minimum SoC.
LiFePO4 Capacity versus Battery Voltage 

Note:
1. There are different battery constructions out there.  Some have as many as 8 internal cells. This is an evolving and improving technology. Low temperature automatic protection by the BMS and internal 12V heaters are relatively new.  Some batteries are well constructed internally, and some are not. Some have steel cases, while most have plastic. Prices range from about $325 to $900 for a 12V, 100Ah battery.

(c) N Retzke 2022


Wednesday, March 30, 2022

Lithium-Ion Battery Update - I changed from AGMs

 

100 Ah LiFePO4 Battery

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During the FMCA Rally, I had an opportunity to attend a battery seminar and also discuss features and current prices with several battery vendors.   In February 2015 I posted here about Lithium-Ion LiFePO4 batteries. In that post I said: "a similarly rated lithium battery system will probably deliver about 45% more power than a similarly rated AGM battery system, and it will probably do so for at least 6-7 years."  

Here's a link to the earlier post:

Are Lithium RV Coach Batteries Expensive?

In August 2017 I posted this comparison:

Comparing AGM and Lithium RV Battery Systems

In March 2018 I posted this:

Lithium Battery and RV DC Power System Developments

In October 2019 I posted an update:

Coach Batteries - AGM versus Lithium-Ion Update

The battery seminar was cancelled. I'd been hoping to get current information and insights. I discussed with FMCA and advised them that I could provide a 1-hour presentation.  For one thing, I have nothing to sell and I'm very familiar with what's available, the costs, as well as the pros and cons of each battery type. However, there were several battery vendors at the rally, and I had the opportunity to talk to Battle-Born, RVConnection, etc.

One of the Roadtrekkers brought up coach batteries in a conversation and told me he was interested in building a LiFePO4 battery.  That piqued my interest.  After the rally I did some additional research.  

My AGMs were installed in January 2017.  They are 5-years old.  I do test the capacity of my batteries from time to time and I had determined it was about time to replace, as the capacity has diminished, which is normal. BTW, using the Roadtrek LED indicator isn't very useful for this.  I measure the battery voltage under load and monitor the decrease in voltage over time under load.  By discharging the battery at a constant current, I can observe the decrease in battery voltage.  That's how I know the battery condition.  I admit it is a rough estimate, but adequate for my needs.  

Some Roadtrekkers use the LED L-F-G-C volt indicators.  I don't. The "G" indicator is illuminated at about 11.9V and above.  My AGM batteries are at 25% SOC at 11.9V OC (Open circuit), which is below the minimum 50% state of charge necessary to achieve rated life. 

Is it time to change to Lithium?

I had once again been thinking about switching from AGM coach batteries to Lithium-Ion.  I too was interested in building my own batteries.  I decided to check prices, which have changed significantly in the past couple of years.

What I discovered was I can purchase a complete battery for about the same price as building one.  In one of those oddities, the price of the components is now nearly equal to the price of some of the assembled batteries; I guess the marketing people in the factories in China figured it out. A year or so ago the parts were less costly than the batteries, but no longer. This is not surprising because a 12V LiFePo4 battery requires these components:

  • (4) 3.2V LiFePO4 Cells.
  • (1) Battery Management System (BMS).
  • Screws, buss-bars, wire and terminators for the BMS.
  • (1) Case, which is sometimes called a Rack, to hold the cells.

Add up the cost of all of the above and compare to the cost of some of the batteries available. At present there isn't much of a difference.  I won't put the component numbers here. However, I decided that while assembling my own battery would me a fun exercise, there was no financial incentive for me to do so.

Note: There are some very low-cost Lithium LiFePO4 batteries available. I've seen prices as low as $300 for a 12V 100Ah battery. However, the price range is from $300 to $900.  I decided to do research into testimonials and specific battery reviews as part of the selection process. I also decided to make a simple specification so I could do a reasonable comparison. 

Be aware that there are different grades of 3.2V LiFePO4 cells; A, B, C, etc. Those cells are assembled in a case with a battery management system (BMS) to make a 12V, 24V or 48V battery.  Grade A cells are more expensive than Grade B cells. Grade B cells are characterized as "unqualified" at manufacture.  Maximum benefit is achieved from a LiFePO4 battery constructed from Class A cells.  But different manufacturers use different standards.  So, a Class A cell from one manufacturer may be the equivalent of a Class B from another.  Confusing, isn't it?  That's why it is a good idea to purchase a battery from a known manufacturer (or assembler) who provides a proper warranty. 

Battery Prices and available capacity:

One thing that is unchanged is the price differential of AGM batteries versus LiFePO4. True, the lowest priced lithium batteries are competitive with AGMs, but may not offer features or warranties. Determining battery quality isn't easy. I decided to avoid the lowest cost lithium because there are different quality levels in the cells.

