LFP and long storage cold temps

ws9876

What is the best way to store Lifepower4 batts for 6 months at -10F to +40F without any input ?
Just sitting there unheated but inside...

BB.

As always, refer to the Mfg's instructions. Here is one example:

https://www.redodopower.com/blogs/learn-about-lithium/a-comprehensive-guide-on-how-to-store-lifepo4-batteries

My two cents... First, determine if your LFP batteries have a battery Monitor System (BMS)... LFP have very low self discharge, however, a BMS can draw a bit of power even if the battery bank is disconnected. Should not be a big issue, but there are BMS with wireless communications, etc. and that can add to the power drain over time.

In general:

1. Cold is better than hot when storing almost anything. The engineering rule of thumb is for every 10C (18F) below room temperature (typically 25C/77F), the "aging" is cut by 1/2 (storing at 15C, battery will age 1/2 as fast; store at 5C, battery will age 1/2*1/2=1/4 as fast)... Conversely, storing at 35C, battery will age 2x faster (this generally applies to both "aging of battery chemicals, and to self discharge--I.e., cold batteries self discharge slower than hot batteries).
2. Thermal cycling causes mechanical stresses/failures. I used to do qualifications for computer disk drives long ago... Best quick test was to run from min to max operating temperatures 2x per 24 hour period (not violate any product specifications). A "poor quality drive" would fail the tests in less than 2 weeks of cycling (28 thermal cycles). A "good drive", would not have any failures in 1+ months.
3. Suggestion is the keep batteries cool if possible (store in cool/dry locations) and keep in an insulated box to limit thermal cycling... Note that insulation may be an issue in warm to hot climates as batteries are used/cycled (Lead Acid batteries generate a fair amount of heat cycling, LiFePO4 batteries typically generate little to almost no heat during cycling--Low battery resistance and "efficient" chemistry during cycling).
4. Typically for LiFePO4, avoid charging below ~10C/50F or so--Check mfg. specifications (and definitely avoid charging below 32F/freezing).
5. Ideally, charge/discharge LiFePO4 batteries to 50% state of charge "before long term storage". Or at least in the range of 20% to 80% SoC is OK. (vs Lead Acid batteries that should be charged to 100% before storage, and recharged again at/before 75% state of charge).

That is my two cents suggestions (not a battery engineer).

-Bill

ws9876

so its ok that they will be freezing for months off and on...they will be totally disconnected and the master bus on the Batt will be turned off. The other alternative is in a box that is 4 ft below grade but its un heated as well and will basically be in the same temps as in the building. Both closed up for the winter.

Horsefly

For LiFePO4 batteries that are completely disconnected, letting them get cold - even below freezing - is fine. The important thing is you should never attempt to charge a LiFePO4 below freezing. For storage, the general rule is that you should have the batteries at about 50% SoC, as Bill said. 

The two things that cause LiFePO4 batteries to loose capacity are high temperature (the worst thing) and time. Calendar aging will always take its toll, but at least on LiFePO4 the lifespan starts out pretty long anyway. Cold temps actually slow the aging, so it actually will help you!

Very important: Make sure the batteries are above freezing before you put them back into service in the spring!