How to Restore a Dead 12V Battery

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How to Restore a Dead 12V Battery
Take the weight of a new battery purchase off of your back by restoring your old battery. (businessman with battery image by Dmitri MIkitenko from

A car battery is designed to last a minimum of five years. The technology used in battery construction has remained basically unchanged in the last 50 years. Some advances in chemicals and packaging have allowed batteries to become essentially maintenance free. However, there are some things that the battery owner can do at home to extend the battery's life. By following a few easy steps the battery can be restored and perform its duties for the vehicle owner.

Skill level:
Moderately Easy

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Things you need

  • Workbench
  • Protective eye wear
  • Plastic funnel
  • Battery hydrometer
  • Battery post/terminal cleaner
  • Screwdriver
  • Volt meter with probes
  • Battery load tester
  • Distilled water
  • 6/12-volt battery charger/starter

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  1. 1

    Open the car bonnet. Put on safety glasses. Remove the battery cables--positive cable first--with a wrench. Remove the battery from the vehicle and place it on a well-lit and sturdy workbench.

  2. 2

    Clean the battery posts. Put a battery post cleaner on a battery post and start turning it back and forth until the post becomes bright and shiny. Repeat the process for the other battery post. For a side mount battery, use the wire brush on the other end of the cleaning tool to scrub the battery's cable mounting surface. This brush will also clean the cable ends on both battery types.

  3. 3

    Check the voltage. Place the positive probe from a volt meter onto the positive battery terminal and the negative meter probe onto the battery's negative terminal. The meter should read 12 volts. Anything less than 12 volts means there is a problem with at least one cell. If the meter reads 12 volts, you can recondition it.

  4. 4

    Remove the cell covers (on non-maintenance free batteries). Place a screwdriver into the slot on top of the cap and turn it counter-clockwise. Remove the cap and set it to the side. Be careful not to spill any of the battery liquid. For maintenance free batteries, proceed to Step 6.

  5. 5

    Test the cells. Place the volt meter's positive probe onto the positive battery post and the negative probe in the first cell. If the cell does not read two or more volts on the meter, there is a problem with that cell. Place the positive probe in the first cell and the negative probe into the second cell. Take the reading. Place the positive probe into the second cell and the negative probe into the third cell and take the reading. Move to the next cell and repeat this procedure, testing all cells. The final reading will have the positive probe in the last cell and the negative probe on the negative terminal post. The reading should be zero.

  6. 6

    Complete a load test. Connect the positive clamp from the load tester to the positive battery terminal and the negative clamp to the negative post. Switch the load tester to the "On" position and push the test button. If the needle drops below 12 volts and remains there, the battery must be replaced. If the needle drops then returns to 12 volts, the battery can be restored.

  7. 7

    Perform a hydrometer test. Squeeze the bulb of the hydrometer several times after inserting the rubber tube into a battery cell. This will stir up the fluid in the cell and give a more accurate reading. Squeeze the bulb and release it, this will suck fluid into the tester. Watch the meter. See which colour the fluid rises to. If it rises to green, the battery is good. If it rises to white the battery is fair. If it rises to red the battery needs a charge.

  8. 8

    Use a funnel to add distilled water to any low cells. The fluid should be at least 1/8-inch over the cell plates. Charge the battery for 24 hours with a trickle charger. Return the battery to the vehicle and reattach the battery cables. Close the bonnet.

Tips and warnings

  • When charging a dead (or very low) battery, use a slow trickle type charger. A fast charge will do harm to the plates in the battery's cells.
  • Always wash any battery fluid off of your skin with plenty of fresh water. The fluid can cause severe burns.

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