How to Charge a 12-volt car battery?
You can charge a 12-volt battery in several different ways. As long as your battery is in good working order and doesn't have a bad cell that would prevent it from holding a charge, you will be able to use all the following methods to charge it.
Use a Battery Charger
Hook up the dead battery to a 12-volt car battery charger. Written clearly on the control panel at the front of the charger, there are between one and three settings that enable you to either: "trickle-charge" (2 amp) your battery; rapid-charge it (50 amp); or use a medium (10 amp) setting. Trickle-charging is the best choice for a lasting charge. Most 12-volt battery chargers have a switchable setting on the front control panel allowing you to charge both low maintenance 12-volt batteries and deep-cycle marine batteries(3PA5015R 12V Car Battery Charger for Motorcycle Lead Acid Battery).
Use your car as a charger. Charge your 12-volt battery by starting your vehicle with a good battery, then manually removing the good battery while keeping the engine running. Hook up the dead battery in its place to charge it while the motor is running. A 15-minute charge should place enough power in the battery for it to be started again.
Reach for the jumper cables if you encounter a dead battery while you are on the road. A jump from another vehicle will enable you to get to a garage or an auto-parts store to have your battery tested. Simply place the positive (red) alligator clamp on the rescue car battery's positive terminal while the driver runs his engine. Clip the negative (black) alligator clamp on his negative battery terminal. Keeping the two alligator clamps on the opposite end of the jumper cables apart to avoid sparking, proceed to place the corresponding positive (red) clamp on your positive battery terminal and the negative (black) clamp on the negative battery terminal or post.
Use Solar Power
Harness the sun's free energy to charging your 12-volt battery (see Resources). A 13-watt amorphous folding solar panel can save the day by providing your 12-volt battery with the charge it needs to power your boat, car or recreational vehicle with no trouble at all. Simply hook the solar panel's negative and positive alligator clamps to your battery's corresponding terminals and place the solar panel in the sun.
Hook up a solar-powered battery maintainer (see Resources) to keep your battery charged by the power of the sun's energy. A solar panel fits on your dashboard and hooks up directly to your battery, keeping it fully charged.
Affix a solar panel to your vehicle's roof and connect it directly to the car's battery for constant charging.