Dead battery

[98taco]

It is up to me to fix my brothers 83 nissan... Anyway his battery keeps dying, completely dead as in new battery dead. The alternator has been tested twice and they say it is good. The only thing I can think is a short but how do you test for that. I do mechanics and welding I hate electrical. Any help would be GREATLY appreciated.

[JJS]

start pulling fuses from the box (while running) and see if any of them arc. that's a sign of a short.