Ford Bronco Alternator 2

Does anyone know what tests are done on the alternator of a Ford Bronco 2? It gives me 13.4 volts output, but it's not enough to keep the battery charged after more than half an hour of use. If I turn off the truck after a couple of minutes and try to start it, the battery dies and it doesn't have enough power to start.

That alternator charge is sufficient to charge the battery; the problem may lie with the battery itself. Put a tester on the battery and ask someone to turn the ignition on and start it. If the battery drops below 11 or 10 volts, it's considered bad. If it doesn't, the problem may be the starter.