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:

  1. 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).
  2. 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.
  3. Remove both battery cables from the battery terminals. First remove the negative (black) cable and then the positive (red) cable.
  4. Check that the charger is not plugged in to an electric outlet and that the power switch is off.
  5. Connect the positive (usually red) charger cable to the positive (+) battery terminal and the negative (usually black) cable to the negative (-) battery terminal.
  6. 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.
  7. 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.
  8. Leave the charger or cables in one place while the battery is charging.
  9. Turn off and unplug the charger when the battery is fully charged.
  10. Disconnect the cables, starting with the negative cable, and then moving on to the positive cable [source: CarsDirect].