LiFePO4 in cold: charging

Question

I have used LiFePO4 batteries, only in discharge mode, by down to -15 deg C (charging was done 'back to the warmth'), and they worked fine. But from what I read it looks like Li-* batteries do not like to be charged by minus temperatures.

I would like to use large LiFePO4 cells with a solar panel by negative temperatures (typically -15 deg C, may be down to -30 deg C). I will have quite low charging currents, max 1 to 1.5Amps for a 40Ah cell. Do you think it may be OK to charge at such low rates (this guy seems to say no: Solar powered single cell LiFePo4 charger circuit ; but here it sounds like very low charge may be ok http://batteryuniversity.com/learn/article/charging_at_high_and_low_temperatures)?

Has anybody more specific experience with this? Or has anybody knowledge of a place where I may get military / space grade cells that accept cold if usual LiFePO4 is a no go, or alternative battery chemistry solution? FYI, here are the cells I use as by now:

http://www.batteryspace.com/lifepo4-prismatic-module-3-2v-40-ah-10c-rate-128-wh-----un38-3-passed-dgr.aspx

Answer 1

When working on the fringes of specifications, it is always best to speak with an applications engineer at the manufacturer. They often have access to data that is not part of their published specifications.

With that being said, you should give careful consideration to thermal management within your design. For example, a large insulated thermal mass in contact with the cells will retain heat generated during the exothermic charging cycle. This will help prop up the ambient temperature of the cells during non-charging periods - such as at night.

The choice of enclosure color can also play a role in raising the average ambient temperature of the cells.

You may also go so far as to use your solar power to warm the cells until they reach an acceptable charging temperature. While this is not the most energy efficient approach, it is a pragmatic one that may enable the application.

Answer 2

There is newer LiFePO4 vendor which allow you to charge battery at negative temperatures. I am sure there is more vendors like this but you have to do some research on them.

If vendor doesn't list negative temperature charging in specs - don't charge because you will damage the battery irreversibly.

Answer 3

Basically to prevent damage to negative electrode that has high ESR at cold temps, you need to heat up the electrolyte evenly before you can push 1C charge or more. This means it is slow. Otherwise if you heat up the layer around the electrode too fast then permanent damage may occur.

REF: https://relionbattery.com/blog/lithium-battery-cold-weather

With the low-temperature RELiON RB100-LT 100Ah battery, it takes about an hour to warm from -20°C to +5°C before charging begins.

A thermal analysis with ESR needs to be done with your pack to before can say for sure how to lower the ESR safely and start charging at rated currents. This can be done by feedback of testing ESR or by design with insulation and computing time to raise temperature safely.

That's my advice only FWIW.

Answer 4

I agree with both answers. This is not advise, but my opinion that you should NOT charge your LiFePO4 battery below its rating. Instead, you can discharge it to some extent to help warm it if you have the capacity or use an external already warm smaller LiFePO4 battery with a temperature sensor to warm your main battery via a heat mat or a current regulated Ni-chrome wire. It will increase it's life, is much safer, and will increase performance. For example, say for an ATV using a LiFePO4 battery, turn on the headlights to discharge the battery for a few minutes, then try to start it, if it does not start, wait a few more minutes then try again. As the battery discharges, it will warm the battery. I would not recommend charging on a cold LiFePO4 battery, call the manufacturer as stated above but they sell heat mats for batteries just for your case. And a "GOOD" BMS is always recommended. Ideally, the charger will have a temp sensor that disallows charging below a certain temp and has a built in temperature curve that increases the charge rate as the temperature increases to optimize charging. I have not seen these, but it makes good common sense. As the battery charges, it gets warmer, but lower charge rates at lower temps are good if the temperature is above the manufacturers rating:) Again, just imho. Food for thought: SSD's (solid state drives) hold data longer when the drive is hot, and have lower long term storage if the drive is cold. Batteries perform best when charged and discharged at specific temperatures.

Answer 5

Alternate thought, why is the temperature so low? If possible bury the battery in the ground as much as possible, and use the higher temperature of the earth itself. The battery does not out-gas like lead-acid, so no explosive mixture. Trying to put the battery high up on a pole will be counterproductive from a charging point of view.