Lead-acid batteries need to be kept charged otherwise the cells can be permanently damaged. If you have a fully discharged lead-acid battery, the best way to recover it is to charge it very slowly using a "trickle" charge. This lets the individual cells recover and means the battery retains its charge. If you charge a fully discharged lead-acid battery quickly it's likely to lose its charge. Check the battery cell caps for removal. If the caps come off, verify fluid level inside the battery.
Wear protective gloves. Lead-acid battery cells contain sulphuric acid which burns if it gets on your skin.
Place the fully discharged lead-acid battery on a battery tray or flat piece of wood. Remove the six cell caps on the top of the battery. Unscrew them using your fingers or pry them off using a flathead screwdriver. Do this carefully as the cells contain sulphuric acid. Some batteries are sealed units. Do not attempt to access the cells if they are sealed.
Check the fluid level. Note the mark in each cell. The fluid should be up to the mark. If the cells require more fluid, top them using distilled water. Ensure the water covers the cells completely. Replace the caps.
Get your battery charger and place it near the battery. Set the battery charger to its lowest charge rate. Some have preset settings, others have a variable switch. It may also have a setting called "trickle charge." This is the lowest setting and is perfect for charging a discharged battery.
Connect the black cable from the battery charger to the negative terminal of the battery using the spring clamp on the end of the cable. Connect the red cable from the battery charger to the positive terminal on the battery. Both terminals are clearly labelled.
Plug in your battery charger then turn it on. Check the battery charger to ensure its charging the battery. The dial indicates the charge rate, or a light illuminates.
Let the battery charge for 24 to 36 hours. Turn off the charger and remove the cables from the battery.
Check the output voltage of the battery using a voltmeter. Touch the red and black wires from the voltmeter on the positive and negative terminal of the battery. If the voltmeter reads 12-volts, the battery should be charged.
Leave the battery for a day or two. Check the output voltage using a voltmeter. If it reads between 11- and 12-volts your fully discharged lead-acid battery is recovered. If the voltmeter reads less than 11-volts the battery is losing its charge. The lower the voltage, the more damaged the battery.