Bottom line is, the AGMs are less costly, out-of-pocket.  However, if one compares the capacity of the batteries and charge-discharge cycles, the LiFePO4 offer superior performance. 

Furthermore, because we seldom boondock, I don't need a lot of Ampere-hours from the batteries.

Here's an "out of pocket" price comparison.  I priced an inexpensive 12V AGM battery as follows:

  • AGM (lead acid) = $219 + shipping + tax
  • LiFePO4 = $569 + shipping + tax

There is a capacity difference and that should be considered.  Considering how I use my Roadtrek, the fact that I do have solar available, and I have an Onan generator, I really don't need two 100 Ah Lithium batteries. However, a single 110Ah AGM could only provide 55Ah which is not sufficient. As a consequence, I need to compare 220Ah AGM to 100Ah lithium.  This would be adequate based on available capacity:

  • (1) 220Ah AGM = 110 Ah available (50% maximum discharge)
  • (1) 100 Ah LiFePO4 = 80 Ah available (80% maximum discharge)

Battery price per Watt-hour (Wh):

The following is a simple cost comparison and doesn't consider lifespan, number of cycles or battery performance degradation. In fact, it would be more accurate to use 50% capacity for the AGM and 80% for the LiFePO4.  Discharging below those levels can ruin the AGM and may reduce the lifespan of the lithium battery.

For example, one 12V, 100 Amper-hour (Ah) assembled LiFePO4 battery which has acceptable specifications has a price of $569 plus shipping plus tax. That's $0.44 per Watt-hour (Wh), as follows:

$0.44 = $569 / (12.8V x 100 Ah)

A 12V, 100 Ampere-hour (Ah) AGM battery has a minimum price of $219.  That's $0.18 per Watt-hour (Wh):

1 battery $0.18 = $219 / (12.5V x 100 Ah)

2 batteries = $0.36 per watt-hour

If I were to purchase a single 206 Ah LiFePO4 battery, the cost per Wh would be less, while the price of two AGMs would be twice the capacity for twice the price:

$0.39   = $1,029 / (12.8V x 206 Ah)

Prices above are "raw" which is to say, a simple cost per Wh.  However, if we consider the life of the batteries, the numbers change.  An AGM battery can provide 500 charge-discharge cycles. A Lithium-Ion battery can provide 4,000 charge discharge cycles. The LiFePO4 battery can be repeatedly discharged 80% and the AGM 50% to provide this cycle lifespan.

Realistic battery price per Watt-hour (Wh):

In fact, the AGM battery I would be inclined to purchase would be the Deka 8AGC2.  This is a 220Ah  6 V battery which closely matches the original furnished with the Roadtrek. Two would be required to get 12V, but at 50% useable capacity they would provide 110Ah at 12.5V. The cost each is $300 + shipping + tax. This is the realistic AGM battery cost per Watt-hour:

AGM batteries: $0.44 = $600 / (12.5V x 110 Ah)

One 12V, 100 Amper-hour (Ah) assembled LiFePO4 battery which has acceptable specifications has a price of $569 plus shipping plus tax. It has a 80% useable capacity, 80Ah. That's $0.56 per Watt-hour (Wh), as follows:

100Ah Lithium: $0.56 = $569 / (12.8V x 80 Ah)

The AGMs when new can provide energy at a lower cost than the Lithium.  However, the Lithium batteries will provide more power over their lifetime because they can tolerate greater number of charge-discharge cycles.

Comparing Charge-Discharge Cycles

If we consider charge-discharge cycles, then these batteries can provide these Wh over their lifespan:

Lithium:   3,840 KWh = 4000 cycles x 1200 Wh x 0.8

AGM:      300 KWh = 500 cycles x 1200 Wh x 0.5

As can be seen above, the lithium can provide 12.8 times greater power (3840 / 300) over its lifespanConsidering battery lifespan, the lithium batteries are substantially less costly.

Cost over the life of the batteries

If I take the initial cost and divide by the number of useful years, I can arrive at an estimated cost to own per year.  I am using my experience with my AGM batteries, which provided 5-years of useful life in my Roadtrek. For comparison I'm using the warranty period of the LiFePO4 batteries. Note that the warranty is 7 years but can be extended to 10 years.  To make this comparison, I'm using two AGM's (100 Ah useful) and one LiFePO4 (80Ah useful).  I'm ignoring shipping and tax, and I am assuming that the existing Tripp-lite charger/inverter will work with both batteries. That's the configuration I'm considering.

