Having a trickle charger can help save a low battery from going completely dead. A trickle charger may even recharge a dead battery as long as the battery cell itself is still chargeable. You can take the car battery into any auto parts store to have the battery cell tested to see if it is still capable of taking a charge. Charging a car battery with a trickle charger is simply a matter of properly hooking up the trickle charger to the battery and waiting.
Turn off the vehicle if it was running and completely remove the keys from the ignition. Open the bonnet to the vehicle.
Disconnect the battery wires from the battery terminals with an adjustable wrench starting with the black negative connection. Usually it does not matter what terminal you disconnect first, but just to be safe, disconnect the negative one first. This is to prevent any accidental electrical injury in case there was an unknown power source still feeding the battery. Repeat this step removing the positive connection. Move the battery connection wires away from the battery terminals.
Attach the red positive clamp from the trickle charger securely over the red battery terminal. Securely clamp the black negative one over the other battery terminal. Make sure the trickle charger clamps are snug and will not wiggle loose without force.
Switch the control switch on the trickle charger to "6" amps if your trickle charger has an option of amp level.
Plug in the trickle charger to an outdoor power source using an extension cord if necessary. Wait 10 to 12 hours for the battery to completely charge or until you see the green light come on the trickle charger.
Some trickle chargers come with a "pigtail" terminal. This is meant to be permanently attached to the battery terminals with the pigtail hanging safely away from the battery. The idea is that you can then just plug the pigtail into the trickle charger instead of disconnecting the entire battery set-up when you need to charge the battery.
Do not put a trickle charger on a battery while a vehicle is running. Do not use a trickle charger meant for motorcycles and snowmobiles to charge a car battery--they were not designed for this purpose. Use only approved trickle chargers for car batteries to recharge your car battery. If you hook up the trickle charger and the green light indicating a full charge comes on within a short time frame--say, 15 minutes--when you know the battery is quite low, then you may have a bad battery. You will need to take the battery to an auto parts store to check the condition of the cell itself.