A lead-acid 12-volt battery is made up of six individual cells. When all six cells are functioning properly, the battery provides approximately 12.6 volts of electrical power. However, if even one of the cells is low on electrolyte, has short-circuited or contains heavy sulphate deposits on its lead plates, the entire battery will be affected. Restoring one or more dead cells in your 12-volt battery is fairly simple, requiring basic tools and materials, as well as a battery charger.
Put on the safety glasses and rubber gloves.
Loosen and remove the battery vent caps. Verify that there are no foreign objects, such as dirt or loose pieces of metal, inside the battery cells. Inspect the metal plates to make sure they are not punctured or warped. If there are foreign objects present in any of the cells, or the battery's metal plates are damaged, reinstall the battery vent caps and dispose of the battery accordingly. If there is no visible damage to the plates, and there are no foreign objects in any of the cells, proceed to the next step.
Inspect the electrolyte level of each cell. If the electrolyte level in any of the battery cells is low but not empty, fill that cell with distilled water until the electrolyte level reaches the bottom of the "Fill" mark. If the battery cell is empty, fill the cell with electrolyte solution instead. (An empty battery cell may have a leak. Check the battery for any leakage around that cell before attempting to charge the battery.)
Replace and tighten the battery vent caps. Attach the negative (black) lead of the battery charger to the negative battery terminal. Attach the positive (red) lead of the battery charger to the battery terminal. Plug the charger into a wall outlet and turn the charger on. Allow the battery to charge for at least two hours. Check the battery every half hour while charging, making sure that the battery does not start bubbling or get hot to the touch. You may choose to remove the safety glasses and gloves while the battery is charging.
Turn off and unplug the battery charger. Remove the battery charger leads from the battery. Allow the battery to cool down for one hour. Put on the gloves and glasses, then remove the battery vents to check the electrolyte levels. For each battery cell that is below the Fill line, add distilled water until the electrolyte level in that cell reaches the bottom of the Fill line. Replace and tighten the battery vent caps.