12.8 volts is absolutely not sufficient. The system must develop between 13.75 and 14.75 volts. A fully charged battery with the engine off will read 12.6 volts so at 12.8 there is no reason for any current to go back into the battery to keep it charged.
Measure the voltages on the small wires on the back of the generator. One must have full battery voltage all the time. If it is missing, look for a blown 15 amp fuse. If it's there, check that the battery warning light is turning on before you start the engine. If it is not, that circuit must be checked for a broken wire or stretched / corroded connector terminal. That is the turn-on circuit to tell the voltage regulator when to wake up and do its thing.
Friday, March 16th, 2012 AT 6:09 PM