How was the generator tested? The preliminary quick test is to measure battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. Anyone can do that test with an inexpensive digital voltmeter. The second half of the test is a full-load output test that requires a professional load tester.
Your description of the symptoms suggests the generator has a defective diode. When one diode of the six is bad, the most current you will be able to get is exactly one third of the generator's maximum design value. 30 amps from the common 90 amp unit is not enough to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down, often within a few hours to a couple of days. You didn't list the engine size so there's no way to know which generator you have.
Even with a bad diode, the output voltage will still usually be within the acceptable range. Sometimes you can identify this by turning on as many accessories as possible with the engine running. High beam head lights take over ten amps. Running / tail lights will draw at least another five amps. The heater fan on the highest setting will draw between ten and fifteen amps. Add brake lights, the radio, and a power window, and you'll be drawing more than the generator can develop, and you'll see battery voltage start to drop below 13.75 volts, even when engine speed is increased slightly.
Load testers also test for "ripple". Ripple voltage will be very high when one diode is bad.
Monday, December 22nd, 2014 AT 1:50 PM