A fellow EV’er commented about using 40 foot long greenhouse soil heaters
to warm his battery packs. Liking his idea, here is what I wrote back.

The ACP cars use battery heaters. It only draws a few hundred
watts when it’s running. It takes a day or two to heat up a cold pack,
but holds it there nicely after it does. The lead packs I’ve worked on
in ACP cars put off heat for a whole day once they are in the 30-40C
range. When we pull a pack out to replace a weak battery you can feel
the heat in a big way.

The pack heaters are also one of two reasons the ACP vehicles get 30k
miles from a lead pack. Heating up a battery to 30-40C also increases
it’s capacity, making it less likely to reverse a cell. The second
reason for such great longevity is having a BMS.

The ACP battery heaters are powered from the pack itself I just
remembered. That’s why they don’t have to worry about what voltage
they are charging from. They do have a simple IGBT circuit that turns
on, off, or pulses the heaters to keep them at the same temp with a
thermal sensor.

A little math shows that the ACP heaters total about 450 watts. So on a
336v system that’s about 1.34 amps. So our packs would supply a little
less as our nominal voltage is 312v. So it’s not a huge drain when
it’s running.