To measure the mAh (milliamp-hours) of a battery, you discharge it at a known, constant current and record how long it takes to drain completely. The formula is simple: multiply the discharge current in milliamps by the time in hours. A battery that delivers 500 mA for 4 hours has a capacity of 2,000 mAh. The method you use to do this depends on your equipment and what type of battery you’re testing.
What mAh Actually Tells You
Milliamp-hours represent how much energy a battery can deliver over time. A 3,000 mAh battery can theoretically supply 3,000 milliamps for one hour, or 1,000 milliamps for three hours, or 500 milliamps for six hours. In practice, the relationship isn’t perfectly linear (more on that below), but the core idea holds: mAh is current multiplied by time.
Manufacturers test capacity under controlled conditions. Industry standards like IEC 61960 (for lithium cells) specify testing at room temperature (20°C, plus or minus 5 degrees) and at a slow discharge rate of 0.2C. That “C-rate” means the current is one-fifth of the rated capacity. For a 2,000 mAh battery, 0.2C equals 400 mA. These gentle lab conditions produce the highest possible number, which is the one printed on the label. Your real-world results will typically be lower.
The USB Meter Method
If you’re testing a power bank or any USB-charged device, a USB power meter is the easiest and cheapest approach. These small inline testers cost around $10 to $30 and plug directly into a USB port. They display voltage, current, and cumulative mAh in real time.
Here’s how to do it:
- Fully charge the battery. Start at 100% so you’re measuring total usable capacity.
- Plug the USB meter into the output port. It sits between the power bank and whatever you’re draining it into.
- Attach a load. This can be your phone, a tablet, or a dedicated USB load tester that draws a steady current. A USB load tester is better because it pulls a consistent amount of power, while phones fluctuate as the screen turns on and off or apps run in the background.
- Drain completely. Let the battery run all the way to empty. The meter’s cumulative mAh reading at that point is your measured capacity.
- Run it more than once. Average two or three full tests for a more reliable number.
Keep in mind that USB meters measure the energy leaving the port, not the energy stored in the cells. Power banks lose some energy to heat and voltage conversion, so the output mAh will always be lower than the rated cell capacity. A power bank rated at 10,000 mAh might deliver 6,500 to 7,500 mAh at its USB output, and that’s normal.
The Controlled Discharge Method
For loose cells like 18650s, AA rechargeables, or other standalone batteries, you need a way to draw a steady current while tracking voltage and time. There are two common setups.
Battery Analyzers and Smart Chargers
Many hobbyist battery chargers (popular in the RC and flashlight communities) include a discharge-and-measure function. You insert the battery, set a discharge current and a cutoff voltage, and the charger does the rest. It drains the battery at a constant rate, stops when voltage hits the floor, and reports the total mAh. These devices typically cost $30 to $80 and handle the math automatically.
Electronic Loads
For more precise testing, an electronic load lets you set an exact constant current and log voltage over time. Professional bench models from companies like Keysight can connect to a computer, automate the entire discharge cycle, and log data into a spreadsheet. You set three parameters: the constant current to draw, the cutoff voltage (so the test stops before damaging the cell), and a logging interval. The software records everything and calculates capacity for you. This is overkill for casual use, but it’s the standard approach in engineering and quality control.
Why Discharge Rate Changes the Result
One of the most important things to understand about mAh measurement is that the number changes depending on how fast you drain the battery. This isn’t a testing error. It’s a property of battery chemistry described by Peukert’s Law.
The principle is intuitive once you see it: the harder you push a battery, the less total energy you get out. You might expect a battery that runs for 20 hours at 10 amps to last 10 hours at 20 amps. In reality, it will last significantly less than 10 hours. Internal resistance increases at higher currents, more energy is lost as heat, and chemical reactions inside the cell can’t keep up.
This is why manufacturer specs always list the discharge rate alongside the capacity. A battery rated at 3,000 mAh at 0.2C might only deliver 2,600 mAh at 1C (a much faster drain). If you’re comparing your measurement to the label, make sure you’re discharging at a similar rate. Testing at a high current will always produce a lower mAh number, and that doesn’t necessarily mean the battery is defective.
Setting the Right Cutoff Voltage
Every discharge test needs a stopping point: the voltage at which you consider the battery “empty.” This cutoff voltage matters both for accuracy and for safety.
For lithium-ion cells, the safe minimum is 2.5V to 3.0V per cell. Going below this threshold causes irreversible damage to the cell’s internal chemistry and can make the battery unsafe to recharge. A typical 12V lithium battery pack (four cells in series) should stop discharging around 10.0V to 10.5V. For a single lithium cell with a nominal voltage of 3.7V, most testers use a cutoff between 2.8V and 3.0V.
For NiMH rechargeable AAs, the standard cutoff is around 1.0V per cell. For lead-acid batteries, it’s roughly 10.5V for a 12V battery.
If your cutoff is too high, you’ll stop the test early and undercount the capacity. If it’s too low, you risk damaging the battery. Most smart chargers and battery analyzers have safe defaults built in, but if you’re using a manual setup, look up the correct cutoff for your specific chemistry before starting.
Software Estimation on Phones and Laptops
You can also check battery health through software, though this gives you an estimate rather than a true measurement. On iPhones, Settings > Battery > Battery Health shows “Maximum Capacity” as a percentage of the original design capacity. On Android, apps like AccuBattery track charge cycles over time and estimate current capacity based on charging curves. Windows laptops can generate a battery report (run “powercfg /batteryreport” in the command prompt) that shows design capacity versus current full-charge capacity in mWh.
These tools don’t perform a controlled discharge test. They use voltage curves, charge counters built into the battery’s management chip, and statistical modeling to estimate how much capacity remains compared to when the battery was new. The numbers are useful for tracking degradation over time, but they’re approximations. If you need a precise mAh figure, a physical discharge test is the only reliable method.
Safety During Testing
Discharge testing is straightforward, but batteries store real energy and demand basic respect. Lithium cells in particular can overheat or vent if something goes wrong. Test in a well-ventilated area on a non-flammable surface. Never short-circuit a battery by connecting the terminals directly. Use a proper load, whether that’s a resistor, an electronic load, or a device designed to draw current safely.
Watch for heat during the test. Warm is normal; hot to the touch is not. If a battery swells, smells unusual, or gets uncomfortably hot, stop the test immediately and disconnect it. Overdischarging (draining below the safe cutoff voltage) is the most common mistake in DIY testing, and it can permanently reduce capacity or create a safety hazard when you try to recharge the cell afterward. Always set or monitor your cutoff voltage.