LiFePo4 - reduced cold weather performance?

[FLBikeDude]

Not sure whose cells you're using, but 40F is not anywhere near dangerously cold for any cells I know about. Discharging at typical rates is not where the problem is anyway...it's in charging cells from a very low SOC at a very low temp. I wouldn't even give this a thought until you are well below freezing unless you typically charge at high rates. Too much charge current in a really cold cell will plate Li onto the cathode, wiping out capacity rather quickly.

My battery is this one... Http://www.endless-sphere.com/forums/viewtopic.php?f=14&t=63009

 

[liveforphysics]

How far below freezing ?.. can you slap a " rough / average " number on it ?

I don't charge at all if cells are below freezing. Most all production EV's will disable charging functions with cells below freezing as well.

However, it's not that it's always explicitly unsafe be charged at all, it's that the charge rate needs to not exceed the thermal kinetics driven lithium ion diffusion rate through the SEI and into the anode materials, or they stack up on the outside and end up plating at metallic lithium (as wb9k mentioned).

If you find yourself in a pinch with a cold pack that needs to be charged, you had better set it to trickle, and/or find a way to warm it even if you need to tuck it under your jacket or something.

 

[Jason27]

The cold has no effect on my battery which disperses the heat evenly over the pack.

 

[wb9k]

Ok, attached is an excerpt of a technical package on the gen 1.5 20Ah pouch cell. This cell is still in use, but only by some customers. The gen 2 cells perform a bit better than this, but the gen 1.5 cell still significantly outperforms any other LFP I'm aware of. I have to apologize for previously misstating some things about low temp charging. You have to stop at a higher SOC; low SOC is not where you have to be extra careful. Bear in mind that these numbers are for CELL temp, not ambient temp. Automotive OEM's enable charging in cold weather by heating the cells to a temperature where they can safely accept charge. The data here is a bit complex. Rather than try to walk everyone through it here, I'll answer questions as they come. In any case, the charts clearly show loss of max power delivery with falling temps. Charging ability suffers more. I have been able to easily observe the power output decrease in both my Hymotion packs and my electric motorcycle (which I commute on in temps as low as the mid-30's F). With the bike, you lose some range, a bit of acceleration, and a bit of top speed. Performance does tend to improve with drive time since the cells will self-heat. With the Hymotion packs, range clearly falls off a bit when it's cold out. It gets better again when the weather warms back up.

ALL batteries are affected in similar ways, at least on the output side of the equation. Chemical behaviors change with temperature, it's that simple. Nobody has yet developed a cell that performs evenly across all possible real-world operating temps.