Battery in cold weather

[Lollandster]

I was riding my bike yesterday in the cold winter weather thinking I could get to the town and back on one charge. I was wrong. On the way home the BMS shut down the battery at 5.54Ah. My battery is rated 10Ah, I haven't tried running it flat before but I have used 7Ah on a charge earlier with still power to go in warmer conditions.

It was -14'C yesterday so I was wondering how much does this usually effect LiFePO4 batteries? I have heard you shouldn't charge the battery in under -10'C but I didn't know it would effect the range that much. Are there any chemistries that works better in the cold, I know lead acids are just as bad.

I would like a graph showing the battery capacity against temperature.

FYI. The battery was charged indoors at 22'C and I checked the cells today after charging and confirmed that all cells are balanced.

[Bikemad]

I found this graph for a Saft 44Ah 3.3V LiFePO4 cell:



Although the capacity is reduced at lower temperatures, it still manages to supply 90% of its rated capacity at -20°C, but it's only supplying current at around 0.2C throughout the test.
This would be equivalent to the GM pack continuously supplying just 2.05 Amps from full to empty.

As you will have obviously be draining your battery at a much higher rate, the consumed capacity is likely to be a lot less than the above graph.

Fortunately I don't have an ambient temperature of -15°C to test my LiPo pack in to provide a rough comparison. 



Alan
 

[Cornelius]

I have used my e-bike during winter for several years, and even at 0deg. C. the capacity are noticeable reduced compared to 15-20deg. C. At -10deg. C., i'm guessing the capacity are reduced to around 60-70%.

At temperatures below freezing, I seldom do long rides; only a couple of miles at a times, and I have to charge more often.

I'd agree with Alan; That graph would not be valid when discharging the battery at higher rates. Also, the battery's capability to deliver the rated continous C will also be reduced below freezing.

On a side note here; it has fallen about 1 foot of snow here the last days, so it's time to put on the studded winter tyres...

[Lollandster]

Thanks for the graph Bikemad. I should have guessed this problem as it is a big problem around here with cars not starting wintertime due to flat batteries, I just forgot to think about it. I'll bring my charger next time. I probably need to buy a bigger battery too if I want to ride wintertime.

Cornelius: I changed to spoked tires a month ago and they work wonderful on snow and ice. But make shure the spokes are the pointed type and not the flat headed type biltema sells.

The max current I can draw from the battery doesn't seem to change, but if the C rating is truly effected I might be hurting my battery when riding it hard in the snowy woods. Its not a GM batery by the way and I am using it at its maximum rated continues current.

[Morgen 3Eman]

Perhaps you can keep the battery in a warm place until you go for a ride....That would help

TTFN,
Dennis

[Lollandster]

Perhaps you can keep the battery in a warm place until you go for a ride....That would help

I always store my battery indoors, but the battery probably got very cold before riding home again. Maybe I need some thermal insulation around the battery?

I'm not dependent on my bike so I'll just use my car until it gets warmer. Buying a new battery pack this month isn't going to happen.