Charging a LiFePO4 Battery in the Roadtrek Class B RV

Tripp-Lite Charger-Inverter DIP switches and LED indicators

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

New LiFePO4 battery, new solar, battery compartment heaters

 

Work station in Arizona

The modifications are substantially complete.  Earlier posts go into the pros and cons of LiFePO4 batteries, and the issues encountered when attempting to charge them in below freezing conditions.  I decided to install small heaters in the outside battery compartment.  I also upgraded the solar system I had installed in 2014. 

I'm of the opinion that components should be sized and selected carefully so as to extract the best benefits from the system.  In other words, the components should be selected to perform as a system.  It is desireable to get the best performance for a specific cost. System component selection includes the type and wattage of solar panels, the solar controller type and capacity, the Ah of the battery, the wattage of the inverter and other components, and even the size and ampacity of the wiring.   To do otherwise means overspending on some components while being performance limited by the weakest links in the chain.

In my experience, too many of us RVers are inclined to spend our money to get what we think we might need, rather than spending it on what we will use.  Experience may be the best teacher. 

When I decided to replace the coach batteries, I concluded this was an opportune time to evaluate the performance of my existing solar-battery charging system and make any alterations. In doing so I unconcealed  the weak link in the chain and decided to so something about it. Of course, this is an iterative process; once the weakest link is eliminated, there is the next one on this particular chain to deal with. 

I suggest some self-control and restraint may be in order. Otherwise, one might build a Roadtrek with a "warp-drive" Lithium-ion battery pack.  Oops, G just reminded me that this has already been done.  LOL.

This is the list of tasks:

1. Remove AGM coach batteries, install Lithium-ion LiFePO4 battery.
2. Mount shunt on the new battery (for the existing remote volt-ammeter display).
3. Remove existing de-sulfating solar controller used with AGM batteries.
4. Install (2) 12VDC battery compartment heaters and controllers.
5. Install (1) 120VAC battery compartment heater and controller. This heater has adjustable wattage.
6. Install MPPT solar controller. I chose a controller which accepts "user" settings which precisely match the recommendations of the battery manufacturer.
7. Install Blue-tooth communications module for MPPT.
8. Install low battery voltage automatic cutoff switch. 
9. Install fuses and wiring for the above.
10. Connect existing solar panel. Convert this to a remote portable solar panel.
11. Prep for a new rooftop solar panel. (2 total panels). This is anticipated to be a 100W solar panel on roof (to do). Wattage limited by the rooftop real estate available. 
12. Retain 120VAC power in battery compartment (installed 2014).
13. Install connector for portable Li-ion battery charger in the battery compartment. This is optional, but permits charging the battery without solar or the use of the Tripp-lite charger/inverter installed by the Roadtrek factory.

Remaining, to do:

1. Complete the solar panel wiring.  Add new rooftop 100W flexible solar panel. The goal is to have one mounted on the roof and one portable.  The existing panel is wired as the portable. This will allow parking in shade while simultaneously acquiring some solar energy via the portable panel.  I expect parking in the shade will be preferred to parking in full sun when the outside temperature is above 100F.  This is based upon our experience. LOL. 

Wiring:

All wiring is properly sized for the amperes which will be carried.  Fuses were added to protect the DC wiring. I did install a fuse for the portable solar panel. 

1. Battery wiring modifications are 4AWG.
2. The main Solar Panel wiring is 10AWG. Portable solar panel wiring is 12/14 AWG.  All solar wiring is new.
3. The 12VDC MPPT output is 10AWG and fused for 20A. 
4. The 120VAC compartment heater is sourced by the Roadtrek installed GFCI outlet under the side door passenger seat.  A power strip with circuit breaker was added. The power strip has an Off-On switch. This heater is controlled by a temperature controller and an adjustable watt control was added to vary the heat.
5. The 12VDC from the battery to power the 12VDC heaters is wired with a SAE connector cable, 16AWG and fused at 10A.  After the fuse each controller and each compartment heater is wired using 18AWG. Each controller/heater combination has an Off-On switch and a fuse. Actual connected amperes per controller = 2.0A (24W), but 18AWG can accommodate a significantly higher wattage heater, up to 200W and the controllers can each manage up to 120W.  None of the components should be stressed as sized.
6. DC power connectors are rated 65A.
7. Shunt for remote volt-ammeter was connected to the new battery.
8. Tripp-lite charger-inverter was retained. 
9. A plug-in connector is wired to the battery for a portable Li-ion charger, if that is desired to charge the battery. 
10. Each controller has a temperature sensor.  These are located in the battery compartment. Two are connected with 18AWG and one uses the factory provided cable, about 1/8 inch OD (AWG unknown).
11. The MPPT solar controller is wired with connectors for quick and easy removal, if that should be required.
12. The MPPT solar controller includes a temperature sensor. This is located in the battery compartment.  This is not necessary for the functioning of the MPPT controller with LiFePO4 battery, but it is a convenient method of monitoring the compartment temperature.  The controller and battery parameters including compartment temperature are displayed on a LCD display as well as via a smart-phone blue-tooth app. 

