driving a vehicle putting out 18 volts when the accessorys are designed for 12 volt (13.8) could seriously affect lifespan of your accessories. Your cd player may have overload protection so the display will turn itself out. did the mechanic load test the battery when he tested voltage in. ? sometimes a battery will be bad from new. I don't know about generators but when we have bad batterys at work the volt gauge will show high charging rate at all times when running as the alternator will try to charge the battery with the bad cell. and the battery with the bad cell will drain the others as we have 3 or 4 hooked together.