I had a similar "What Happened?" incident, I just my car down on the track, when I noticed sparks coming from inside.. I ripped open the lid, and found that a brush wire was hanging down and just touching the chassis, causing a constant stream of sparks.. had to unplug the battery (turning it off did no good) to get it to stop.. my dropping the car to the ground must have been enough to get the wire to contact..
I was very confused about it until I realized this (the QC2) was the first ESC I had that shared the positive wire between the battery and motor, and it was indeed the negative side of a batt was bare, touching the chassis, and the positive brush wire of the motor was completing the circuit..
The 2 cells that were shorting were pretty weak when I tried the pack out, but after a couple cycles they're back to normal..
I think after that, my ESC lost its reverse capability.. sent'er back to get it fixed $60 bucks later.. ohwell, now I'm super-diligent where these electrics are touching..
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