[bilsbie]

Dumb question but I’ve always wondered if we could make a giant reusable “hand warmer” type chemistry around the battery and use that to get it going in cold environments.

Looking into it more. Maybe something like supersaturated solution of sodium acetate (plus water) in a sealed pouch with a metal disc. Bending the disc triggers crystallization, releasing stored heat (around 130–140°F for 20–60 minutes). Boil them to reset.

So you could boil and reset them during charging and click them off if needed in cold weather.

[0_____0]

One way I've seen of doing this is to include a PTC heater. It's a heating element that you feed DC. It has a positive coefficient of resistivity vs. temperature, so it'll asymptotically approach a temperature defined by the structure of the material. No PID controller required, it's just a sheet of material you include in the battery structure.

Granted, you have a minor bootstrapping issue wherein you need the battery to be warm before you use battery power, but at very low % of the battery's power capacity I suspect it's less of an issue.

[gpm]

I don't think it's a dumb question at all. Storing thermal energy separately from electrical energy would make plenty of sense if we could store the thermal energy better (cheaper, lighter) than the electrical energy.

A quick search suggests that sodium acetate used like this stores 230kj/kg (i.e. 64 Wh/kg in the units used for batteries) [1] which is significantly worse than the sodium ion batteries being discussed. Same order of magnitude though, so maybe there's a better material that would make it work.

[1] https://www.sciencedirect.com/science/article/abs/pii/S13594...