How to Test a Cordless Pack With a Multimeter

A basic multimeter tells you more about a cordless pack than the charger lights do. It won't tell you everything, but it's the right first check before you buy a new battery.

1. Before you start

Only test on the pack's outside terminals. Don't open the pack.
Never let the probes or anything metal bridge the + and − terminals. A short can cause sparks, burns or a fire.
If the pack is swollen, cracked, leaking, hot or smells, don't test it, charge it or use it.
Wear eye protection and work on a clear, non-flammable bench.

2. Set up the meter

Set the meter to DC volts. On a manual meter choose the range above the pack voltage, such as 200V.
Red lead in the V socket, black lead in COM.
Touch the red probe to the pack's + terminal and the black probe to the − terminal. Check the markings on the pack, as layouts differ between brands.

3. What the reading means

Most tool packs use lithium-ion cells in series. An 18V or 20V Max pack has 5 cells in series. As a rough guide for a 5-cell pack resting off the charger:

About 20 to 21V: fully charged.
About 18 to 19V: part charged.
About 15 to 17V: close to flat. Charge it.
Below about 15V: very flat. It may have been left too long. Try the maker's charger and watch it closely.
Well below 12V, or zero: the pack may be deeply discharged, the protection board may have shut it off, or there's a fault. Some brands also show little or no voltage at the terminals until the pack is in a tool or charger.

For other packs, divide the reading by the number of cells in series. Full is about 4.2V per cell and flat is about 3V per cell.

4. A simple three-reading test

Charge the pack fully on the maker's charger. Note the voltage straight off the charger.
Leave it an hour, then note it again.
Leave it a day or two, then note it again.

A healthy pack holds close to its full voltage over a couple of days. A pack that drops noticeably while sitting unused may have a weak cell or a fault and should be looked at before you rely on it. Record your readings on the free Pack Testing Record Sheet.

5. What a multimeter can't tell you

Capacity: a pack can show full voltage and still run out quickly. A capacity tester or a timed run in the tool shows how much it really holds.
Cell balance: the reading is all the cells added together. One weak cell can hide behind the others.
Behaviour under load: a pack can read fine on the bench and cut out under load.

See Battery Testers for the Bench for the next step up from a multimeter.