The most common cause of bright or dim lights failing completely is a bad dimmer switch. It's also possible that the same filament on both headlights is burned out, so test that first with an ohmmeter across the bulbs or by putting voltage directly to the lights with jumper leads.
If you're running a generator, it's not uncommon for the voltage to rise noticeably as the engine is revved up. There's really no fix for that, as DC generators don't turn fast enough at idle to develop max voltage. Be sure all the connections are clean and tight, especially the battery post terminals. Try checking the voltage at the battery posts at idle, then again with the engine running 1500 RPM or more. I wouldn't be surprised to see 1/2 volt or more of difference. If an alternator is doing the same thing, there's a problem with the alternator or the regulator.
Jerry
