The rated capacity on the box is a polite fiction. We measured what these batteries actually deliver, and the gap was bigger than expected.
The number printed on a power bank is one of the most optimistic figures in all of consumer electronics. It tells you how much energy the cells inside theoretically hold, under ideal conditions, before any of the messy realities of actually charging a device get involved. The number you care about — how many times it will genuinely refill your phone — is always lower, sometimes dramatically so. We bought seven popular banks and measured the gap.
The method was deliberately boring, because boring is what reveals the truth. We charged each bank fully, then used it to recharge a drained phone over and over until the bank gave out, logging how much real energy reached the device each cycle. We did it at room temperature, with good cables, giving every bank its best possible showing. Even then, the results were humbling.
The cells are honest. The marketing is not. The job of a review is to stand between the two.
— our notes from the test bench
The shortfall is not always fraud; mostly it is physics dressed up as marketing. Energy is lost converting voltage up and down, lost to heat, and lost to the simple fact that you can never safely drain a battery to absolute zero. A well-made bank loses a predictable, modest fraction. A poorly made one loses far more, and a dishonest one prints a capacity its cells could not deliver on their best day.
Of the seven, only two delivered close to what an informed buyer should expect, and they shared the same unglamorous virtues. They ran cool. They were honest about their real-world capacity in the fine print, not just the front of the box. And they held up over repeated cycles instead of fading after a few months, which is the failure mode the test bench cannot see in a day but the brand's reputation can.
The five that did not survive failed in familiar ways: capacities that evaporated under real load, banks that grew alarmingly warm, and one that simply stopped delivering a stable charge partway through the test. None of them were the cheapest on the shelf, which is the uncomfortable lesson — price is a weak signal here, and the logo on the front tells you less than you would hope.
You cannot run a test bench in a store, but you can shift the odds. Ignore the headline capacity and look for a brand that publishes its real-world output. Favor a bank that feels reassuringly dense, because energy has weight and a suspiciously light battery is light for a reason. And read the reviews that mention longevity, not just the first week, because the gap between a good bank and a bad one widens with every month you own it.
The capacity on the box is a ceiling you will never reach. Buy banks that run cool, publish real-world output, and feel dense for their size — and weigh longevity over the headline number.