NiMH Explosion - Causes and Consequences?

[aesmith]

Hi,

My flight pack blew up in flight yesterday. See pictures ...





There was no prior warning, power and climb rate all seemed normal, and in previous flights the battery never got more than warm. This time there was a lot of heat in the wreckage, enough to soften the plastic covering of the pack which sort of came apart as I eased it out of the fuselage.

Any ideas about possible causes? Also, should I assume that it would have damaged the other electronics to the extent that it would be advisable to scrap say the ESC or the receiver? The receiver appears to work, but the ESC is untested.

Thanks, Tony S

[RCModelReviews]

I've never seen that before. Seen a few go bang on charge but never while actually in use.

[aesmith]

After dissecting the pack, I'm not a lot wiser. There's a cut in the red wire, which might have caused a short, but that would have shorted out the whole pack and not just the one cell.

[RCModelReviews]

What can happen with a short circuit in a multi-cell pack is that the weakest cell discharges first and then becomes reverse-polarized by the current flowing through the system. When reverse-polarized, the cell heats up far more quickly and releases a great deal of gas -- hence the explosion.

[aesmith]

Could that happen without a short? I mean if one cell died but the others were happily pushing 8 or 9A through it. Looking again at the cut in the wire, I think it was caused by the burst as it was right alongside the cap that flew off, but there is no burning of the insulation around the cut or any matching scorch mark on the cell casing.

Anyway I resoldered it as a 7 cell pack (reading 8.9V off load) to test the ESC. As far as I can see the ESC is dead - no 5v supply to the receiver. I would expect it to at least power the receiver give that voltage.