A fully charged battery at my house on an average day is 12.75v at least 24 hours after it has seen a charger which allows the electrolyte to stabilize. If the voltage is down around 12 you need a charge. Put a voltmeter across the battery terminals and get a reading. Start the engine and take another reading. The max voltage from a regulated output will be is 14.5v but that is on a charged battery. If the battery is defective or discharged, it can't pump it up that high initially, but whatever the reading it will exceed what you read with the engine off.
Mark
