[ELI5] Why are battery capacities expressed in Ah (Amperes-hour) and what does it mean, for example, a AA battery that is 2500 mAh?
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Amp is the SI unit of electric current. A coulomb (unit of electric charge) is defined as 1 amp second. 3600 seconds in an hour so 1 amp hour is 3600 coulombs. One other thing that’s useful is the electric charge every electron has 1.602*10^(-19) coulombs. That means if you group about 6 quintillion electrons together, you will have 1 coulomb or 1 amp second. Do that 3600 times to get 22 sextillion electrons and you will have 1 amp hour. There you have it. A battery with a 1 amp hour rating can shove 22 sextillion electrons through a wire before it runs out of charge… (Sort of, there’s some physics around how energy is actually carried from the source to the load but for eli5 it’s good enough) And now you also know why they call it ‘charged’ batteries vs ‘discharged.’
The rating is also useful in another sense. Let’s say you know your circuit draws 0.1 amps from a AA battery with a 2.5 amp hour rating. Well, if we divide amp hours by amps, the current cancels out leaving us with 25 hours of use. You can drain 100 milliamps from this battery for just over a day before it runs out of charge. Or, you can draw 1 amp for only 2.5 hours. It’s an easy conversion between loading and duration… But the loading isn’t constant. A Duracell AA battery will start at 1.6V and draw down to maybe as low as 1V before most electronics will stop. And since current is Voltage divided by ohms, if the ohms of your load is constant, the current draw will lower over time as the batteries deplete. So it’s a bit messier than that. However, it’s a good back of the envelope calculation for most things.
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