Older automotive batteries had removal caps so the electrolyte, or acid, could be checked and topped off if needed. However, the removable caps could leak and possibly cause the battery to explode. An advantage of maintenance-free batteries is that the electrolyte is sealed inside the battery case and cannot leak or create fumes outside the battery. Modern maintenance-free automotive batteries will last many years, but may need to be charged if the vehicle's lights or other battery-robbing accessories are left on after the car has been shut off.
Open the bonnet of the vehicle and locate the battery.
Turn off the battery charger (if equipped with an on/off switch) and unplug the power to the charger.
Connect the red, or positive, battery charger cable to the positive battery terminal by squeezing the alligator clip with your hand to open the jaws. Place the open jaws over the terminal and release the clip so it attaches firmly to the terminal.
Connect the black, or negative, battery charger cable to the vehicle's heavy gauge metal frame or engine block as far away from the battery as the cable will allow. Face away from the battery when making this connection.
Turn the charger voltage adjustment to the proper voltage for your vehicle (6 or 12 volts). Set the charger to the "automatic charging" setting for maintenance-free batteries. Plug the charger into an electrical socket as far away from the battery as possible.
Unplug the charger once it has charged the battery and has shut itself off. Remove the negative (black) charger cable first by squeezing the alligator clip. Remove the positive (red) charger cable from the battery in the same fashion. Close the bonnet and start the vehicle.
Some battery chargers have a fast or slow charging adjustment. A slow charge will more thoroughly charge the battery; a fast charge will enable you to start the vehicle sooner if you are in a hurry. Battery charging times will vary from between one and 20 hours depending on the size of the battery, the battery's state of charge, and whether you are slow charging or fast charging. Most modern vehicles made since the 1950s have 12-volt electrical systems. Check your owner's manual before attempting to charge the battery if you are not sure of your battery's voltage.
Batteries are very explosive if not handled correctly. Do not connect a charger improperly or lay a tool across the negative and positive terminals. Always wear wraparound safety glasses when working near automotive batteries. Keep any flame and sparks away from the battery.