Why a 10,000mAh power bank will not charge a 5,000mAh phone twice

What follows is the working version of power bank capacity: the decisions in the order you actually meet them, with the reasoning attached.

Milliamp-hours are half a measurement

Charge in milliamp-hours multiplied by voltage gives energy in milliwatt-hours, and only energy can be compared meaningfully. A power bank rating is quoted at the internal cell voltage, typically around three point six or three point seven volts. Your phone charges over a link at five volts or more, so the same energy corresponds to fewer milliamp-hours at the higher voltage.

That conversion alone accounts for a large share of the apparent shortfall before any inefficiency is considered.

Conversion happens twice

The bank steps its cell voltage up to the output voltage, and your phone steps that back down to charge its own cell. Each conversion is efficient but not perfect, and the losses appear as the warmth you feel in both devices.

Higher output voltages under fast charging reduce current for a given power, which reduces cable losses and can be more efficient overall. The regulator in a cheap bank is usually the least efficient component in the chain.

Watt-hours are the honest figure

Reputable manufacturers print an energy figure in watt-hours alongside the charge figure, and that is the number to compare. Airline restrictions are specified in watt-hours for exactly this reason, since energy is what determines the hazard. Dividing the bank energy by the phone battery energy gives a realistic upper bound on recharges before losses.

At the protocol level, expect meaningfully fewer full recharges than the raw ratio suggests, and treat any bank that only advertises the charge figure with suspicion.

Output capability matters as much as capacity

A large bank that can only deliver low power charges a laptop very slowly or not at all. Power delivery negotiation determines what voltage and current pairs are available, and a device will not draw a profile the bank does not offer. Total output across multiple ports is usually shared, so charging two devices halves the rate of each on many models.

For laptops, check the maximum wattage on a single port rather than the total.

Ageing and storage

Power bank cells age like any others, losing capacity with cycles and with time spent at high charge in heat. A bank stored full in a hot car for a year will have lost noticeably more capacity than one stored around half charge in a cupboard. Storing at roughly half charge and topping it up every few months is the standard advice for lithium cells in storage.

Swelling, heat when idle or refusal to hold charge are signs to stop using it rather than problems to work around.

Firmware updates change this behaviour more often than hardware does.

Safety and travel

Airlines generally require lithium banks in cabin baggage and impose watt-hour limits, and the exact thresholds vary by carrier and country. Protection circuitry against over-discharge, over-current and short circuit is what separates a reputable bank from a cheap one, and it is invisible from outside.

Certification marks and a named manufacturer are the practical proxies available to a buyer. A bank that gets hot rather than warm in normal use should be retired.

The takeaway

Compare watt-hours, then subtract for conversion. The printed capacity was never the energy you receive.

Once you know what it is trading away, the design stops looking arbitrary.