How to recharge old dead car batteries

Updated March 23, 2017

You may find that your car battery may not hold its charge like it used to if it's more than two years old. In fact, only 30 per cent of batteries sold today last 48 months, partly because today's cars have increased energy requirements, according to But a battery's lifespan can be greatly increased by merely reversing the reason for most battery failure: sulfation. This is the deposition of sulphur on metal plating, a process that causes 80 per cent of battery weakening. You can reverse sulfation at a fraction of the cost of getting a new battery by replacing the acid and fully charging--but not overcharging--the battery overnight.

Turn off the car's ignition.

Put on safety glasses and rubber gloves to protect against blinding or acid burn.

Detach the battery cables. Set the battery in a well ventilated area.

Lift off the plastic cell caps on the top of the battery if it's the serviceable type. If the cells are sealed, drill holes into the plastic--one hole per cell. You can tell where to drill by the six "shadow marks" that the six cells will have.

Microwave 1 quart (litre) of distilled water to about 65.6 degrees Celsius. Do not use tap water as you don't want the minerals inside to coat the metal battery plates.

Dissolve 10 tablespoons of pure Epsom salts (magnesium sulphate) into the quart of distilled water.

Pour the salt solution into the six holes using a funnel. There should be marks on the side of the battery to indicate how high to fill the cells.

Charge the battery overnight with a "smart charger," which varies voltage to avoid overcharging and minimise future sulfation. Specifically, use a three-step charger to achieve a full charge. If you have been charging the battery with your car alternator alone, it can leave the battery charged only 90 per cent. According to, the 10 per cent of electrolyte left uncharged leads to sulfation. You can purchase a three-step charger for as little as £26 at auto parts stores or online.

Plug the six holes with "hole caps" after the battery has fully charged. The caps are available at most hardware and auto supply stores.


Not charging a car battery for a few days in the cold or for merely 24 hours in the heat can lead to sulfation. You can avoid this problem by installing a solar trickle charger, which charges the battery in small amounts without overcharging. Disconnect the battery if storing the car for more than two weeks.

Things You'll Need

  • Epsom salts
  • Distilled water (1 quart)
  • Gloves
  • Safety glasses
  • Funnel
  • Battery charger
  • Drill
  • Three-step recharger
  • Hole caps
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About the Author

Paul Dohrman's academic background is in physics and economics. He has professional experience as an educator, mortgage consultant, and casualty actuary. His interests include development economics, technology-based charities, and angel investing.