Recondition Power Tool Batteries DIY
How to balance battery cells in a cordless tool pack
You balance battery cells in a cordless tool pack by metering every cell in the 18V or 20V string and bringing the low cell up with its neighbors, because the charger stops on the string rather than on the weak cell — start from how to recondition dead power tool batteries.
An 18V or 20V pack is a string, not a brick
The number on a cordless shell is a pack name. Inside, cells sit in series so their readings add up to something the tool can use. The charger watches that sum, and so does the tool. The new fact on this page is what that arrangement does to a single weak cell: the charger can stop when the highest cell is ready, while the lowest cell is still behind. The tool then dies early, because the low cell drags the string down under load. Balancing means getting those cells to similar readings. It does not mean zapping a lithium cell, and it does not mean jumping the output if the cells are already far apart.
If the pack will not run at all and the cells you can read sit near one another, you may be looking at a sleeping board instead. That is a different page. Balance is for the pack that runs, then quits, or that charges "full" and still feels short, with one cell obviously lower on the meter.
Find the low cell before you move any charge
Use the cell-by-cell meter method. Write the row in order. Circle the lowest cell and the highest cell. If they are close, balance is not your problem today. If one cell is the outlier, mark it on the shell with tape so you do not treat its neighbor by mistake.
Also read the chemistry. A lithium string and a NiCad string are both series packs, and both can be uneven, but the way you add charge to one cell is not interchangeable. A nickel outlier may be a whisker short. A lithium outlier may be a tired cell or a cell the board has been protecting. The label picks which guide you are allowed to use. When the label is unclear, stop and return to the main reconditioning page rather than inventing a chemistry.
Do this on a cool pack. Heat means you wait, and a swollen cell means you stop for good. Balancing is a reading job plus a careful charge of the low cell, not a reason to lean on a damaged case.
Bring the low cell toward the others
- Meter every cell and keep the written row.
- Identify the one cell that sits clearly lower.
- Charge that cell on its own only with a method that matches the chemistry printed on the pack.
- Stop if the cell warms, swells, or will not move.
- Meter the whole string again.
- Repeat only while the low cell is moving toward the others and staying cool.
- When the row is close, charge the pack on its own charger and run the tool on the bench.
This page does not name a charger model or a target number, because the listing does not print one. "Close" means the outlier is no longer the obvious stranger in the row you wrote. If the low cell will not climb, it is tired. Replacing that one cell is a repair beyond a balance, and buying a new pack is the other door. Both are decisions for the comparison page, not another hour of hoping the number moves.
Do not borrow the other fixes
A jump wakes a board. It does not fill a low cell. Read how to jump start a dead lithium power tool battery before you borrow that step. A zap targets nickel whiskers. It is not how you even a lithium string. Using either one because the word "balance" sounded slow is how packs get hurt. If you came here from a drawer of mixed shells, sort them by the printed chemistry first, then by the meter row.
The listing tied to the button at the end of this page is Last Battery. The page title is Last Battery™. The listing says about three minutes per battery, materials you already have at home, and it names power tool batteries along with car batteries and old AA and D cells. This site did not balance a pack with that video playing. The row of cell readings is the part you can do with a meter before you decide the video is the next hour.
The order that keeps jump, zap, and balance from getting mixed is how to recondition dead power tool batteries.
Last Battery
Last Battery is the full video when the string is the problem and you want the listing that names power tool batteries, household materials, and about three minutes per battery.
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Questions
Why does an 18V or 20V pack need balancing?
The cells are in series. The charger and the tool see the string. One low cell makes the tool quit early and can keep that cell from catching up when the higher cells end the charge.
How do you find the cell that is behind?
Meter each cell and write the row. The low cell is the one that sits apart from the others. If they all match, balance is not the fault.
Can you zap a low lithium cell to balance it?
No. A zap is for crystal whiskers in a NiCad cell. A lithium string that is uneven needs the low cell brought up carefully, or it needs to be retired if it will not move.
When is the string close enough?
When the cell you circled is no longer the obvious stranger in a fresh row of readings, and it stayed cool while you charged it. A cell that will not climb is finished.