I have a brushed DC gearmotor (rated for up to 14.4V) such that if I hook up appropriate batteries for it, it runs for a bit and rapidly decelerates and stops. If I watch the voltage across the battery / motor terminals with a meter while this occurs, it quickly drops from 14V (the original battery voltage) to the 1V to -1V area. 1V is not enough to drive the motor, so, given that, I'm not perplexed that the motor stops, but where is the extra energy going?
If I unplug for a while and then reconnect it repeats the same behavior. When I unplug and watch the voltage across my batteries they seem to go back up to 14V at a roughly linear pace (maybe this is just because my digital voltmeter is averaging over time - I suspect that's the case).
Also, this is hooking the battery up to the motor directly, as opposed to interfacing the battery to the motor with a motor controller. When I used the motor controller it seemed to work fine. Any ideas what could be going on here? Did I just fry my motor by flooding it with a whole lot of amps? This seems unlikely, because the stall current is rated at 85A, and my batteries aren't even capable of putting that much out.
Update: Actually, the motor doesn't work with the motor controller either. It's possible I didn't let it run long enough earlier with the motor controller to see the problem occur. The motor is also making an unusual, grinding-like noise. It's a planetary gear-motor, and something is clearly going on inside there but the shaft is not turning.