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