How to Rebuild an 18V Battery Pack

The 18-volt rechargeable battery pack is the choice for many cordless power tools. Battery packs are a major cause of tool failure. A rechargeable battery pack is only capable of a few hundred charge cycles before it becomes trash. Corrosion, moisture, and temperature extremes shorten a battery's life even more.

The typical 18-volt battery pack consists of 12, 1.5-volt rechargeable batteries soldered together in a series. Batteries are joined in a series by connecting the positive terminal of one battery to the negative terminal of another. Rebuild a battery pack by replacing individual batteries that have failed.

Things You'll Need

  • Soldering iron

  • Solder

  • Battery tester

  • Screw driver set

  • 12, 1.5 volt rechargeable batteries

  • Copper strips

  • Tin snips

  • Utility knife

  • 1/2-inch foam board

Disassemble the Battery Pack

Step 1

Unlatch the battery pack and separate it from the power tool.

Step 2

Remove the small screws around the perimeter of the battery pack case with a small screw driver, and separate the two sections.

Step 3

Unsolder the two wires that are attached to the positive and negative electrodes of the battery pack's terminal plug. The terminal plug is identified by the two metal strips that slide in and make contact to power the tool.

Step 4

Remove the cluster of small batteries by lifting them out of the bottom section of the case.

Step 5

Mark the connection sequence of the old battery cluster with a felt tip marker. Begin at the positive terminal that was joined to the battery pack's terminal plug and denote this as battery number one. Turn the cluster over to see which battery is connected to the bottom of battery one and mark this battery two.

Follow the connection sequence around the old battery cluster, marking the connection order of each battery until it ends at the battery that was connected to the negative terminal plug.

Rebuild the Battery Pack

Step 1

Cut a piece of 1/2-inch-thick foam board with a utility knife. Make it larger than the bottom of the battery pack case to form a template. Place the old battery cluster on top of the foam board and trace a line around it. Denote the foam board with a plus and minus sign next to the spot of the two batteries in the cluster that were attached to the positive and negative terminals of the pack's terminal plug.

Step 2

Push a new battery, with the positive terminal facing up, into the spot where battery number one belongs on the template. Mark this battery number one.

Step 3

Push another new battery, negative side up, in the spot where battery number 12 belongs, Mark this new battery number 12.

Step 4

Number the remaining new batteries two through 10 and push them into the proper places of the template. Match the proper terminals and follow the sequence number of the batteries in the old cluster.

Step 5

Tape the batteries together that are pushed into the foam template and fit the new battery cluster into the battery pack case. The new cluster should form the same shape as the old one.

Step 6

Join the batteries in the new cluster by soldering copper strips across the correct terminals. Connect all terminals in the new cluster the same way they are joined in the old one.

Step 7

Solder two wires, one from the open positive and one from the open negative battery terminal, to the positive and negative terminal of the battery pack's terminal plug.

Step 8

Insert the new battery cluster into the bottom of the battery pack case.

Step 9

Rejoin the two sections of the battery pack case, then replace and tighten all the screws around the perimeter of the case.


Observe the physical size of the individual batteries in the old cluster and replace them with new rechargeable batteries of the same dimension.

Divide 18 volts by the number of individual batteries in the cluster; this will yield the voltage of each individual battery.

Push the new batteries into the foam board like a craft person would push a flower stem into it. The foam board holds the batteries in place to form the proper pattern.

Remember that the series connection alternates negative and positive from bottom to top.

Use the old battery cluster as a visual guide to connect the batteries and terminals in the proper order.

Cut and trim the copper strips to size using tin snips.

Test the voltage in any battery series at the open positive and negative terminals and it will match the total voltage of all individual batteries connected.


Wear safety goggles when operating a soldering iron. Copper strips may contain sharp edges. Do not touch the areas that were heated up with a soldering iron. Do not overheat the new batteries when soldering on the copper strips. Never solder two batteries together of the same terminal. For example: never solder positive to positive or negative to negative terminals.

References & Resources