Lowest cost AGM:  $88 per year = (2 x $219) / 5 years

Deka AGM: $120 per year = (2 x $300) / 5 years

LiFePO4: = $82 per year = $569 / 7 years

If one can get a 10-year warranty for the LiFePO4 batteries, the cost per year is further reduced, and the Lithium battery is less costly than the AGM:

LiFePO4: = $57 per year = $569/10 years

Advantages and disadvantages

Lithium LiFePO4 batteries have these characteristics:

  1. Can be repeatedly discharged to 80% with no reduction in battery life. In other words, a 100 Ah battery (1200 Wh) can provide about 960 Wh safely without decreasing battery life. A similar AGM battery should not be discharged more than about 50% repeatedly.  The AGM can provide about 600 Wh, which is 63% of the power provided by the LiFePo4 battery.
  2. The Lithium can be charged-discharged about 4,000 to 8,000 times, or cycles.  The AGM battery, if discharged repeatedly to 50% can be charged-discharged about 500 times, or cycles. The lithium can be charged-discharged at least 8 times more cycles than the AGM.
  3. The Lithium battery provides power at 13.2VDC.  This may gradually decrease to 13.1V when discharged to 40% capacity.  At 20% remaining the battery will be about 12.9V.   The AGM begins at about 12.9V and decreases to about 12.3V at 50% state of charge.  At 20% remaining the battery voltage will be about 11.7V.  Actual output may vary by manufacturer.
  4. The Lithium cannot be charged at temperatures below 32F (0C).  To do so the battery will be ruined.  Using Lithium batteries in below-freezing conditions takes some forethought.  However, they can be discharged at any temperature. Adding a battery heater can solve this problem.
  5. The LiFePO4 battery may be mounted in any position (but check with specific manufacturers for their limitations and recommendations).

My Specifications  & Tripp-Lite Charger-Inverter Settings

To make an apples-to-apples comparison of LiFePO4 batteries from various suppliers I had a simple specification:

  1. Minimum 4,000 cycle life.
  2. Internal Battery Management Systems (BMS).
  3. Low temperature cut-off to protect the batteries.
  4. Over charge & over discharge protection
  5. Over current & short circuit protection.
  6. High temperature disconnect.
  7. Storage as low as -22F. Discharge temperature range -22°F to 140°F.
  8. Charging temperature range 32°F to 140°F.
  9. Charging Current range 0 to 20A (maximum 50A).
  10. Maximum output 100A.
  11. Be compatible with my Tripp-lite charger (14.4V Charging Voltage and 13.5V float, maximum 45A charging current). - Note 1, 2, 3 below.
  12. A minimum 7-year warranty.
  13. Prefer a removeable cover and replaceable BMS, but not mandatory.
  14. Blue-tooth (r) communications optional.

Note 1: The Tripp-lite has two charging settings for two different types of batteries:

  • "Wet Cell (vented)" (DIP Switch A1 in the DOWN position) and
  •  "Gel Cell (sealed)" or AGM (DIP Switch A1 in the UP position).  

Note 2: The Tripp-lite has three stages for charging: BULK, ABSORPTION and FLOAT.  The "Absorption" and "Float" stage voltages are adjusted by selecting the battery type:

  • Wet Cell: 14.4 VDC "Absorption", 13.5 V "Float".
  • AGM: 14.1 VDC "Absorption", 13.6 V "Float". 
Tripp-lite BULK Stage: In this stage, the battery is brought up to about 80% capacity using a constant charge current; the charge voltage can vary in this stage.

Note 3: The Tripp-lite has two charging current rates:

  1. 11 Amperes (DIP Switch B4 in the UP position).
  2. 45 Amperes (DIP Switch B4 in the DOWN position).
Battery Selection & Sizing Considerations 

To choose LiFePO4 batteries it would require these decisions:

  • Choose (1) or (2) 100Ah batteries (total 100Ah or 200Ah capacity; 80Ah or 160Ah useable).
  • Or choose (1) 200Ah battery (160Ah useable).
  • Determine mounting location: (1) or (2) 100Ah mounted outside or (1) large 200Ah battery mounted inside the coach (the 200Ah battery is too large for the Roadtrek battery tray). Inside mounting would add cost, as additional 6AWG wiring would be required.
  • A single 100Ah battery could provide 20A for 300 minutes (about 240W for 5 hours).
  • A 200Ah battery could provide 20A for 600 minutes (about 240W for 10 hours).
  • The Tripp-lite inverter is rated 750W continuous.  Ignoring losses, the inverter input for 750W AC output would be about 60A at 12.8 VDC.  The battery should be capable of providing that current output.  The inverter can provide 150% output for a short period of time, so a battery capable of 100A output would support that. Of course, higher battery current decreases available battery time; 60A output could deplete the 100 Ah battery within 100 minutes.

