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