Top view - Heater Controls installed beneath rear-side entry passenger seat.
These are accessed by flipping up the seat bottom which is on a hinge.
When not in use or not required heater power is OFF using the switches.

Under side wiring - Heater Controls Shelf

Initial Heater settings (adjustable, using battery compartment temperature). System design can accommodate higher wattage heaters if this is determined to be necessary.)

. 

Battery Installation - Heater wiring and temperature sensors exposed,
prior to being covered.

Silicone heat pad cover and 12VDC heater controls fuse

Top view - 12VDC from MPPT Solar controller-
Mounted adjacent to interior water tank - Not yet installed:
portable solar panel fuse blocks

Battery Monitor DC Voltmeter-Ammeter mounted adjacent to RT power center. Voltmeter added in 2014. This is connected to the coach battery via a shunt and the circuit is fused. Connections are before any disconnect, so the battery voltage can be read even with the Roadtrek DC disconnect in the "off" position. 
I repeat, the circuit is independently fused!
An Off-On switch turns off power to the meter when not in use.

MPPT Solar Controller smart-phone App. 
Other screens provide more detail, control of load and history

Issues, Observations & Procedures:

1. I installed the MPPT solar controller, but two days later the LCD screen went blank.  I thought it might be some sort of "screen saver" but pushing the front buttons got no response.  The Blue-tooth (r) smart phone application worked fine and indicated the controller was functioning normally.  I contacted the factory and they suggested a hard reset (disconnect solar panels and power down the controller).  After 30 minutes I powered it back up. No change.  Renogy had me take some voltmeter readings to confirm all was properly connected. I sent photos to them and even several smart-phone screens at their request.  They agreed that the controller was performing normally but the LCD screen was inexplicably blank. They concluded it was a failure and the controller was replaced at no cost to me by the supplier.  
2. Making certain wiring changes in a class B can be challenging.  It took a bit of thinking and disassembly to determine how to do this; where to mount components, route the wiring, etc.  I determined a course of action prior to purchase of the various components.  Purchasing the battery was the easy part, after I had decided upon 1) Manufacturer, 2) AH, and 3) Where to mount it.
3. I didn't want to remove the side fabric panel in the inside rear of the Roadtrek.  To do this would have required more deconstruction than I wanted to do.  As it is, I had to temporarily remove some of the freshwater plumbing to gain access.  It took a bit of effort, but I was able to fish a stiff wire behind the fabric panel and pulled the new wiring for the portable solar panel into the space between the liner and the exterior fiberglass coach shell.  A new plug-in connector for the portable panel will be installed inside the passenger side rear exterior compartment. 
4. The solar panel system will be designed to accommodate using one or two panels, one fixed and one portable. The reason to have a portable panel is this will allow adding solar when the vehicle is stationary.  It also permits parking the Roadtrek with the rooftop panel in the shade while the portable panel is placed in full sun. However, if one panel is in full sun and the other in partial/full shade, series wiring is not optimal.  The design addresses this.
5. I built and wired the battery heater controls and tested them with the heaters on a bench.  This proved the wiring and functionality.  I wanted to bench test so that if any issues occurred after installation in the Roadtrek it would be attributed to the coach wiring and more easily isolated and corrected. 
6. I used ring terminals throughout which is prudent in an installation subject to vibration and jolts.  I used heat-shrink tubing to protect, insulate and support smaller wires at the connectors. I installed the heat-shrink tubing where appropriate. 
7. The 12VDC for the heaters is wired directly from the batteries with an ATC fuse. The fuse is within a foot of the battery + connector. In this manner the 12V heater system is protected and can operate independently of the Roadtrek power disconnect. I used an automotive SAE connector dis-connect cable.  There is no acceptable way to install a terminal block and I won't use a butt-splice for power. I joined the coach battery cable to the SAE cable using ring terminals bolted and insulated with shrink-tubing. 
8. The solar controller for the AGM batteries was installed by me inside the battery compartment in 2014.  This was disconnected when I installed the LiFePO4 battery. The replacement controller is larger, and I wanted it installed inside the coach. This required a change in DC wiring.
9. I decided upon a more costly MPPT solar controller so as to extract as much out of the solar panel(s) as possible.  I don't plan on living off the grid with solar.  But I do want to have sufficient solar to keep the battery charged and sufficient 12VDC for the basics of the coach (refrigerator controls, hot water heater controls, overhead fan, propane alarm, lights, PC, phone charging, etc.  But not all at once, LOL.).  
10. I oversized most of the heater circuit electrical components. Temperature controllers are rated 10A or more, wiring for the heaters has greater ampacity than required. Cabling for longer runs is 16AWG multi-conductor with jacket. This did increase the cost, but should provide trouble-free operation. Wiring outside the coach is protected and is installed in wire-loom split tubing which is properly supported.
11. I used 65A protected connectors for the battery connection to the MPPT solar controller.  This is an independently fused circuit, but I wanted a means to easily and safely disconnect battery power at the controller.
12. Solar panels are connected with MC4 connecters. 
13. I made several simple wiring sketches of how to add the low-voltage battery disconnect and placement relative to the existing 50A circuit breaker and the inverter.  I was able to mount the disconnect adjacent to the Tripp-lite inverter/charger.  I was able to re-arrange the 12VDC+ wiring for the disconnect and was able to add 12VDC wiring from the solar controller using available space.
14. The heater wiring was designed in my head, no sketches made.  I made a mental list of what was required, compared this to my inventory in Arizona and purchased what was needed.
15. I marked various power conductors and other wiring clearly.  I'll make a drawing for posterity and future maintenance.
16. I have a bit of clean-up to do in the battery compartment, but the project is essentially complete.
17. I'll add the second solar panel when convenient.  I'd like to see how this performs before I do that.
18. When not is use all heater controls are turned off using the switches I installed for this purpose. 
19. With adequate solar, the battery separator can be in the OFF state when travelling.  

