A 12-volt car battery must maintain a full charge not only to start your vehicle, but also to ensure that the battery lasts as long as possible. A battery that is left to drain or remain discharged for long periods will inevitably lead to problems such as sulfation or corrosion. This, in turn, will shorten the life of the battery. If you notice the battery cannot accept or sustain a full charge, this is a sign that you will need to purchase a new battery in the near future.
Check your car or truck to make certain all electrical accessories and switches have been disabled or turned off. Then release the latch on the vehicle's hood.
Remove any metal jewellery you may be wearing. Put on a pair of heavy-duty gloves and some safety goggles or glasses.
Access the battery by removing all shields and/or protective guards that conceal the battery terminals.
Examine your digital voltmeter. If it is equipped with a changeable voltage switch, ensure it's set at 12 volts. You will also see a red, positive clip and a black, negative clip. You must attach these clips to the corresponding terminals on the battery. Connect the positive clip first, followed by the negative.
Observe the voltage readout on the meter. This indicates the charge of your battery. Should you see a reading of at least 12.6 volts, then your battery is fully charged. Any number lower than 12.6, however, indicates a battery that is not holding a full charge.
Be aware that if you are testing your battery in cold weather, the voltage reading of a fully charged battery will be slightly lower. For example, if the temperature is -1.11 degrees Celsius, the reading will be approximately 12.5 volts. If your battery is not fully charged, try recharging it with a quality, 12V battery charger. Then allow the car battery to rest, unused, for about 12 hours or overnight and retest. If the battery tests low once again, in all probability you need to replace it.