To protect the battery, a low voltage cut-off device at 11.2V is recommended.  The BMS of the battery I am considering will cut-off at 10.4VDC. However, this very low cut-off voltage may reduce battery life.  An in-line fuse is also recommended.  I do have a digital voltmeter installed, so I can rely upon that at the lowest cost approach.  Keep in mind that battery voltage is an indicator of the battery capacity, but the battery must be at rest for this method do be accurate.  "At rest" means nothing drawing a load.  In my Roadtrek I can use the battery voltage as an indicator if 1) The chassis/coach battery separator is in the "off" state, 2) the battery disconnect is "off" and 3) the inverter is "off".  About 15 minutes after disconnecting the LiFePO4 batteries I'll consider the voltmeter reading to be indicative of the battery capacity.  

Because the LiFePO4 can't be charged below 32F, mounting inside the coach has an advantage. The interior coach temperatures can be maintained above 32F while the coach is in use. However, if the battery were in the outside compartment a heating pad could be attached to the battery to provide some supplemental heat in the winter, thereby keeping the battery or batteries above freezing. It would be my preference to mount outside to conserve interior space.   The heater will add cost. 

Notes: 

  1. Certain RV organizations including FMCA and Escapees may offer price discounts to members for specific batteries.
  2. Certain manufacturers offer batteries which have internal heaters and have blue-tooth (r) communications.  However, this is at greater cost.
  3. The Roadtrek battery tray is outside of the coach.  Care must be taken to assure that the LiFePO4 batteries mounted within stay dry.
  4. The charging requirements of the battery selected must be compared to the capability of the Tripp-lite charger/inverter to assure compatibility.  Otherwise, a new charger and inverter would be required.  
  5. I decided to purchase a single 100AH SOK battery pictured at the beginning of this post.  If purchased directly from the U.S. supplier, the warranty is extended from 7 years to 10 years.  My cost for (1) battery was $624.93.  This included tax and shipping.
  6. If I decide to add a thermostat and 12V heater to keep the battery warm, my out-of-pocket additional expense would be about $40 (fuse, pad, thermostat).  A second 120VAC 84W heating pad with internal 45F thermostat is about $39. Why two? Well, if I decide to winter camp on 120VAC, I can heat the battery using shore power or the generator.  Alternately, I can use the chassis alternator to heat the battery using 12VDC while the Roadtrek is in motion or I can use coach 12VDC to maintain the LiFePO4 above freezing.  I have not purchased any parts, but I do anticipate installing at least the 12V heater.
  7. My Solar system will work with the LiFePO4 battery.
  8. Low voltage disconnect at 11.2V is recommended for maximum battery life. If I want to add an automatic cut-off that would be at additional cost. I currently have a DC display with alarm. Here's the post link:  New Voltmeter-Ammeter-Wattmeter for AGM batteries  

(c) N. Retzke 2022



Tuesday, November 5, 2019

Coach Batteries - How to get the most out of them


DC Voltmeter and Ammeter to monitor the coach batteries.
I added this to improve upon the Roadtrek LED battery indicator.

If left on, it also provides a power reading and energy consumed.
Roadtrek L-F-G-C battery display -
4 LEDs lit indicates charging or fully charged.

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The post points out what I've learned  about my AGM batteries including limitations of the L-F-G-C  battery display which is simple to read but can be misleading, as I discovered.

How do we use the Roadtrek if we're not in a campground? This is important because it influenced our battery decision. We do overnight in the Roadtrek off of the grid, but we don't do a lot of boondocking. If the temperatures are above freezing but cold we may run the generator and the heat pump. However, G and I have also slept in the Roadtrek in sub-freezing temperatures while off the grid and below the limit for the heat pump. We will then run the propane furnace if necessary and that requires 12VDC.  If temperatures dip into mild sub-freezing overnight and we haven't winterized we will run the hot water heater on propane and the furnace. If we are on propane I don't think it is practical to run the generator continuously overnight and we usually don't. We rely on battery power.

What I discovered. I did not understand the limitations of that L-F-G-C Battery indicator and learned that "F" or "Fair" is really "Poor" because the batteries may be nearing depletion at that point.

I also learned that the Inverter Off-On switch didn't always turn off the inverter function of the Tripp Lite inverter/charger; that was a switch malfunction. It caused unexpected battery depletion.

I realized I had been mis-using my batteries. I didn't understand that the entire 220Ah or rated battery capacity is not available unless I want to seriously reduce the lifespan of the batteries. I also learned that repeatedly using the batteries below the "F" Fair indicator and until only the "L" or "Low" indicator was the only one illuminated was not good for the batteries, if I want longer battery life. My lack of understanding coupled with an inverter switch problem led to the early demise of my first set of AGM batteries.

