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