How to Slowly Discharge 12 Volt Batteries


Ordinary 12-volt lead acid batteries are reliable and usually last for three or four years in a car or motorcycle before you need to replace them. Fully charged 12-volt batteries will show about 12.6 volts on a meter. You may need to discharge and then recharge batteries that have been overcharged or lose their charge rapidly. Discharge a 12-volt battery slowly to equalize the sulphate on the lead plates. Allow a battery to discharge on its own, or discharge it slowly by connecting a car headlamp to the battery.

Things You'll Need

  • Volt meter
  • Car headlamp
  • Electrical wire
  • Set a volt meter to its 12-volt scale.

  • Connect the positive (red) terminal on the volt meter to the positive (+) terminal on the battery. Connect the ground (black) terminal on the volt meter to the negative (-) terminal on the battery. Note the voltage displayed on the volt meter.

  • Cut two lengths of electrical wire long enough to reach from the battery terminals to a car headlamp. Strip once inch of insulation off each end of the electrical wires.

  • Connect the negative (-) side wire from the car headlamp to the negative terminal on the battery by inserting the wire between the volt meter clip and the battery terminal. Repeat for the positive (+) side of the battery and headlamp.

  • Monitor the volt meter. Disconnect the positive (+) wire and then the negative (-) wire attached to the battery when the battery is discharged to no less than 11 volts. Recharge the battery before use.

Tips & Warnings

  • Keep battery terminals clean and free of corrosion and oil for maximum performance.
  • Never discharge a 12-volt lead acid battery below 10.5 volts. Severe damage to the battery lead plates may result.
  • Discharge a 12-volt battery in a well-ventilated area.
  • Always use protective eye wear.
  • Avoid touching the battery terminals. Electrical shock is possible. Wear gloves when connecting battery terminals.
  • Never touch any fluids leaking from a lead acid battery. The acid can burn your skin.

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