If your car’s battery isn’t holding a charge or otherwise is not up to par, you may be able to fix it. The most common cause of degraded battery performance in lead-acid batteries is sulfation, which occurs when sulphur collects on the lead plates in the battery, blocking the electric current. It’s not difficult to recondition a car battery at home. However, sulfation causes irreversible corrosion of the lead plates, so this process will work only three to five times.
Test the battery to see if it is likely to respond to reconditioning; it needs to register 12 volts on a voltmeter. If it’s between 10 and 12 volts, you may be able to restore the battery to full function, but if it tests at less than 10 volts, you’re probably wasting your time.
Heat a half quart of distilled water to 65.6 degrees C, and dissolve 198 or 227g of Epsom salts in the water.
Remove the battery cell caps. If you have a sealed battery, find the “shadow plugs” that cover openings to the battery cells--you’ll need to drill through these.
Drain any fluid out of the battery and use a plastic funnel to pour enough of the Epsom salt solution in to fill each cell of the battery properly.
Insert plastic plugs in the drill holes or replace the battery caps and shake the battery to make sure the Epsom salts solution is well distributed.
Recharge the battery on a slow charge for 24 hours and then reinstall it in your car.
Put the battery on the charger for three or four nights to restore it to maximum capacity. To minimise loss of performance, put car batteries on a trickle charger if you won’t be using them for several weeks or longer. A stored battery gradually loses its charge, allowing sulphur to form on the lead plates.
Car batteries contain sulphuric acid, which is among the most powerful of acid compounds. Work only in a well-ventilated area and don’t have open flames nearby. Wear safety goggles and rubber gloves. If you get acid on your skin, irrigate it with water immediately. Don't use tap water because it contains chemicals that will damage a battery.