Buying a new car battery can be fairly expensive, but it seems there is not much choice when a battery's performance plummets. However, in many cases you can refurbish car batteries and extend their usefulness. Doing so does not require pricey chemical additives. You can achieve the same results with magnesium sulphate, better known as Epsom salts. Car batteries are "lead acid" batteries with a solution of water and sulphuric acid. When sulphur collects on the lead plates of the battery, performance degrades. Magnesium sulphate can remove the sulphur and can restore a battery's performance.
Loosen the battery cables with a crescent wrench and remove the battery. Use baking soda to clean the terminals and battery cables so you get good contact when the battery is reinstalled.
Mix a solution of 1/2 gallon distilled water and 0.227kg. Epsom salts. Stir until the salts are dissolved (if you use warm water this is a lot easier). Always use distilled water because the chemicals in tap water can damage a battery. Remove the battery cell caps and drain the water from the battery. If your car battery is of the sealed type, you'll need to find the shadow plugs (they'll be marked on the top of the battery) and drill through them.
Use a funnel to fill each cell of the battery with the Epsom salt solution. Once you have filled each cell, shake the battery to make sure the solution is well distributed.
Place the battery on a charger using a slow (trickle) charge for 24 hours. Remove the battery from the charger and replace the battery caps. For sealed batteries, buy plastic plugs from an auto parts store and insert them in the drill holes. Reinstall the battery and make sure the battery cables are properly fastened. Your battery should now work properly.
Repeat the charging process two or three times over the next week to complete the refurbishing process. This helps to remove any remaining sulphur from the lead plates inside the battery. If you store the battery for a long time, place it on a trickle charger to prevent sulphur from accumulating on the plates again, as the battery drains while not in use.
Avoid deep discharges (draining the battery) as much as possible. This causes sulfation (sulphur accumulation). Each time this happens it corrodes the lead plates. After 10 times or so, the plates will be too badly damaged for refurbishing, and you'll have to buy a battery.
Be safe when you refurbish car batteries. Sulphuric acid is highly corrosive, so wear safety glasses and protective gloves and work in a well ventilated area.