I've learned about AGM batteries and when I replaced them I also investigated Lithium-Ion batteries (LiFePO4). I'll point out what I discovered about the AGM batteries in this post, but first, I decided that I would continue with AGMs because:

  • 220Ah AGMs are sufficient for my use (approximately 110Ah to 170Ah useable - explained in this post).
  • I have an Onan generator which I can use to provide power for coach appliances and charging.
  • I was concerned about the low temperature charging restrictions of LiFePo4 batteries because my AGMs are mounted outside in the rear of the coach. That is the location I would use for LiFePO4 replacements.
  • I did not want to open my wallet for the more costly LiFePO4 batteries.
  • I decided to add a small 50W portable solar panel with de-sulfating 180W controller to help charging of the batteries. I concluded this was a better use of my money as compared to the LiFePO4 batteries. I may eventually put a 100W panel on the roof.
  • I decided that adding a good DC voltmeter/ammeter to monitor the batteries would assist me in using them and avoid the earlier problems.
  • There have been improvements in technology, and who knows, I may change my mind in 2-5 years.
This post is about a few things that may help you improve the life of your AGM batteries. 
I include a typical 12VDC energy audit at the end of this post.

My 2013 210P Roadtrek has two 6-Volt AGM lead-acid sealed batteries rated 220Ah. I use these on our treks in both warm and cold weather. We don't boondock a lot, but we do spend overnights off the grid. The numbers in this post are with those batteries in mind.

Some things I have learned
These lead-acid batteries can provide 4-6 years of good life, and sometimes more. But how they are used is a significant factor and determines how long they provide good service. Here are a few things to be aware of, and more on what the indicator in the Roadtrek is attempting to display and how it relates to what we observe with our batteries.

I don't think it is possible to reverse the aging of these batteries. It is possible to damage them.

How the batteries are used and discharged/charged is probably the single greatest determinant of how long these lead-acid batteries can provide good service. Battery life is determined by age and these cycles. We sometimes don’t get the life out of the batteries we may expect. I have two 6-volt AGM batteries. They are wired in series to provide 12V DC and can provide a maximum of about 220Ah (Ampere-hours).

What are DC Watts and Volts and Amperes?
These are important because they relate directly to our batteries. If we know the amperes our appliances and lighting are using we can estimate how long on a charge we can run them.

You can skip this and move on to the next section, but I think you will want to read this before you do an audit of your DC appliances and get a better idea of how much battery power you are using.
  1. Watts are Volts multiplied by Amperes. In our Roadtreks with 12V AGM batteries:
  • 12V x Amperes = Watts; for example 12V x 5A = 60 Watts.
The battery voltage does stray but for doing calculations using 12V is handy.
  1. Because we know watts and the voltage, we can calculate Amperes. For example:
  • Watts divided by volts = amperes. A 100 watt DC appliance: 100W/12V = 8.33A
An Ampere-hour (Ah) is a measure of amperes used per hour. Batteries are rated in Ampere-hours.
  • An Ampere-hour is using one ampere continuously for an hour: 1A x 1 hour = 1 Ah
Ampere-hours let us determine how much battery capacity is being used. For example, let's assume we three things that are using DC electricity:
  • 1.0A lights
  • 0.8A charging phone
  • 2.6A furnace and fan.
If we add these up, we can see how many Ampere-hours they would use in one hour:
  • 1.0 + 0.8 + 2.6 = 4.4A
  • 4.4A x 1 hour = 4.4Ah
If we multiply by the number of hours these appliances and lights are "on" we can determine how many Ah will be used in total, and as you will see this is useful for estimating how long our batteries will provide us with electricity between charges.

For example, let's say we left those appliances and lights on overnight, for 8 hours. Here is how much battery capacity they would use:
  • 4.4A x 8 hours = 35.2 Ah
Later in this post I provide some estimates which can be used to determine how long your batteries could last when providing DC power.

What use is knowing Ampere-hours? Ampere-hour (Ah) rating of a battery is a measure of how many amperes a battery can provide when discharging. One Ampere-hour is one ampere for one hour.

Unfortunately, a battery rated 220 Ah cannot provide 220 Amperes continuously for one hour.  In watts that is 220 A x 12V = 2640 Watts total.

