My battery is about to arrive, Im getting started with a case for it. It will be acrylic. I was wondering if heat is an issue with these packs? Its 40ah 48v so a pretty big thermal mass. Im thinking about making one face of the box aluminum with thermal pads to conduct heat from the battery out of the case. Do you guys ever have issues with heat? Im also wanting to do some fast charging possibly right after a ride.
It depends on the cells and on the current you draw as well as ambient temp. and time. I use Molicel P42A high current cells in a 12S8P configuration. If I torture the battery with 100A batt current for some time, they can get as warm as 60 degrees celsius. If I use them reasonably and draw less than 80A batt current (10A per cell), they stay below 50 degrees which is acceptable.Temp is measured with a 10k beta 3950 ntc in the mittle of the pack. The ntc is connected to the motor temp vesc input to get a log (as motor temp.)
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Lots of posts here about Vesc overheating but I don’t recall anyone being too concerned with just the battery pack.
Obviously cooler is better so building a box with heat dissipating capability won’t hurt but likely not necessary based on the volume of riders here who haven’t experienced a problem. 
Fast charging a recently used battery is perhaps going to “harm” the battery more than the actual riding. The word “fast” can mean a lot of things so hard to comment on without numbers.
By “harm” I mean that the lifespan of the pack can be shortened by any kind of “abuse”
Yeah thats kinda what I was thinking. Should’ve specified, about a 40a charge rate. So i could have it filled back up in the hour. But i think the heat from that would be far too much
I don’t profess to be an expert in any of this but if you research the specs on the cells used for your pack you will likely find info on acceptable charge rates.
Might be a reasonable idea to have two packs. So no need to fast charge and have some redundancy if a pack fails.
Yes check the battery specs for charge rate. I noticed that my cell LG217050mlt have a much reduced rate below 25celcius. Normal rate for these 1.5amps. You say a 40amp charge rate for an 8p so 5amps per cell which would kill mine and probably create a fire.
My calculator can give you an approximation of cell temperature at specific currents:
https://lbre.de/hp/BPC/BPC.html
Enter the details at the top (cell count, int. resistance, weight), then set peak current to the actual average current you expect while foiling and time to something like 30 minutes (1800 seconds)
The result below (delta cells) will be how much the cells would heat up just by their own thermal capacity, in a vacuum basically without any ambient cooling. So worst case, but if thats already looking OK then no need to worry about cooling
Yeah, maybe in the future. It’s hard to drop 700 on a second battery!
Yea the cells are technically rated for 6a per cell. So theoretically they could but I think the heat is the issue. I’m not too worried about getting thousands of cycles out of the pack, either.