Generator/Alternator

Regulator = possible.

Battery = bingo! The only effective way to test a battery is to do a "load test". Most garages and even auto parts stores do them for free. Another approach is to substitute a known good fully charged battery. If the problem still occurs, replace the regulator.

However if the voltage falls below 6.5 volts as the ammeter reading increases suspect a failing battery.

Note: A cheap DVM (digital voltmeter) like sold by Radio Shack, etc. often give false readings on this type of testing. They are "fooled" by the electrical noise inherent in these systems. Better to use an old style analog meter, or a quality industrial DVM like a Fluke.

Maybe we should have mentioned to him not to let the thing run full charge for any long length of time.

Those systems can literally cook themselves to death.

Allan

It almost sounds like maybe it's the battery.

If it comes up gradually as the temp increases, this indicates that the system is trying to charge what it 'sees' as a low battery.

But, if it just "jumps" up there, might be a short in the field windings, related field wire, or the regulator points are sticking when hot.

Next time you start it, give it plenty of throttle to assure generator speed and watch that gauge to see how quickly it jumps in there; if it comes up slow with the temp, I'd suspect & test that battery.

Tryin to help,

Allan