A 220 Ah AGM battery cannot be used that way.  The more we attempt to pull from our batteries, the less Ah they can provide in a short period of time. I'm using the data provided by the manufacturer of my batteries, which is relative capacity. Relative capacity takes into account "battery fade" which normally occur to batteries as they age and are used:
  1. To get good battery life, these batteries should not be repeatedly discharged more than about 50%, and when discharged should be immediately recharged. In other words, one cannot get 220Ah out of these batteries if we want good life from them.
  2. A 50% discharge of 220Ah is 110Ah that we can use. The batteries must then be recharged. AGM batteries discharged repeatedly 50% and then recharged can provide about 1200 charge-discharge cycles. That’s the service life. Keeping batteries fully charged and not storing them partially discharged also is an aid to improved service life. Temperature also impacts service life, and temperatures above 77F reduce battery life.
  3. These batteries can be discharged repeatedly by 80% but that will reduce the life. A 80% discharge of 220Ah is 176Ah that we can use. AGM batteries repeatedly discharged 80% can provide 700 charge-discharge cycles. That is much lower than the number of cycles if we only discharge to 50%.
  4. Many battery manufacturers use a 20 hour rating for batteries. That is a more realistic and useful measure of battery capacity. For example, my AGM batteries are rated 220Ah, which implies continuous 220A for one hour. The actual 1-hour rating of the batteries is only 130A.
  5. The battery can, however provide a total of 220Ah over 20 hours of discharge.  That is a continuous 11 amperes for 20 hours.  But we should only go for 50% which is 5.5 A for 20 hours. This is to get better battery life. 
  6. Over 10 hours, a typical night, the battery can provide a total of 210Ah according to my battery manufacturer. In other words it can provide 210Ah/ 10 hours = 21 amperes continuously each hour for that 10 hours. But, if we only discharge the battery 50%, of the 220Ah full rating, that implies about 110Ah/ 10 hours or only 11.0 amperes continuous discharge for 10 hours.

    Conclusion: Over a typical night, my batteries could provide 11.0 Amperes each hour and provide good service life.
  7. These are the characteristics of the batteries in my Roadtrek when they were new. This capacity diminishes with battery age and use. 
  8. There are differences between manufacturers, so it is best to check the specifications for your batteries.
What are cycles?
  1. What does 1200 charge-discharge cycles mean? That's the number of times we can discharge and immediately recharge the batteries. For example, if you discharged the batteries to 50% and then fully recharged them once a day and every day, they could have a useable lifespan of 1200 days. 1200 days/365 days per year= 3.2 years battery life.
  2. If you did this every three days, then we would complete a cycle every three days, or 365/3 days = 122 cycles in a year.  1200 cycles/122 cycles per year = 9.8 years life.
  3. However, because of other factors including temperature, length of time the batteries sit in a partially discharged state, battery age, etc. it is unlikely we will ever achieve this in our Roadtreks.  The manufacturer’s battery data is based upon ideal situations, including a temperature of 77F
What occurs as batteries age? As batteries age there are internal changes and that reduces the capacity. For example, after 600 cycles of charging and discharging the battery to 50% what occurs?
  1. As the batteries age, even under ideal conditions, we may not get that 110Ah because the capacity of the battery diminishes as it ages. This is called battery fade. In other words the battery initially can provide 220Ah, but the actual capacity decreases over time.  We notice this as a more rapidly falling terminal voltage, which we can see on the Roadtrek L-F-G-C display, which spends less time in the “Good" area. In other words, the display falls from C to G to F more rapidly than one would expect. That is an indicator of aging batteries with diminished capacity. Eventually we decide that the batteries don’t provide us with enough power to get through the night or whatever while powering our devices. We then get new batteries.
What happens if we repeatedly discharge the batteries below 50%?
  1. Frequently discharging the batteries below 50% will further reduce the service life. For example, we can repeatedly discharge them to 80% (20% remaining). If we do this,  the service life will decrease. My battery manufacturer states that discharging repeatedly 80% will reduce battery life to about 700 cycles. That is considered to be the lowest acceptable service life.
  2. At 700 cycles if we discharge and charge every 3 days (122 times a year), the batteries will have a useful life of about 700 cycles/122 cycles per year = 5.7 years. But we must also consider the aging of the battery, temperature and so on. These also reduce battery life.

There is a trade-off.
  1. We must decide between longer battery life, or more power from each charge, or a compromise. In practical terms we must choose between how long we want to power our DC appliances each time we discharge the batteries and how long a battery service life we want. We can’t get both maximum power for maximum time because the deeper we repeatedly discharge the batteries the shorter the service life. We notice this as how quickly the available power diminishes. Of course, we can replace the batteries every three years or so. That is a financial decision.

