A phone battery is advertised in the one unit that cannot be compared
Milliamp-hours measure charge, not energy. Two batteries with the same mAh figure hold different amounts of energy if their voltages differ, which is exactly what happens between a single-cell and a dual-cell phone.
Two phones advertise 5,000 mAh. One holds appreciably more energy than the other, and nothing on either box says so.
The reason is that a milliamp-hour is not an amount of energy. It is an amount of charge: how many electrons move, not how much work they do on the way. Work depends on the pressure behind them, which is voltage. The same quantity of charge at a higher voltage is more energy, in the same way the same volume of water does more work falling from a greater height.
This is the phone version of the confusion kW and kWh cause on an electric car, and it has the same fix: convert to an energy unit before comparing anything.
The conversion, which is one multiplication
Watt-hours equal milliamp-hours times nominal voltage, divided by a thousand.
A 5,000 mAh pack at 3.87 volts is 19.4 Wh. A 5,000 mAh pack built as two cells in series at 7.74 volts is 38.7 Wh, twice the energy from an identical-looking number. Dual-cell designs became common because splitting the pack lets each half charge at a lower current for the same total power, which is how very fast charging got safe enough to ship.
Makers know this. It is why the mAh figure on a dual-cell phone is sometimes quoted as the sum of both cells' charge and sometimes as the pack's charge at the higher voltage, which are different numbers describing the same battery. Neither is wrong. Only one of them is comparable to a single-cell phone, and the packaging rarely says which you are looking at.
Where to find the watt-hour figure
Regulators have been more useful here than marketing. Airlines set their lithium battery limits in watt-hours, so the figure is printed somewhere on almost every device that flies. EU energy labelling for phones requires a capacity declaration and a rated endurance per cycle, which puts a comparable number on a label rather than in a press release.
Failing that, the teardown figure is usually etched on the cell itself, and regulatory filings carry it.
Why even watt-hours do not predict battery life
Energy is the supply side. Runtime is supply divided by demand, and demand is set by the display, the chip's sustained power draw, the radios and how the software idles. A larger, brighter, higher-refresh screen can spend the whole advantage of a bigger battery and more.
That is why a smaller battery in an efficient phone routinely outlasts a larger one in a hungry phone, and why the honest comparison is a measured screen-on time under a stated workload rather than any capacity figure at all. Capacity tells you what is in the tank. It does not tell you the consumption.
Sources
- IATA, Lithium Battery Guidance Document, on watt-hour limits for passenger devices, read 17 September 2026
- European Commission, Ecodesign and energy labelling for smartphones and tablets, Regulation (EU) 2023/1670, read 17 September 2026
- BIPM, The International System of Units, on the ampere, the watt and the watt-hour, read 17 September 2026
Terms defined here
- Milliamp-hour (mAh)
- A unit of electric charge: a thousandth of an amp flowing for an hour. It becomes an amount of energy only once you know the voltage it flows at, which is why it cannot be compared between batteries of different voltage.
- Watt-hour (Wh)
- A unit of energy: one watt drawn for one hour. mAh times nominal voltage, divided by a thousand. It is what airlines regulate, what the EU requires on a label, and the only battery figure that compares across devices.
- Nominal voltage
- The average voltage a cell holds across a discharge, not its peak. A single lithium-ion cell is usually quoted at about 3.8 volts; two in series are quoted at roughly twice that.
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