Delphi is one of three major automotive battery manufactures. Delphi not only sells batteries under its own label, but also ACDelco and EverStart. It's smart to check your Delphi battery's voltage output regularly to be sure it's strong. You can't rely on the alternator to continue charging a battery once it's passed its peak, and a jump start only lasts a short while.
Pop the bonnet of the car and prop it up. Don't crank the engine during the test, otherwise the voltmeter will read the voltage of the alternator instead of the battery.
Attach the voltmeter leads to the battery. Connect the positive lead to the positive battery post and the negative lead to the negative battery post. The positive post on the battery is red and is marked with a "+" sign.
Turn the voltmeter on and check the reading. A healthy Delphi battery should read between 12.5 and 12.8 volts. Anything less is an indication of a weak battery, in which case you should take the battery to your local auto part dealer for further testing.
Crank the engine if you want to check your alternator as well. The reading on the voltmeter should be between 13.6 and 14.3 volts.
Be careful when handling a battery. Wear gloves, and check for acid spills. Apply baking soda to neutralise acid spills.