Does the Roadtrek “L-F-G-C” tell me when the battery is at 50%? The information I have about this is the “G” indicator is “ON” if the battery in my coach is above 11.9 volts, which is about 29% state of charge for my batteries.
  1. If I use the "G" indicator and recharge the batteries when it goes out, then my battery manufacturer indicates that I can get about 800 cycles from my batteries if I discharge them to about 20-30% repeatedly.
  2. Important Note: I have not verified the Roadtrek indicator with actual measurements comparing the battery voltage to the indicator LED thresholds (I use a voltmeter and no longer pay much attention to the indicator). I'm using published information and it is my understanding that the “F” indicator is “ON” if the battery is above 11.2V,  but the table for my batteries indicates a 0% Relative state of charge when the voltage decreases to 11.6V and completely depleted at 10.5 volts. The table provided by the manufacturer of the AGMs in my Roadtrek indicates 50% relative State of Charge = 12.35V and 100% State of Charge = 12.9 volts.  I’ve seen other AGM charts and those batteries were in the range of 11.66V (20%) to 12.05V (50%) to 13.0V (100%).
What else should I know about the Roadtrek “L-F-G-C” indicator?
  1. It measures battery voltage. This is an approximate indicator. A battery will give two different voltage readings. One reading if it is being discharged; the another is if it has rested (no discharge) for about 6 hours.  The battery specification for “State of Charge” are usually for a resting battery. If a battery is discharged and then allowed to rest the voltage will usually increase. In other words, the battery may have more capacity remaining than the “L-F-G-C” indicator represents when we are in our Roadtrek and discharging the coach batteries.
What is the best approach for the batteries?
  1. Use a voltmeter if we want a better idea of the condition of the batteries, so we avoid excessively discharging them. Avoid high temperatures because battery life decreases at higher temperatures.  Measured  life is usually at 77F and many batteries will lose half of their life if the temperature is 95F. Don’t charge if above 120F. Charge the batteries after every period of use. Don’t discharge more than 50%. All of these things improve the life of the batteries.
  2. We can add a voltmeter in the rear cabinet of a 210P to monitor the voltage. 
  3. 12VDC socket in the rear of my 210P

    Inexpensive 12VDC digital meter
    plugs into socket in photo above
    Inexpensive 10 inch Splitter Cable for 12VDC
    Use if we need two devices from a single 12VDC source,
    such as the photo of the cabinet, above. 

    Dual, fused splitter with digital voltmeter
    and USB sockets


    As battery capacity diminishes what does that mean in practical terms if I want to boondock? 
    1. There is a trade-off. Longer battery life, or obtaining more power from each charge. This is further complicated because as the batteries age, their capacity is diminished. The actual capacity of a 200Ah battery will gradually decrease to 190 Ah, 180 Ah, 170 Ah and so on. This is because the internals of the battery change as it ages. We usually notice this change because the voltage decreases more rapidly as the battery discharges. 
    2. The beginning voltage of a fully charged battery will be that of the charger which is about 14.7 volts, this is called “surface charge” and this charge dissipates quickly as we discharge the battery. Both good and faded batteries will usually show a "C" when on the charger, and immediately after they are disconnected.  That is deceiving and is not indicative of the actual state of the batteries.
    3. If an older 220Ah battery has a capacity that begins at 160Ah capacity then it won’t take very long to get to that 110Ah level which is the 50% capacity level of the new battery. We see this as a more rapid movement of the indicator as the indicator moves more rapidly from “C” to “G” to “F”.

