Originally Posted by TTsixxer
Ohms law. Measure the alternator resistance with the wires disconnected, then divide it by 13.5 volts. The answer will be your amp output.
My gut tells me this won't work - correct me if I'm wrong, but isn't an alternator just an AC generator with a rectifier? If so, I don't see how measuring the input resistance of the output of the generator would allow you to determine the maximum current output.
The easiest way is to find the maximum power output of the generator (in Watts) and divide that by 13.5 volts. I've always been able to find the max power rating of any of my bikes generators in the service manual or owner's manual.
edit: Saw you already looked in the service manual - try looking on the web. Or you could just get a beefy potentiometer and keep increasing the load on the system until the voltage started to drop down under the charging system's voltage. Then you'd know how much of a load you could handle - therefore how much current you could safely draw with your electrics.