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Small Cabin Forum / Off-Grid Living / Keep an LFP battery above freezing
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# Posted: 27 Feb 2021 13:55

To Charge.
Been reading a lot on proper care and management of the LFP batteries if continuously hooked up to solar. Some people are building insulated boxes, installing all manor of little heaters to keep the box above 32*f (but not too hot). This can get fairly complicated if it gets to thermostatically controlled micro-mini heaters run off the battery itself (as it needs to be). And depending upon the ambient temps involved.
My thought is perhaps suitable for the 'shoulder seasons' where the temp come morning (when the solar starts to charge) may have been below freezing long enough that the battery in a plain insulated box may have chilled off too low).
The typical little inverters put out some heat and many have a fan to dump it out of the inverter box.
How about putting one in the box with the battery?
The inverter will be the micro-heater, the fan will circulate the air in the box and the battery may stay above freezing for some of those cold nights/mornings.
Ive got a 'spare' msw unit that never was all that good, it could go in the box semi-perm, nothing hooked up but maybe my powerhouse/toolshed led lights, and just switch it on of an eve before a cold night then switch it off next morning. With a cheap thermometer inside I could get a sense of just how to run my switching for whatever ambient temp.

# Posted: 27 Feb 2021 15:26

I have LFP and other lithium based cells in a few applications. My approach has been a little different.

The home LFP battery bank is in a utility room off the shop. Never goes below 45 F or so. No problem there with our normal use pattern. If I was to leave home for extended periods in the winter I would likely take the approach I do with the remote cabin, as noted below.

The cabin LFP cells are inside the cabin in a drawer box that slides out from under the bed when I want to see them. The cabin will sit for a month or more in winter with no use. Temperatures may remain below freezing for extended periods. During that time I leave nothing running. No power is used, no need to charge. I pull the PV array disconnect when I leave. The LFP cells don't really care much what the SOC is, but the SOC is usually above 50% and less than 80%. That is in the range for safe long-term storage and sufficient to be ready for use the next time. I warm the cabin with the wood burner or the propane or both and monitor the internal pack temperature before re-connecting the array. With the cabin occupied the batteries are warm.

The RV is treated much the same. The shelter it is in will drop into the freezing zone. When in storage mode the PV and shore power is disconnected. Water drained, etc. The batteries at some partially full SOC. Ready to be used.

Lithium cells can be used or discharged when frozen. The discharge warms them up. I do not subject them to big loads until they are warmed; just a light or two, maybe a small water pump or fan. I do have a temperature probe in the pack so I can monitor the actual temperature before re-connecting any incoming power.

I have EarthX LFP batteries in the ATV, the tractor, and the SuperCub. When I go to start any of those in cold weather I first turn on the bright lights for a few minutes. That discharge warms the battery to where it can supply enough energy to crank the engine. I will admit doing that takes a leap of faith as I am more used to hoarding the stored power in a cold-affected lead-acid battery. Turning on bright lights before cranking the starter just seems all wrong. That seems to make the batteries warm enough to be ready to accept a charge from the engine-driven charge system as well.

# Posted: 27 Feb 2021 16:20

That is certainly real world experience , and no need to build an insulated box!
Knowing how deep cold saturation goes all the way to a core Im leery of the time it takes for the pack to fully reach 'above freezing' before introducing a charge, but as you say, they are useable for at least light (pun intended) loads and plenty of 'reserve'.
Guess Im just too used to FLA bats and the maintenance & techniques to use them. Somebody said unlearning FLA to transition to LFP is the hard part.

# Posted: 27 Feb 2021 17:17

I "buried" a temperature probe in the middle of the packs. The LFP cells are assembled into battery "bricks" in series; units that are clamped between plates in blocks. Those are side by side and connected in parallel. The temp probes are inserted between the parallel units. So, I think they give a pretty good indication of internal temperature.

And remember that light discharge loads help the warming and that works from inside. It's a by-product of the exothermic electrochemical discharge reaction.

Unlearning is about as hard as new learning

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