How to fix cordless drill batteries with car batteries

Written by hank macleod
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How to fix cordless drill batteries with car batteries
Revive old drill batteries to save money (drill image by Albert Lozano from

Whether you have a small tool bench or a full-scale workshop, cordless drill batteries can get costly fast. Luckily, there is a technique to restore nonfunctioning Ni-Cd batteries to nearly new condition using common household items. Over time, nickel-based batteries develop a crystalline formation when not fully discharged. Using a 12-volt car battery to shock the individual cells inside the drill battery, you can break down the crystallisation and recharge the battery to full strength.

Skill level:

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Things you need

  • Safety gloves and goggles/face-shield
  • Screwdriver
  • Electrical tape
  • Multimeter
  • Black permanent marker
  • Car battery
  • Alligator clamps (14 gauge)

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  1. 1

    Put on all safety gear. Remove the screws from the top cap of the battery pack and set them aside. Lift off the cap carefully, as most battery packs are spring-loaded. Set the cap and spring aside.

  2. 2

    Wrap the protruding metal band terminal with electrical tape, taking care not to complete the circuit between it and any other metal. Turn the battery pack upside-down into your hand and pry out the bundled batteries carefully. Remove the plastic shield from the top and bottom of the bundle, taking note of their original positions. Peel back any tape covering the top and bottom of the individual battery cells.

  3. 3

    Lay the bundle on its side so that both ends of each cell are accessible. Turn on the multimeter and set it to "2V," as each cell should be 1.2 volts. Touch the red test lead to the positive terminal of a cell (the positive side has the protruding button) and the black lead to the corresponding negative terminal (the negative side is flat). Mark the negative terminal with an "X" if the readout is below 1.1 volts. Repeat for each cell.

  4. 4

    Connect the red alligator clip to the positive terminal of the car battery and the black to the negative, taking care not to touch them together or to anything conductive. Touch and hold the black clip to the negative terminal of one of the "X" marked cells. Touch and hold the red clip to the positive terminal of the same battery for no longer than two seconds. Repeat this procedure for each "X" marked cell.

  5. 5

    Check each cell again with the multimeter, marking any sub 1.1-volt cell with a star. Repeat the restoration procedure above until each cell is repaired. Reinstall the plastic shields and pack the bundle back into the housing. Remove the tape from the terminal band. Screw the cap back onto the battery pack and charge it overnight.

Tips and warnings

  • Do not hold the alligator clips for longer than two seconds at a time, as the cell may rupture and cause injury.
  • Never use this method on lithium batteries, as they can explode. Only restore Ni-Cd battery packs.

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