How to Test a Power Tool Battery with a Multimeter – Step-by-Step Guide

You grab your drill, squeeze the trigger, and… nothing. Or it runs for thirty seconds and dies. Before you toss that battery in the recycling bin, grab a multimeter. Nine times out of ten, a quick voltage test tells you exactly what’s going on inside the pack — whether it’s a dead cell, a tripped protection circuit, or a battery that’s genuinely cooked.

I’ve been testing and building battery packs for years on job sites and in my home shop. Here’s the step-by-step method I use to check any lithium-ion power tool battery, no matter what brand it is.

What You’ll Need

• A digital multimeter (any cheap one works — $15 at Harbor Freight is fine)
• The battery you want to test (fully charged if possible)
• Safety glasses (lithium packs store serious energy)
• A clean, dry work surface
• Your tool (for the load test in Step 4)

Don’t have a multimeter yet? Any basic model with a DC voltage setting will do the job. Set it and forget it — you’re only measuring DC volts here.

Step 1 – Safety First: Prepare Your Battery

Before you probe anything, inspect the battery physically. Look for:

• Cracks or bulging in the plastic casing
• Corroded or bent terminals
• Any leaking fluid or weird smells
• Excessive heat (if it just came off the charger)

If you see any of those, do not test it — recycle it properly. A swollen lithium cell is a fire risk, period. Bulging means the internal layers have delaminated from overcharging or old age, and there’s no coming back from that.

Assuming the battery looks clean and intact, set it on your bench with the terminal end facing you. Most power tool batteries have a row of three or four metal contacts — a positive, a negative, and one or two sense/communication terminals. You only care about positive and negative for this test.

Step 2 – Set Up Your Multimeter

This is where most DIYers mess up, so pay attention.

1. Turn the dial to DC voltage (look for a “V” with a straight line above it — not the wavy line, that’s AC).
2. If your meter has multiple DC ranges (200mV, 20V, 200V, 600V), select 20V for an 18V/20V Max battery, or 200V for a 48V/60V pack. Always pick a range higher than the battery’s nominal voltage.
3. Plug the black lead into COM and the red lead into the VΩmA port.

I’ve watched guys chase their tails for half an hour because they were reading AC voltage on a DC battery. Double-check that dial position before you probe anything.

Pro tip: If your multimeter has a “hold” button, don’t use it for this test — you want live readings, not a frozen number.

Step 3 – Measure the Voltage

Now the actual test:

1. Touch the black probe to the negative terminal (usually the spring-loaded contact on the side, sometimes marked with a “−”).
2. Touch the red probe to the positive terminal (the center contact, often marked with a “+”).
3. Hold both probes steady and note the reading.

Here’s what the numbers mean:

A 20V Max battery at full charge should read about 20V–21V (six 3.6V cells in series = 21.6V theoretical max, but protection circuits usually cap it around 20.5V). An 18V Makita LXT pack at full charge reads about 20V. A 12V Milwaukee M12 pack at full charge reads around 12.6V.

If your reading falls in the “dead cell group” range, the battery isn’t holding full voltage across all series cells. That’s one of the clearest signs your power tool battery needs replacing.

Step 4 – Check Under Load (This Is the Real Test)

A battery can show perfect voltage sitting on the bench but collapse the second you pull the trigger. That’s why the no-load test only tells half the story.

Here’s how to test under load:

1. Insert the battery into your tool.
2. Set your multimeter to DC voltage again.
3. Touch the probes to the battery contacts while squeezing the trigger on the tool (full speed).
4. Read the voltage while the tool is running.

A healthy battery should drop by maybe 1V–2V under load. If it drops by 4V or more, or if the tool cuts out immediately, the internal resistance is too high — the cells are worn out.

This is super common on old batteries that have been through hundreds of charge cycles. The cells still hold surface voltage but can’t deliver current anymore. Think of it like a garden hose with a kink in it — plenty of water in the pipe, but nothing comes out the nozzle.

Common Mistakes to Avoid

Mistake #1: Testing with the wrong meter setting. I said it once, I’ll say it again — DC, not AC. Also, if your meter reads “OL” (over limit), you’ve got the range set too low or the probes touching the wrong terminals.

Mistake #2: Only testing positive to negative. On some tools (Milwaukee M18 especially), the battery’s internal BMS (battery management system) can trip and show 0V across the main terminals even though the cells are fine. Try short-probing the sense terminal to wake the BMS, or put the battery on the charger for 30 seconds to reset it.

Mistake #3: Skipping the load test. A battery that passes no-load voltage can still be useless under load. Always, always test with the tool running if you can.

Mistake #4: Assuming 0V means dead forever. If your battery reads 0V, try this: put it on the charger for 5 minutes, then retest. Some BMS boards go into deep protection mode when the pack drops below a certain voltage, and the charger can “wake” them back up. If it’s still 0V after 5 minutes on the charger, it’s done.

When to Replace vs. Recycle

If your battery reads at or near full voltage under both no-load and load tests, congratulations — your battery is fine. The problem is probably the tool, the charger, or a dirty contact.

If it reads well below spec or collapses under load, it’s time for a new one. And honestly, for modern lithium-ion packs, replacement is almost always the better option than trying to rebuild them. Cell welding requires a spot welder, nickel strips, and a whole lot of patience. A quality replacement battery compatible with your tool costs less than the equipment and cells you’d need to rebuild one yourself.

Frequently Asked Questions

Q: Can I test a power tool battery without a multimeter?
A: Not really. You can guess by how the tool runs, but the multimeter is the only way to get actual numbers. Borrow one from a neighbor or grab a $15 meter — it’s a tool every DIYer should own.

Q: What voltage should a 20V Max battery read when fully charged?
A: Around 20V to 20.5V. The “Max” label is marketing — these are 18V nominal (5 cells) or 20V nominal (6 cells) systems, and the protection circuit typically stops charging around 20.5V.

Q: Why does my battery show voltage but won’t power the tool?
A: Dead cells with high internal resistance. The battery has surface charge but can’t deliver current. The load test in Step 4 will confirm this — if voltage drops 4V+ under load, the cells are gone.

Q: Can a completely dead lithium battery be revived?
A: Sometimes. If the BMS tripped into protection mode, putting it on a compatible charger for 5–10 minutes can wake it up. But if it’s been sitting at 0V for months, the cells are chemically damaged and should be recycled.

Q: Is it safe to test a swollen battery with a multimeter?
A: No. A swollen lithium battery is a fire hazard. Recycle it immediately — don’t charge it, don’t test it, don’t use it.

Need a replacement? Check out our selection of high-quality compatible power tool batteries tested for safety and performance.