Are there other things I should be aware of?
  1.  Here is the short list. Batteries must be properly charged in accordance with the manufacturer’s instructions, automatic chargers are best. For golf cart batteries at 12V that usually means an Absorptive/bulk charge at 14.7 volts and a Float charge at 13.5 Volts. AGM batteries may be slightly different with an Absorptive/bulk charge at 14.4 volts and a Float charge at 13.5 Volts. My AGMs recommend an Absorptive/bulk charge range of 14.4-14.7V and a Float charge of 13.2-13.8V. Because the lead-acid batteries are chemical devices, there are other noticeable issues. 
  2. Battery capacity is reduced at lower temperatures. A 200Ah AGM battery may have a capacity of only about 80% at 32F or 160Ah.  However, that is at a discharge of about 20Ah (about 4A per hour for 5 hours).If we increase the to 40Ah, the capacity reduces to about 70% or 140Ah. (That provides a rate of about 8A per hour for 5 hours). However, we still have to watch the battery voltage to determine the actual condition, or state of charge. We can’t simply watch a clock.
  3. A 200Ah AGM battery is rated to provide that amount, 200Ah over 20 hours. It can provide 200Ah/20 hours = 10 amperes per hour continuously for 20 hours. But we would not want to exceed that 50% discharge limit if we want acceptable battery life.  In practical terms, a 200Ah battery should be considered 100Ah. if that is so, it can provide 100Ah/20 hours = 5 amperes per hour continuously for 20 hours.
  4. If we increase the amperes used, then the time the battery can produce it will decrease. The same batteries will provide only about 170Ah over 5 hours, not the 200Ah rated over 20 hours. In other words, the more power we demand, the total amount decreases. That’s because these are chemical devices. If the batteries could provide 170Ah over 5 hours, that’s 170/5 or 34 amperes each hour at 100%. But if we allow only 50% discharge, we then can use only 34A x0.5 = 17 amperes per hour over the 5 hours. 17 amperes x 12 volts = 204 watts connected to the battery. 
  5.  If we use an inverter to get 120VAC from our batteries, we need to take into account the inefficiency of the inverter. In other words, if we run a 200W appliance at 120VAC on the inverter, the amount of DC power going into the inverter is greater than the AC power coming out. Simply turning on the inverter uses power, perhaps as much as 200W. To maximize battery power, leave the inverter Off if we don't need AC from the batteries.  Tripp Lite suggests using 1.2 as an inefficiency multiplier:
  • Begin with the AC amperes or watts of the 120V appliance. If 2.5A, then the watts are 120V x 2.5A = 300W.
  • Then to determine DC amperes divide by 12V.  300W/12V = 25 DC Amperes.
  • To estimate the battery Ampere-hours (Ah) required, multiply the DC Amperes x time x inefficiency.  25A x 4 hours x 1.2 =  120Ah. That’s a rough estimate and it exceeds the 50% capacity of a 200Ah battery.
  • To determine amperes from an appliance using DC watts (such as a TV), simply divide the watts by 12 volts.  For example for 100 watts: 100W/12V = 8.33 A
How can I recharge my batteries? The sources of electrical energy for recharging are determined by what’s installed in your Roadtrek. These may include the following:
  • Vehicle engine.
  • Shore power.
  • Onan generator.
  • Underhood generator (GRU) - I don't have this.
  • Solar panel.
How long does it take to re-charge batteries?
The more depleted the batteries, the longer the charging time required. 


  • Using 120VAC and the Tripp Lite 750 Watt Power-Verter DC-to-AC Inverter/Charger, it can take 12 hours to completely recharge discharged coach batteries. This was confirmed by a factory technician. Furthermore, the charger has two settings: 10A and 45A.
  • Using the Onan generator is the same as shore power because the Tripp Lite is used.
  • Solar may take longer. This is determined by the amperes available from the solar panels. If a 45A Tripp Lite requires up to 12 hours, it is reasonable to assume solar may require more time because a 200W solar panel is providing about 17A. However, most RVs on solar don't draw down the batteries because during daylight hours the solar provides a part or all of the DC power requirements of the RV.
I need a 12V DC power budget.  To do this we need to do an energy audit. That’s a way to determine how far my batteries can go before I run out of DC power, and then have to recharge
  1. To get maximum power available and maximize your batteries, it is better to use 12VDC appliances than 120VAC connected to the inverter because of the inverter losses. 
  2. Add up the DC amperes to determine how much you use. Then calculate the Ampere hours to see how long your batteries will last before requiring a recharge.
  3. Add up the ampere requirements of all of the DC appliances and things in your RV. Product labels are reliable sources. Because there are differing options, I can't provide the Amperes required for each and every device. But I do provide a list with some approximations if I know what they are:
  • Lighting 12VDC (LED uses less DC energy than bulbs and fluorescent) - Varies.
  • Each LED light may require 0.25A.
  • Lighting 12VDC fluorescent, single 20W bulb - 1.3 to 1.8A. 
  • Lighting 12VDC fluorescent, dual 20W bulbs - 1.8 to 2.2A. 
  • Propane detector 12VDC about 0.1A.
  • 12VDC TV (about 32W or 2.7A).
  • Amplified TV antenna - varies.Macerator 17A 12VDC when running.
  • Water pump (varies with pressure and flow, possibly 5A 12VDC when running).
  • Inverter 2A losses or more if ON.
  • Roof fan - Fantastic Fan 3350 - 2.5A when running (varies with speed).
  • Bathroom fan.
  • 3-way Refrigerator 12VDC for controls - approximately 1 to 2A.
  • 3-way Refrigerator on 12VDC cooling - approximately 20A.
  • Dometic LCD single zone thermostat.
  • 16,000 BTU Suburban Propane Furnace when running - 2.8A.
  • 12,000 BTU Suburban Propane water heater - 12VDC Module board - 1.0A.
  • 15 inch Laptop - estimated 90 watts 120 VAC.
  • Smartphone charging - estimated 5 Watts @120V,  0.5A @ 12VDC.
  • CPAP machine - varies.



(c) Copyright 2019 Norman Retzke "All Rights Reserved".  See disclaimer notices.