Choosing an open-box power bank: capacity, output and real output
Why a 10,000mAh bank does not deliver 10,000mAh, how to read the output rating that matters, and what to check on arrival.
5 min readDifficulty: EasyUpdated 2026-09-25
Capacity is the number on the box and the least useful figure on it. Conversion losses mean a 10,000mAh bank delivers roughly 6,000–7,000mAh to your phone, and that is normal physics, not a defect.
What actually decides whether a bank is any good is its output rating and whether it can hold that rate.
What you’ll need
- The bank and its cable
- A phone to charge
- The listed capacity and output figures
Step by step
- 1
Read the output, not just the capacity
A 20,000mAh bank at 10W charges a modern phone slower than a 10,000mAh bank at 45W. Match the output to your phone’s fast-charge standard.
- 2
Check the port mix against what you own
USB-C PD in and out is what you want. An older bank with micro-USB input takes hours to refill itself, whatever it puts out.
- 3
Expect roughly 65% of the rated capacity to reach your phone
Two full charges from a 10,000mAh bank on a 4,500mAh phone is a good result, not a shortfall.
- 4
On arrival, charge it fully and note how long it took
A bank that reaches full unusually fast is not holding its capacity. This is the single most telling first-day check.
- 5
Run it down once through a real charge
Charge your phone from it until the bank is empty. Compare what you got against the 65% rule.
Common questions
- Can I take an open-box power bank on a flight?
- Under 100Wh (about 27,000mAh) is allowed in cabin baggage on most airlines, never in the hold. Check the printed Wh figure on the unit.
- Does an open-box bank have a degraded battery?
- No more than a new one. These have not been cycled — that is the point of open-box.
- It gets warm while charging. Is that a problem?
- Warm is normal, especially at high output. Too hot to hold comfortably is not — stop using it and report it.