How to test a power tool battery with a multimeter

The pack terminals can lie by omission

A meter across the outer terminals of a cordless pack tells you the sum the tool might see. It does not tell you whether one cell in the string is far from the others. That hidden cell is why a pack can show "some" voltage and still die in the tool, or refuse a charger, or come back for ten minutes and quit. The new fact on this page is the place you measure: each cell, not only the pair of terminals on the outside of the shell.

You need a multimeter that can read DC volts, the pack, a pen, and enough light to see the straps between cells. You do not need a second product to take the reading. The reading is what picks the next page.

Open only a pack that is quiet

Look before you pry. If the shell is swollen, split, hot, or leaking, you do not open it to satisfy a meter. Those packs are finished. For a cool, intact pack, remove the screws that hold the shell and stop as soon as the cells and their straps are visible. You are not peeling cells out of the plastic unless a later step on another page says a single cell is the whole job.

Set the meter to DC volts. The range only needs to cover a single cell, which is a small number next to the 18V or 20V printed on the outside. If your meter is manual, pick the low DC volt range that can show a cell without overloading. Touch the red probe to the positive side of one cell and the black probe to the negative side of that same cell. Write the number. Move to the next cell. Do not measure from the first cell's positive to the last cell's negative and call that a cell reading. That is the string again.

Keep the probes on metal, not on painted plastic and not on a strap you have not identified. A slipping probe that bridges two cells is a short you made. If a cell is too corroded to see the terminal, do not dig. Note it as unreadable and stop if you cannot tell positive from negative.

What the row of numbers is for

You are not matching the row to a spec this site does not have. You are comparing the cells with each other. A row that sits together, on a lithium label, with a tool that still sees nothing, points at a sleeping board. A nickel label with one cell near a dead reading and the others still showing numbers points at whiskers in that cell. An 18V or 20V label with one cell clearly lower than the rest points at a string that needs balance, not a new charger story.

Write the numbers in order around the pack so you can find the same cell again. A photo of the row is useful if your handwriting is not. Do not trust memory for six similar numbers.

After the row exists

If the cells are close and the label is lithium, the jump page is the next read, not a zap. If one nickel cell is the dead one, the whisker page is next. If the string is an 18V or 20V pack and one cell lags, go to how to balance cells in a cordless pack. If you cannot tell the chemistry, do not pick the most dramatic step. Go back to how to recondition dead power tool batteries and stay on the label until it is clear.

A meter reading is not a repair. It is the permission to choose a repair, or the permission to stop. This site did not publish a "good cell" number, because the listing does not print one and a single number would hide the comparison that matters: each cell against its neighbors.

When the row says the pack is a candidate for a home method rather than a new shell, the listing on these pages is Last Battery. Its page is titled Last Battery™. It says the method uses materials you already have at home and about three minutes per battery, and it names power tool batteries. The meter work above is the step this site can describe without pretending the video was run here.