backchannel: charging rates and self heating batteries

From a reddit thread. OP wants to know how hard he can charge LiFePO4 (4x 100Ah Renogy smart self-heated batts) from shore power.

Full disclosure: unless there’s some odd time limitation¹ I see little upside in charging as fast as possible from shore power. But this is an attempt to answer OP’s question.

TL;DR

I agree with [clueful commenter] that 0.4C is acceptable, especially at human-comfortable ambients. Colder temps might warrant gentler charging, even with self-heating batteries.

temperature and charge rate

If one uses the rig in cold weather we might want to be less aggressive, even with the self-heating function.

Stated charge rates for LiFePO4 typically assume the cells are at human-comfortable temps like 25C (77F). Below and above this² the charge rates decrease. This information is available in cell datasheets but, due to the high likelihood of causing confusion, not shared by the sellers of drop-in LiFePO4 batteries.

Since we don’t know what cells are in the Reno batts let’s use something where the data is known. Here is the relevant info for the cells in my particular bank (REPT 280):

Temp (T)	Charge rate (C)
<0°C	No charging allowed
0°C ≤ T <10°C	0.1C charge to 80% SOC
10°C ≤ T <15°C	0.2C
15°C ≤ T <45°C	0.5C
45°C ≤ T <55°C	0.2C
>55°C	No charging allowed

Per the manual, the Reno’s self-heating logic is:

The self-heating function will start operating automatically once the battery and the battery temperature drops below 41°F (5°C) and stop operating automatically once the battery temperature rises above 50°F (10°C)

which means

1. the Reno cells could get as chilly as 5°C.
2. and (if the info above is representative) should not be charged harder than 0.1C nor higher than 80% SoC at that temp

a real-world example of longevity with 0.4C charging

The oldest in-use and instrumented LFP bank I know of is Rod/MaineSail’s bank from 2009. He has charged at 0.4C to moderate voltages (13.8v) and the bank still meets capacity spec.

conclusion

Not telling anyone what to do. Given this setup and assuming I’d be on pedestal power at least overnight, I would charge at 0.1C. It’s safe at any (low) temperature the self-heating would allow and it will charge overnight whether we are doing 0.1C or 1C. Since time is not our friend with solar or alternator charging we might have to take the hit and charge more aggressively with those.

If a self-heated battery allows setpoint configuration one might increase the operating temps of the heaters to avoid the problems of charging cold cells. In my own setup I keep the bank externally warmed to 15C (~60F) to allow safe 0.4C charging at will, solar and alternator gods permitting. I also limit my charging to ≤13.8v, with occasional pseudo-Equalization jaunts to 14.0v to combat battery monitor drift.

comments

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1. Only staying on shore power in a day-use scenario where charge time is limited? Power grid going down? ↩

2. and at higher SoC ↩