Two batteries quoted at the same amp-hour rating can hold very different amounts of energy, because amp-hours measure charge and say nothing about the voltage it sits at. Multiplying by the voltage gives watt-hours, and watt-hours are comparable.
It is the reason a power bank rated at 10,000 mAh does not put 10,000 mAh into a phone: the rating describes cells at around 3.7 volts while the output is at 5, so the deliverable charge is smaller before any losses at all. How much of a battery is usable in practice, and how fast it can be drawn, is a separate question.
How do the units relate?
One multiplication, and the voltage is the part that gets left out.
| To get | Multiply |
|---|---|
| Watt-hours | amp-hours × volts |
| Amp-hours | watt-hours ÷ volts |
| Watt-hours from mAh | mAh ÷ 1000 × volts |
So a 10,000 mAh cell pack at 3.7 volts holds 37 watt-hours. That figure does not change when the output voltage changes, which is exactly why it is the honest one.
Conversion losses take a further cut. Stepping 3.7 volts up to 5 costs efficiency, so the energy that reaches a phone is meaningfully less than the energy stored — a bank delivering two thirds of its rating to a device is performing normally rather than badly.
Why do airlines quote watt-hours?
Because the hazard scales with energy rather than charge. Lithium battery limits for air travel are set in watt-hours, with a threshold at 100 for carrying without permission and a higher band requiring approval.
That is why a large power bank prints its watt-hour figure on the case, sometimes in small type next to a much larger milliamp-hour number. The small one is the number security is reading.
Working it out from the printed rating needs the cell voltage, which is usually 3.6 or 3.7 for lithium-ion. A pack that lists only milliamp-hours and no voltage cannot be assessed, and that is itself a reason to be cautious about it.
What does Ohm’s law actually let you do?
Find any one of voltage, current and resistance from the other two, and power from any pair. It is the most reused equation in electrical work because almost every practical question reduces to it.
The power forms are the ones that get used most in sizing. Power equals voltage times current, and substituting gives two more forms — current squared times resistance, and voltage squared over resistance — which is how you work out how much heat a component has to dissipate.
The current-squared form is worth internalising. Doubling the current through a resistance quadruples the heat, which is why undersized wiring fails suddenly rather than gradually.
Why is a signal strength negative?
Because it is a ratio expressed logarithmically against one milliwatt, and the signals involved are far weaker than that. A negative number simply means less than a milliwatt, which every wireless signal at a receiver is.
The scale is not linear. Every three units is a doubling or halving of power, and every ten is a factor of ten — so the difference between minus 60 and minus 70 is ten times the power, not a sixth more.
| Signal | Quality |
|---|---|
| −30 dBm | as good as it gets |
| −50 dBm | excellent |
| −67 dBm | reliable for streaming and calls |
| −70 dBm | usable, marginal |
| −80 dBm | unreliable |
| −90 dBm | not usable |
The practical threshold is around minus 67, which is the level generally treated as the minimum for anything real-time.
What should you compare when buying?
Watt-hours, and the output the device actually supports. Two banks with identical milliamp-hour ratings can differ in energy if their cell chemistry differs, and a bank that cannot deliver enough current charges slowly regardless of capacity.
For a battery in a fixed installation the same rule applies with an extra term. Usable capacity is less than rated capacity, because discharging a battery fully shortens its life — so a sizing calculation has to allow for the depth of discharge the chemistry tolerates.
The honest comparison is always energy in, energy out, and the loss between them.
Does capacity fall over time?
Steadily, and the rate depends on how the battery is used. Lithium cells lose capacity with each full charge cycle and lose it faster when kept at a full charge, at a very low charge, or warm.
That is the reasoning behind the charging limits some devices now offer. Holding a battery at eighty per cent rather than a hundred slows the degradation noticeably, at the cost of the top fifth of the range on any given day.
A pack rated at 37 watt-hours when new therefore holds less after a couple of years, which is worth allowing for when sizing anything that has to last.
Questions people ask
Why does my power bank charge my phone less than twice? Voltage conversion and heat. Two thirds of the rated energy reaching the phone is typical.
Does a higher mAh always mean longer life? Only at the same voltage. Across different voltages it means nothing on its own.
Can I add batteries together? In series voltages add and amp-hours do not; in parallel the reverse. Watt-hours add either way.
Is a stronger Wi-Fi signal always faster? Up to a point. Past around minus 50 the limit is elsewhere — the connection behind the router, or congestion.
Convert to energy before comparing. Ah to Wh, Wh to Ah and mAh to Wh handle the battery arithmetic, the Ohm’s law calculator and resistance calculator cover the electrical relationships, and the Wi-Fi signal dBm converter turns a signal reading into something interpretable.