I have a fairly new battery in my 1978 Cadillac Seville. If I measure its performance, it brings 12.8 volts. If the car is running, it delivers 14.8 volts, what means, alternator is okay. When I turn the ignition key, the car will not start, it just clicks, as if the battery is empty. Starting only works, when I connect a second battery. what can I do, what might be wrong?