Wire gauge vs. current

[travis]

Sounds like you are working on a beetle weight fightin' robot, and I'll weave this tale in that light.

210W sounds high for 6oz of battery, or perhaps 1lb of motor. Kinda like my 85W computer speakers with 24ga wire going to them (and 40ga inside). I had a 60lb spinner in the old days that had a theoretical max current draw of 400A, mostly from a grossly oversized weapon motor. I would have had maybe 3 seconds of 160A before a battery fire started. An amp clamp never registered more than about 100A, and that was a transient from the weapon motor start up.

Breakers have no place in this sort of machine. Your typical cheapie low voltage beetle type motor is a heater that happenes to produce torque, they are a perfectly good fuse. Sizing a breaker below your motor burn out current can leave you vulnerable at key moments. Even the 500ms of a quick breaker is enough time to sustain major frame dammage, a more costly repair than a fried motor. Test drive with breakers, but remove them for the competition.

You might want to find out who actually made the cells in those batteries, they'll tell you the straigh dope on discahrge characteristics. I don't think you could expect 30C out of a Li poly, shorted with a silver crowbar. The A123 litium ion battereis (availible here http://www.battlepack.com/A123.asp ) are quoted by the manufacturer as sourcing 70A continous, which is semi terrifying.

The quick answer is to use the above referenced wire sizing guides. The long one is to start with a smaller than expected gauge, lock the motor shaft in a bench vise, and keep bumping up the gauge until the fires stop (safety glasses + ABC rated fire extinguisher on hand). This is a good time to break out the amp clamp and determine what the worst case loads are. Note the voltage depression as well. The extra cost of silicone wiring is always worth it.

In closing, I never went to a robot fight where fires were frowned upon. These things only need to work for 3 minutes, 30 seconds if the audience is lucky.

Travis