My A123 cells are dying... help?

[wulfgang]

I have an outrunner setup on a HB Lightning buggy. I'm running direct drive to the rear wheels only (no center diff) using a Turnigy 50-55 outrunner, kv=600. Running 6S A123 packs that I build and using a Turnigy 100A ESC.

So I balance the pack and then put it on the buggy. Within about 3 minutes at low throttle with some full throttle blips, the ESC cuts out for low voltage. So I shut it down. The pack comes off usually pretty cold with maybe one or two warm cells. When I check the voltages, it always turns out that only ONE cell is dead. The rest read 3.3 V. My charger won't let me charge that cell now because it has discharged too far.

I would say it's just a bad cell, but this has happened to 4 cells individually! Which means I have only had 4 runs on this thing, because every time I run it, a cell dies. Also, everything else I've read about A123s says that they usually maintain excellent balance.

Any ideas?


Also, just want to throw this out there for others who might be thinking direct-drive with an outrunner is a good idea: On paper, it looks ok, and you can find motors that seem to fit the bill if you use enough cells. My setup, for example, *should* be very hot if I could fix the pack issue. On the track, it would probably be pretty good.

But not when bashing. Here's why: consider that these motors are sensorless and so the ESC cannot do a hard, high-torque start from stall like with a brushed motor. The ESC has to roll the motor over a bit to find out where the poles are before apply full torque. This problem is worse on the outrunner because it has no gearing. So if an inrunner setup can apply full torque at say, 2 mph, then the outrunner won't be able to apply full torque until 7 mph (using a common 46/13 spur/pinion ratio). I can see this effect when I hit full throttle with the wheels off of the ground. Even with no load, the outrunner has a bit of a dead spot before it really revs up.

So no wheelies from a stop, even with 2WD. So I think my next setup will use the RC-Monster motor mount with an inrunner setup.