A lot of folks will disagree weth me but an alternator is not intended to charge a weak or dead battery, it is intended to maintain the charge of a strong battery to prevent the battery from loosing its charge from the voltage draw while the engine is running, lights being on, electronics operating and so on. Nothing changes a weak or dead battery better than a slow charge using a good battery charger with an adjustable amp switch. Slow charge at a low amp setting charges the weak or dead battery back to full charge with its rated cold cranking amps back i it. Voltage is the power but amps is the torque. Think of it this way, drinking a car head on into a brick wall at 30 mph……… the speed you hit that wall is your voltage , but the amount of damage done to you car and to the brick wall is the amps