How to Use a Car Battery Charger

An automotive mechanic using a jump cable to jump start a dead car battery. — A car battery charger is an essential tool to have in your car; it may save you when you least expect it. warodom changyencham / Getty Images

A car battery contains chemicals that produce hydrogen gas during use. Hydrogen gas is volatile and has been known to explode under certain conditions, causing serious injuries. For example, a car battery may explode while starting the car, while jump starting or by carelessly shorting the terminals with a screwdriver. In fact, under certain conditions a car battery may explode while just sitting in a parked car or on a table [source: battery-chargers].

Keeping this mind, here's how to safely use a car battery charger:

Use the correct charger for your particular car battery. Check the car owner's manual for information (or ask your car dealer or trusted auto mechanic).
Read the charger manual and make sure you understand how the charger works and what precautions are necessary. Pay close attention to all the warnings.
Remove both battery cables from the battery terminals. First remove the negative (black) cable and then the positive (red) cable.
Check that the charger is not plugged in to an electric outlet and that the power switch is off.
Connect the positive (usually red) charger cable to the positive (+) battery terminal and the negative (usually black) cable to the negative (-) battery terminal.
Decide whether you want to charge the battery slowly (trickle charge) or quickly. Select a lower charging voltage and current for a trickle charge and a higher setting for a quicker charge. Check the charger manual for the proper settings. Note that trickle charging gives the best results.
Select whether the charger will run for a specific amount of time or automatically shut off when the battery is charged. Not all chargers have this feature.
Leave the charger or cables in one place while the battery is charging.
Turn off and unplug the charger when the battery is fully charged.
Disconnect the cables, starting with the negative cable, and then moving on to the positive cable [source: CarsDirect].

Originally Published: May 11, 2011

Car Batter Charger FAQ

Can a battery charger jump-start a car?

A battery charger is designed to deliver a small charge over a long period, allowing the battery to absorb the charge and maintain a voltage and amperage. The charger does not create enough amperage immediately to satisfy the needs of the starter motor, and will be unable to jump-start the car.

Can I start my car while it's on the battery charger?

Yes, but getting enough charge into a flat battery might take some time.

What should I look for when buying a car battery charger?

When buying a car battery charger make sure that the charger is 10% minimum of your battery's Ah rating. For example, if your battery is a 100Ah battery, your charger should be a 10 Amp charger minimum. That said go for a greater amp charger if you want to charge the battery faster.

How do you charge a battery with a battery charger?

Usually, most batteries do not need a recharge for at least five years of their use, provided you are taking good care of your car. Before anything, first, check that the charger is unplugged. Now remove the battery cables by removing the negative cable first. Connect the positive charger cable to the positive battery terminal and the negative to the negative terminal. Select a charge setting (low or high). Leave the charger and cables while the battery is charging. Now turn off the charger and disconnect the cables.

Do you need to disconnect a car battery to charge it?

It's always a good idea to disconnect a car battery before charging.