I work on the manufacturing side of power banks, so I will say the unpopular thing first: the number on the box is not the number you get, and this is not always the manufacturer lying to you.
A 10,000 mAh rating describes the battery cells inside, measured at their own nominal voltage of 3.7 V. Your phone does not take power at 3.7 V. It takes it at 5 V, or 9 V, or 12 V when fast charging. So the power bank has to boost the voltage upward, and that conversion is not free.
Here is the arithmetic, and it is worth doing once because it explains almost every complaint about power banks.
The cells hold 10,000 mAh at 3.7 V, which is 37 watt-hours of energy. The boost converter runs at roughly 85 to 90 percent efficiency on a decent board and closer to 75 percent on a very cheap one. So at the USB port you actually have around 30 to 33 watt-hours available.
Now the phone side. A 4,500 mAh phone battery is about 17.3 watt-hours. But the phone does not absorb that energy cleanly either: its own charging circuit and the voltage conversion inside it wastes another 10 to 15 percent. Filling it takes roughly 20 watt-hours drawn from the power bank.
Thirty divided by twenty is one and a half full charges. In a laboratory, with the phone switched off.
In real use you get one, and sometimes less. There are four reasons, and none of them are defects.
The first is that almost nobody charges a phone that is switched off. If the screen is on and you are using the phone while it charges, a meaningful share of what the power bank delivers is going straight into the screen and the processor instead of the battery. Charge a phone overnight with the screen off and the same power bank suddenly performs like the one in the advert.
The second is what happens near the end. Power banks regulate their output, but as the cells drop towards empty the output sags, and most phones will silently drop out of fast charging and fall back to a slower rate. The last 20 percent of the power bank's charge arrives slowly enough that most people give up and assume it is finished.
The third is protocol negotiation. A power bank that does not speak the same fast-charge language as your phone falls back to 5 V at 1 A, which is five watts. That fills a modern phone at a genuinely painful rate, and it is the single most common reason someone concludes their power bank is broken when it is merely mismatched.
The fourth is time and temperature. Lithium cells lose capacity as they age, and a power bank that has been through a few hundred cycles holds meaningfully less than it did new. Cold does the rest: below about 10 degrees Celsius the internal resistance rises and usable capacity drops noticeably. A power bank left in a car overnight in winter will underperform for reasons that have nothing to do with its quality.
Wireless charging makes all of this worse in a way that surprises people. A magnetic wireless power bank typically loses another 30 to 40 percent in the coil-to-coil transfer, on top of everything above. If you rely on wireless, treat a 10,000 mAh bank as a partial top-up rather than a full recharge.
So what is actually worth checking when you buy one.
Look for a rated capacity figure rather than only a cell capacity figure. Responsible makers print both: something like 10,000 mAh for the cells and 6,000 to 6,500 mAh as the rated output at 5 V. If a product only ever quotes the big number and never the small one, you have learned something about the seller.
Match the fast-charge protocol to your phone. If your phone charges over USB Power Delivery with PPS, a bank that only speaks an older quick-charge dialect will work but will not be fast.
Decide whether you need wireless at all. It is convenient and it costs you a third of your capacity. A cable plus a magnetic mount is usually the better trade.
And buy slightly bigger than you think you need, because the arithmetic above means the honest usable figure is around 60 percent of whatever is printed on the front.
On the manufacturing side, the capacity ratings we print are measured, not invented, and the reason a 10,000 mAh bank and a 20,000 mAh bank feel further apart than they look on paper is that the fixed losses are nearly identical and only the cells scale. If you want two full charges for a modern phone, you want 20,000 mAh and a cable, not 10,000 mAh and a wireless pad.
ODMKit develops power banks and car chargers for brand owners, on OEM and ODM programmes from Shenzhen. Full product range and manufacturing capability.
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