Spread a very thin coat on the terminals. A fully charged battery will measure around 12.6 - 13.2 volts with the car off. With the car running, you can measure the battery voltage at idle with lights, and accessories off. Most cars will charge from around 13.2 to 15 volts. If it is lower, check your connections, belt tension on the alternator, and the alternator.
Alternators can fail gradually, with a bad diode, for example.
When this happens, the car battery may slowly run down. For more information, see the AutoTips Alternator page . This page has been accessed times.