Golf-cart batteries are deep-cycle batteries, meaning they are designed to operate for long periods without losing voltage. However, batteries left in an almost discharged state and not used or charged for fairly long periods may develop sulfation. This is a chemical process that causes lead-sulfate crystals to form on the plates in the battery cells, which prevents correct charging. Your battery can be permanently damaged if you don't desulfate it in time.
Open the battery cover panel so you can access the battery. It's usually under the seat, but it could also be located under a panel on the side. Disconnect the black battery cable from the negative battery terminal using a wrench. The negative battery terminal has a minus sign etched on it. Disconnecting it isolates the electrical supply, making it safer.
Remove the battery cell covers so you can look inside. The covers unscrew, so use your hands or a screwdriver to remove them.
Check the state of your battery cells to determine how bad the sulfation is. Start from left to right. Look in the first cell. If you see tiny crystals, a kind of dirty yellow color on the lead-plates, and you can still see the lead, then it's likely you can desulfate the cell. However, if there are large crystals completely covering the plate so you can't see it, and if the walls of the cell are coated as well, there's little you can do except get a replacement battery.
Continue to check each cell. Even though the first one may not be too bad, the others may be different. The amount of fluid in the cells can affect the formation of sulfation. If a cell has a low fluid level, sulfation will form quickly. If all the cells only have small deposits of lead sulfate, then continue. However, if one or more cells has high deposits, it's not worth proceeding; just get a replacement golf-cart battery.
Top-up the cells using distilled water. Each cell has a mark indicating the maximum level. The fluid needs to be up to this marker. During the charging process, the submerged plates warm up and electrons flow between them. Small deposits of sulfate will disperse.
Screw the covers onto the cells, but don't completely tighten them; just leave them halfway unscrewed so gas can evaporate from the cells. Once the plates get warm, the fluid produces gas and it needs to escape -- otherwise pressure can build.
Insert the plug from your charging unit into the charger socket on the golf cart. Set your charger to a low level if it has variable settings. A slow charge disperses sulfation better than a fast charge.
Turn on the charger and leave your golf-cart battery to charge. Periodically check the charger so you know when it is done. Most have a charge light or indicator that changes color or comes on when the charge process is complete. If it has a meter, you will see the charge rate reducing until it's not charging anymore.
Turn off the charger once the battery is charged. Disconnect the plug from the charging socket on the golf cart. Remove the battery cell covers again and look inside the cells. You will see that a lot of the sulfation has dispersed if the process has worked. If there's little change, you may still find your battery performance has improved, so don't discard your battery until you've tried it.