What the USB power bank really gives
A 10,000 mAh bank should fill a 4,000 mAh phone two and a half times; it does it twice, and the bank gets the blame. Two things are tangled here, and this bench entry pulls them apart. The mAh on the label are counted at 3.7 V, those on the tester at 5 V: the number shrinks when the voltage rises, without a single joule being lost. And two converters take their share along the way, the bank’s, which steps up to 5 V, and the phone’s, which steps back down to 3.85 V. In ten minutes, the tester gives the Wh, and it is in Wh that both counts come out right.
You need
- From the drawer: a USB power bank with its label (the mAh, and often in small print the Wh and a "rated capacity" at 5 V), its cable, a half-empty phone, a USB lamp of at least 1 W.
- To buy: an in-line USB tester with the same connector as the bank (USB-A or USB-C), showing voltage, current, cumulative mAh and Wh and the time (10 to 20 $ on Amazon.ca; the models that only give U and I will not do).
- Optional: a second bank, no-name, to compare.
Do
BeforeThe USB port gives 5 V and does not hurt. What hurts is a bank that gets too hot to hold, swells or smells: unplug it, set it outside on concrete, and never charge it again. Never put a bare wire across the two contacts of the port; the tester plugs into the socket, nothing else. A fast-charging phone may ask for 9 V instead of 5: the tester shows it, write down the voltage you read and calculate with that. Step 7 leaves the bank working alone overnight: on a surface that does not burn, away from paper.
Read the label. Note the mAh, and the voltage if it is printed (3.6, 3.7 or 3.85 V). Convert: Wh = mAh ÷ 1,000 × V. If the label also says "6,000 mAh at 5 V", that is the same content counted at the port, efficiency included; it is not a second capacity.
expected37 Wh for 10,000 mAh at 3.7 V, and the same number as the printed Wh if there are anyCharge the bank fully the night before. Run the phone down below half.
expectedall the bank’s lights on and steady; the phone between 20 and 50 %Plug the tester into the bank, the cable into the tester, the phone at the end. Reset the counters. Read U and I as soon as the charge settles.
expected4.8 to 5.2 V and 0.5 to 3 A; or 8.5 to 9.5 V if the phone negotiates fast charging, and then calculate with 9Calculate the power, P = U × I.
expected2.5 to 10 W at 5 V; up to 18 W on fast chargingAt ten minutes on the tester’s clock, read the cumulative mAh and Wh, then check: the mAh counted, divided by 1,000 and multiplied by the voltage read, give back the Wh counted.
expectedat 1 A under 5 V, about 167 mAh and 0.83 Wh, proportional to the current you readCount the charges. The label’s Wh times 0.85 (what the bank delivers at the port), then times 0.9 (what the phone’s charging circuit lets through), divided by the phone’s Wh (its mAh ÷ 1,000 × 3.85 V).
expected37 × 0.85 × 0.9 ÷ 15.4, that is 1.8 charges, not 2.5. The runtime, on the other hand, only extrapolates on a constant load: that is step 7Optional, swap the phone for the lamp, reset, and leave it until the bank shuts off: a night, or more, at 1 W it is thirty hours. Read the total Wh and mAh.
expected28 to 33 Wh for a 37 Wh label on a brand-name bank, that is 75 to 90 %; 60 % and less on a no-name bank; and mAh at the port around 6,000, far from 10,000
What next?
- 1.2 The quantities: why the amp-hour is a charge and the watt-hour an energy.
- The "5 Ah" pack that doesn’t last: the same reasoning, on the chainsaw.
- 6.3 The charger: why the phone’s current drops near the end, and what the bank does with its 3.7 V to make 5.