Measure the battery voltage with the engine off and with it running. You should find 12.6 volts with the engine off. Closer to 12.0 volts means the battery is good but discharged. Around 11 volts means it has a shorted cell and must be replaced.
With the engine running you must have between 13.75 and 14.75 volts. If it is low the generator won't fully charge the battery. If it is acceptable or even a little high, have it professionally load-tested. If "ripple" is high and the most output current you can get is one third of the generator's rated output, suspect a defective internal diode. That can leave you with insufficient current to run all the electrical stuff on the car and it will slowly run the battery down.
Wednesday, October 3rd, 2012 AT 12:16 AM