Many no longer short and burn out when punctured. Also, since the fire is not primarily limited by outside oxygen, but high temperature transfer, having cells loosely arrayed makes them less likely to fail in a chain reaction.
A shorted cell is more of an issue than a puncture. In extremely rare cases, Tesla battery packs have spontaneously started on fire. A shorted cell will get hot enough to start shooting out flames (flammable chemical boiling). Depending on cell layout, this can cause a cascade effect. Electric unicycles use the same battery cells as a Tesla, 18650 or somewhat larger 21700 cells, tightly packed, but single or double enclosed in a case. The battery packs use multiple cell groups, 20 (2|4|6|8 cells in parallel per group) for 84 volts, 24 for 100.8 volts, ..., and now 30 groups, 4 cells per group, 120 total cells, for 126 volts, which is what the prototype is using. In the case of this recent electric unicycle prototype, apparently something on the control board shorted (probably one of the mosfets used to drive the dc motor), causing it to cutout. This was at slow speed, so the rider just stepped off to the no longer self-balancing unit, and somehow the fuse failed or some other issue shorted the pack long enough to trigger a cascade event in less than a minute after failure. Normally worst case is the circuit board burns up, but the fuse blows and protects the battery pack. This was an unusual exception. Skip to 10 minutes and 10 seconds into the video (I put a starting time stamp on the url, but it's not working here). Note - I still ride my small electric unicycle, (35 lbs, good for about 20 mph, the big ones weigh 77 lbs and can go about 50 mph, not a speed I would want to face plant at due to electrical failure). Again, note this was a prototype, loaned out to a series of advanced riders that push the limits of the unit to check for flaws. In this case, the rider was testing acceleration and braking torque limits when the unit cutout, and normal failure mode would be blowing a fuse, in some cases burning up the control board, and very rare, a battery fire.
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Topic: Battery fires - rare but possible due to short, not puncture (Read 2642 times)