Sulfation is a natural chemical process that takes place, if lead-acid battery plates are exposed to air, or the specific gravity goes below 1.225. Sulfation occurs when soft lead sulphate, which is a combination of lead and sulphur, cystalizes into hard lead sulphate. It results in the battery cells being unable to retain an electrical charge so the battery goes dead. If the sulfation is too advanced, then you can't remove it from the lead plates, but if it has only just started to occur, you can remove it by gently recharging the cells.
Check the fluid level in the lead-acid battery cells. Remove the cell covers by unscrewing them with your fingers or a flathead screwdriver.
Check if the fluid level is below the minimum marker on the side of the cell. You can probably see the lead plates are exposed. Sulfation is apparent when you can see hard lumps of crystals on the plates and around the cell walls. Provided the cystalization isn't covering the walls, and the plates only have small deposits, you may be able to remove sulfation during a slow recharge.
Fill the lead-acid battery cells up to the maximum marker using distilled water. Leave the cell covers off. You will heat the plates during the recharge process, which will help dissolve the sulfation.
Connect the two battery-cable clamps from the battery charger to the lead-acid battery terminals. The red cable connects to the "+" terminal and the black cable to the "-" terminal.
Set your battery charger to the lowest rate of charge. The slower and longer you charge your lead-acid battery the more likely it is that the sulfation will be removed.
Turn on your battery charger. Charge your lead-acid battery for 6 hours then take a look inside the battery cells. Don't turn off your charger. If you see tiny bubbles rising to the surface in each cell, that is a good sign and means your battery cells are charging. The charging process starts to dissolve the sulfation. If you can't see bubbles rising in a particular cell, it may mean the cell cannot recharge, but wait until the full charge time is complete.
Continue charging the battery for at least another 18 hours. Look in the cells again, but don't turn off the charger. The cells should be producing rapid bubbles, if they are taking a charge. If any of the cells is not producing bubbles by this time, the cell can't recover. You will need to purchase a replacement battery.
Feel the side of the battery using your hand. It will feel warm, which is good. The heat and the bubbles are removing the sulfation on the lead plates.
Leave your battery to charge for another 6 hours. Turn off the charger. Remove the charger's battery-cable clamps from the battery terminals. Replace the covers on the cells.
Never use tap water to fill the cells in the battery, as it contains minerals that harm the cells.
Ensure you put on protective gloves and goggles. Sulphuric acid will burn your skin, or may blind you, if it splashes onto your skin or into your eyes.