How to Desulfate a Lead Acid Battery

Updated March 23, 2017

Sulfation is a mixture of lead and sulphur that forms into crystals on the lead plates of your battery and, if the condition is at its worse, on the sides of the battery cell walls. Sulfation occurs in batteries that are left to stand for some time, especially if they are left in a state of relative discharge. It also occurs if you let the electrolyte fluid level drop below the top of the battery cell's lead plates. Providing the sulfation is not too bad, it is possible to desulfate your lead-acid battery, but you won't know until you examine the battery cells.

Wear old clothing and put on plastic goggles and a pair of lightweight rubberised gloves. You need to ensure your eyes and skin are protected from any accidental spillage or splashes from the acid-based electrolyte fluid in the battery cells. The acid burns clothing and skin and will potentially cause blindness if it gets in your eyes.

Look on the top of your battery; you will see six plastic cell caps. Unscrew the caps using your fingers. If there is a slot in the top, use the flathead screwdriver to unscrew the caps and then lift them off with your fingers. Place the caps to one side upturned so the threaded end is facing upward.

Look inside the cells from left to right. If the area you are working in is dark, use a flashlight to illuminate the cell, as you need to clearly see the fluid level and the state of the sulfation.

Check the condition of the lead plates and cell walls. If you can see tiny, yellow-brown crystals but can also see the plates and there is no or only minor crystallisation on the walls, it's likely you can desulfate the battery. However, if one or more of the plates is coated in large crystals, you can't see the plates and the cell walls are covered in lead sulphate, you won't be able to remove the sulfation. You will need to replace the battery.

Look at the fluid level. If it is below the minimum marker on the cell wall or the lead plate can be seen above the fluid, fill up the cells using distilled water. Use a container with a spout or a water bottle with an built-in spout. Carefully pour the water into any cells that are low on fluid, filling them to the maximum marker on the side of the walls. Don't overfill them; you need to leave room for expansion. Cap the container to keep the water pure and put it aside.

Attach the battery charger cable clamps onto the battery terminals. The clamp on the end of the red cable attaches to the "+" or positive terminal and the clamp on the black cable attaches to the "-" or negative terminal. It's possible that your battery terminals have red and black coloured rings for easy identification.

Select "trickle charge" on your battery charger if it has this setting. If it does not, select the lowest possible charge rate. Don't use fast or boost charge.

Plug in your charger and turn it on. Let your battery charge for 36 hours. This process removes a small amount of sulfation so the longer the charge time, the better the results.

Turn off your charger after 36 hours. Remove the battery cable clamps from the battery terminals.

Put on your gloves and goggles. Look inside the cells; you will see tiny bubbles rising to the surface of the fluid indicating the cells are charged. Check the plates; the sulfation will be gone or greatly reduced. Check the fluid level again as some of the fluid may have evaporated during the charging process. If any of the fluid levels are below the maximum marker, pour in more distilled water until it reaches the marker.

Replace the cell caps. Screw them loosely into place then tighten using your fingers or use the screwdriver if they have slots on the top. Don't overtighten the caps as you can damage the plastic threads.


Make sure you check your battery in a well-ventilated area. The hydrogen gas from the battery may react with other elements and cause an explosion. Do not use tap water in the battery cells as it contains impurities that will damage the components.

Things You'll Need

  • Lightweight rubber gloves
  • Goggles
  • Flathead screwdriver
  • Flashlight (optional)
  • Distilled water
  • Container with a spout
  • Battery charger
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About the Author

Stephen Benham has been writing since 1999. His current articles appear on various websites. Benham has worked as an insurance research writer for Axco Services, producing reports in many countries. He has been an underwriting member at Lloyd's of London and a director of three companies. Benham has a diploma in business studies from South Essex College, U.K.