Parts and Costs:

I used off- the -shelf components.  To reduce the cost of the battery compartment heaters, I used 12VDC temperature controllers which display in degrees Celsius.  The 120VAC control does display degrees F.  This list is not necessarily all-inclusive; see Note 1 at the end of this post.

Temperature control and compartment heater components:

120VAC temperature controller: $19.00.

12VDC temperature controllers: $7.00 each. (Total $14.00)

120VAC heater: $13.00.

4-outlet AC power strip with circuit breaker: $9.00.

12VDC heaters: $9.00 each (Total $18.00).

Heat Resistant Thin Silicone Grade Rubber Gasket Sheet $9.00.

Off-On toggle switches: $2.00 each (Total $4.00).

5-pair 65A connectors: $7.50 (one used).

SAE Quick connect bulkhead fittings 2-used, $5.00 each (Total $10.00)

MC-4 to SAE portable solar panel connector, 35A, 10AWG: $15.00.

Five 4-point terminal blocks: $2.40 each  (Total $12.00).

Two ATC/ATO inline fuse holders $6.00 ($3.00 each, one shunted for 12VDC negative). 

Solar and battery related components:

Automatic Low-voltage battery disconnect: $83.00.

MPPT solar controller:  $111.00.

Li-Ion LeFePO4 battery, 100 Ah: $575.00.

6-terminal buss bar (battery negative to MPPT): $13.00.

60 ft. 10 AWG wire for solar: $40.00

Two ATC/ATO inline fuse holders $6.00 ($3.00 each). 

4 AWG cables with lugs for automatic low-voltage battery disconnect: $13.00.

Battery manual disconnect switch: $15.00.

MC-4 connectors for solar cables w/ tool.  10 pairs $16.00.

Hardware, misc. wire and terminations (some from my inventory):

2/C 18AWG, 65 ft: $13.00.

2/C 16AWG, 33 ft. $23.00.

#16-14 butt splice connectors.

#22-16 butt splice connectors.

Thermal adhesive tape, about 5 ft. used.

20 ft. 1/2" wire loom split tubing: $13.00.

M8 bolt, nut, washer.

M4 screws, nuts, washers.

8-32 pan head machine screws, nuts, washers.

Heat shrink tubing, various diameters.

Nylon screw mounting cable clips, various sizes.

Zip wire ties and adhesive mounts, various sizes.

3/14" wide double coated foam tape.

Ring type wire connectors, various sizes #18-#10AWG.

#8 x 3/4" self-drilling pan head screws.

#8 x 1-1/4" wood screws. 

1-1/2 x 1-1/2 aluminum angle.

3/4 x 3-1/2 wood slat, length as required.

3/4 x 7 wood shelving. 

Gorilla glue.

Notes:

1. This is not a how-to-do-it post.  I'm providing it as-is and it is not a recommendation or a procedure manual.
2. When not in use all heater circuits are turned off using the switches I installed for this purpose. The 12V heaters are fused and controlled independent of the Roadtrek battery disconnect switch.
3. My solar panels for test purposes are (1) 30A and (1) 50A.
4. I'll be installing a rooftop panel and have wired for a portable panel. 
5. Every trekker has goals and expectations.  It is useful to outfit the Roadtrek so that their personal goals can be realized.  This included comfort expectations, the available heat, 12VDC and 120VAC power, cooling and water. 

(c) N. Retzke 2022

Adding LiFePO4 cold weather heaters, solar, etc.

 

3- Stage battery compartment heating

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

Transitioning to LiFePO batteries

LiFePO4 Battery on shelf, during installation

 

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

Lithium-Ion Battery Update - I changed from AGMs

 

100 Ah LiFePO4 Battery

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 lifespan.  Considering 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