[nudephil (OP)]

Here's a question sent in by Elric:

Batteries lose capacity in cold weather. Say I have a Li ion battery that's fully charged under room temperature, then I drop the battery's temperature to a low enough temperature (let's say -10C) and let it sit for a while so it loses capacity temporally. After that, I reheat the battery up to room temperature such that the battery is functioning normally. Ignoring all minor losses, will the reheated battery be 100% charged? Or does the electricity stored go away as the capacity of the battery lowers?

Can anyone help?

[chiralSPO]

Interesting questions.

I suspect that cooling down and then warming up a battery may indeed lead to a slight reduction in available charge (probably all due to what were called "minor losses" in the OP), but to make better predictions, we need to know more about what is going on:

When cooling the battery down, are you sure that there is actually a drop in stored charge? The circuit is not completed by change in temperature, so the battery isn't actually passing current. When the battery is cold, there will be a significant increase in its internal resistance, and could get to the point where it is functionally dead, but this would likely return to normal when the temperature is normal. This effect is probably largely due to reductions in ion mobility, which should be perfectly reversible.

One could also imagine cases in which cooling the battery down results in irreversible physical damage (ie the different materials in the battery all have different thermal expansion coefficients, so would all change in size slightly differently during the cool-down and warm-up, potentially leading to leaks, or loss of electrical contact etc.) In the most minor cases, the rewarmed battery would still be functional, but with permanently reduced capacity or increased internal resistance. In the extreme cases, the result is just the warmed up corpse of a battery.

Hope this helps!

[LaurenaS]

I also think that batteries kind of lose their ability to function at their full ability after the excessive temperature in this case temperature change. Usually, cool batteries can hold a charge longer than warm batteries and also perform better than hot batteries.

[alancalverd]

Lead-acid starter batteries work better when warm because they are required to deliver a high current for a short period, so need a low internal impedance. I've not made any actual measurements but there is some folklore about switching on the headlights for a few seconds before attempting to start an engine on a cold day, literally to warm up the battery.

It's a bit of an "iffy" calculation because the starter motor load may reduce the available voltage to the point that the ignition system fails. In the bad old days when cars still had starting handles I used an auxiliary dry battery and diode system to get the engine to fire and then run on its dynamo (definitely pre-alternator and electronic ignition days!).

Light aircraft engines use a mechanical "impulse snapper" to drive a magneto, so should fire on the first full revolution, however slow, regardless of battery voltage. When the battery is very cold it often happens that the first push of the starter button won't overcome the compression and complete a revolution, but if you wait 30 seconds or so, the second push gets everything going. Maybe the effect of sticky oil, but I suspect there may be some battery heating involved. 

[Juan25]

Cool batteries hold a charge longer than warm batteries. Cold batteries discharge faster than hot batteries.
Most batteries can be damaged by excessive temperature and may ignite or explode if it's too hot.
Refrigerating charged batteries may help them hold their charge, but it's best to use the batteries near room temperature to ensure they last as long as possible.