I bought a used mower and I charged the battery to 100 percent.cranked it maybe six seconds and let it run for a few minutes and go to restart it and the battery is on 80 percent on the charger.i did this a few times and now the battery wont even charge.put another batt on and it did the same thing.being used could it be a wire not connected or is it the alternator.model 407777-0124-e1 19 h.p briggs