How to Test a Power Tool Battery Capacity in Amp Hours
A pack that dies after eight screws does not always mean a pack that belongs in the bin. The label says 5.0 Ah, and the only way to know what is actually left is to make the pack push current against a load and time how long it lasts.
You can do this on a bench with a battery analyzer or a hobby charger that has a discharge function. What you cannot do is read it off the label, or off a voltage meter, and expect the number to mean anything.
Step 1: Work out what number you are chasing
Two units get thrown around and they are not the same. Amp hours measure charge. Watt hours measure energy, and Watt hours are what actually translates into runtime under a tool load.
Battery University defines the Watt-hour rating as rated capacity in ampere hours multiplied by nominal voltage. A pack rated 5.0 Ah at an 18V nominal rating lands near 90 Wh. A pack rated 2.0 Ah at 18V lands near 36 Wh. Same chemistry, wildly different staying power.
So before you touch a tester, write down the rated amp hours and the nominal voltage from the pack's own label. That rated figure is the baseline everything else gets compared against. Write it down and keep it in front of you.
Step 2: Charge it fully and let it settle
Put the pack on a proper charger and take it to full. Then let it sit.
This step is not superstition. Battery University notes that a voltage reading taken immediately after charging no longer tells you the true state of charge, and that a clean open circuit reading needs at least four hours of rest.
You are not measuring capacity off voltage, so the rest matters less for the final number. What it does for you is confirm the pack actually reached full and give the BMS a chance to balance the cells.
Step 3: Pick a discharge current, and be consistent about it
This is where most home tests go sideways, because the current you choose changes the number you get.
Battery University lays out the convention plainly. A 1C discharge means the pack should run for one hour. A 0.5C discharge runs two hours, a 2C discharge runs thirty minutes, and a 0.2C discharge runs five hours. For a 5.0 Ah pack, 1C is 5A, 0.5C is 2.5A, and 0.2C is 1A.
Pick one rate and stay with it across every pack you test, or the numbers are not comparable to each other. That is the whole trick. Battery University notes that DIN and IEC mark starter batteries in amp hours at a typical discharge of 0.2C, the five-hour rate, so that is the figure to use when comparing against a manufacturer rating.
Step 4: Set the cut-off voltage before you start
A discharge test needs a finish line. Without one, you will run a lithium cell past the point it should ever go.
Battery University gives the standard end-of-discharge figures: 3.0V per cell for lithium ion, 1.0V per cell for NiCd and NiMH, and 1.75V per cell for lead acid. For a five-cell lithium pack, that works out to a 15V pack-level cut-off at 3.0V per cell. Confirm the cell count before you assume the number.
Set the cut-off in the tester, not in your head. A tester that beeps at you while you are doing something else is a tester that over-discharges a pack.
Step 5: Run the discharge and time it
Start the load and note the clock. Let the tester terminate at the cut-off voltage on its own.
The math is simple: amp hours equals the discharge current in amps multiplied by the time in hours. A pack discharged at 5A that runs for 54 minutes works out to 4.5 Ah. Expressed against a 5.0 Ah rating, that is 90 percent.
Most analyzers display the result directly as a percentage of the nominal rating if you enter the rated amp hours up front. Run the test at room temperature and write the room temperature down next to the result.
Step 6: Read the result, and decide
The number you want to compare against is not 100 percent. It is 80 percent.
Battery University states that a replacement is called for when packs fade to about 80 percent, and that lithium and nickel chemistries typically deliver between 300 and 500 full discharge and charge cycles before they drop below that line. A pack reading 85 percent is tired but still working. A pack reading 60 percent is a pack that will strand you on a roof. I'd retire that one before the next job rather than after.
Cold is the other variable to correct for before you blame the pack. Battery University reports capacity losses for a lithium ion energy cell of about 17 percent at 0°C (32°F), 34 percent at -10°C (14°F) and 47 percent at -20°C (-4°F), with power cells losing less. A test run in an unheated garage in January is not a verdict on the pack.
If the pack is genuinely tired, what you do next depends on the format. A cylindrical-cell pack can sometimes be rebuilt or diagnosed cell by cell, and the fundamentals for that are the same ones covered in the guide to choosing individual cells for a DIY pack. If you are testing packs for outdoor equipment rather than hand tools, the battery options for cordless mowers and blowers sit in a different runtime bracket entirely.
When the number comes back low, the choice is a new pack or new cells. Ready-made packs are in the DeWalt and Makita collections, and loose 18650 cells are there for anyone rebuilding a pack by hand.
Common Mistakes
A pack tested straight off the charger shows an elevated voltage, and the surface effect skews the start of the discharge. Let it rest first.
Using a multimeter as a capacity tester. Battery University notes that the discharge curve of lithium ion and nickel chemistries is very flat, and that 80 percent of stored energy sits in the flat middle section. Voltage tells you full and empty and very little in between.
Comparing a fast test to a slow rating. Battery University states plainly that a higher C-rate produces a lower capacity reading, so falling short of a 0.2C rating after a 2C test is normal behaviour rather than a bad pack.
Discharging a series pack without watching the cells. In a pack of five cells in series, the tester sees the total. One weak cell can hit the cut-off first while the others still have charge left. If your tester reports per-cell data, use it.
Repeating the test back to back. Two full discharges in an afternoon add cycle wear to a pack you are trying to measure.
Testing a swollen pack. A pack with a distorted case or a bulging cell does not go on a tester. It goes out of service.
FAQ
Can I test capacity with just a multimeter?
Not meaningfully. Battery University explains that lithium ion chemistry has a flat discharge curve, so voltage cannot estimate capacity across the middle of the range.
What discharge current should I use?
For comparison against the rated amp hours, use 0.2C, the five-hour rate. For a realistic tool-load estimate, use 1C or 2C. Never mix rates between packs you intend to compare.
What voltage should the test stop at?
3.0V per cell for lithium ion, according to the standard end-of-discharge figure Battery University lists. Multiply by the cell count in the pack.
At what capacity should I replace the pack?
Around 80 percent of the original rating. Battery University notes that most lithium and nickel chemistries deliver between 300 and 500 full cycles before reaching roughly that level.
Why did my pack test low in winter?
Cold costs capacity. Battery University reports about a 17 percent loss at 0°C for a lithium ion energy cell, rising to roughly 47 percent at -20°C. Retest at room temperature before condemning the pack.
Frequently asked questions
Can I test capacity with just a multimeter?
No. A multimeter reads voltage, not the charge a pack delivers over time. You need a load that holds a steady current plus a timer, which means a battery analyzer or a hobby charger with a discharge function.
What discharge rate should I use?
Match the rate you are comparing against. DIN and IEC rate starter batteries at 0.2C, the five-hour rate, so a 5.0Ah pack is tested at 1A. Pick one rate and use it on every pack or the results are not comparable.
What cutoff voltage should I set for an 18V pack?
3.0V per cell for lithium ion. A five-cell string works out to 15V at the pack terminals. Count the cells first, because a six-cell string lands somewhere else entirely.
Why does my 5.0Ah pack test below 5.0Ah?
The label figure comes from a new cell at a defined rate and temperature. Age, heat and cycle count all take capacity back, and roughly 80 percent of original is the usual replacement point.
How long does a capacity test take?
At 1C the discharge runs about one hour. At 0.2C it runs five hours. Choose a rate your analyzer can hold steady for the whole run, then stay with it.