B2B guide · Power bank capacity test · 9 min readHow to test a power bank's real capacity, and spot fake mAh

The short answer

• Compare watt-hours, not mAh. The rating is at the cell's 3.7 V; the USB port works at 5 V or more.
• A good pack delivers 80–90% of its rated Wh through a cable. Wireless output delivers less.
• Use a USB meter and a steady load. Discharge from full to empty and read the Wh total.
• Weight is a quick check. Lithium cells store roughly 0.2–0.25 Wh per gram, so a real 37 Wh cell weighs around 150 g or more on its own.

Why 10,000 mAh is not 10,000 mAh

The capacity on the label is the cell's rated capacity at its nominal voltage of 3.7 V. A 10,000 mAh cell therefore stores 10,000 × 3.7 ÷ 1,000 = 37 Wh. To charge a phone, the power bank raises the voltage to 5 V or more, and the converter loses some energy as heat. At 5 V, 37 Wh becomes at most 7,400 mAh, and after conversion losses typically 6,000–6,600 mAh.

That is why testing in Wh is the only fair comparison. A meter that shows 6,200 mAh at 5 V from a 10,000 mAh pack is reporting a healthy result, not a fake one.

What you need

• A USB-C power meter that records mAh and Wh (many show voltage, current and a running total).
• A steady load: an electronic USB load set to a fixed current, or a device that keeps drawing power, such as a tablet.
• A short, good-quality USB-C cable.
• A room at normal temperature, around 20–25 °C.

The test, step by step

1. Charge the power bank to full and let it rest for an hour.
2. Connect the meter to the power bank's output and the load to the meter. Reset the meter's counters.
3. Set a moderate, steady load, for example 5 V at 1–2 A. Very high or very low currents change the result.
4. Let it run until the power bank switches off.
5. Read the total Wh from the meter and compare it with the rated Wh on the label.
6. For samples, repeat on at least three units from the same batch.

What result is normal

A result between 75% and 90% of the rated Wh is normal. Results well below 70% suggest a weak cell, high losses or an overstated rating, and are worth testing again on more units.

Testing wireless output

Wireless charging adds coil losses, so a magnetic power bank delivers less energy to a phone than by cable. On our test bench, about 58% of the energy the cell delivers reaches the phone battery wirelessly. The best way to test a magnetic pack wirelessly is to start with a nearly empty phone, charge it to a known level and compare the percentage gained with the phone's battery size. We show the method for battery cases in how much a 2500 mAh battery case really charges an iPhone.

Six signs of fake capacity

1. Too light. A "20,000 mAh" pack weighing under about 250 g, or a "10,000 mAh" pack well under 150 g, deserves suspicion.
2. No Wh on the label. Airline and transport rules require the energy to be known; honest packs print it.
3. Capacity beyond the size. A very thin, small pack claiming 20,000 mAh or more.
4. Tests far below 70% of rated Wh on several units.
5. A UN 38.3 summary that names a different capacity or cell, or no summary at all.
6. A price far below the market for the same capacity and certification.

For buyers checking samples

Ask the factory for its own discharge test data along with the UN 38.3 summary, and test samples yourself on arrival. Agree in writing on the test method and the minimum acceptable result, for example 80% of rated Wh by cable at 25 °C, before mass production. A third-party inspection can repeat the test on random units from the finished batch. See our wholesale guide and certification guide for the rest of the checklist.

Frequently asked questions