This article draws an analogy between electricity flow and water flow; the analogy holds up quite well, as far as I’ve taken it… but take it any further and it starts to fall apart. It is not intended to be a description of how electricity actually works; it’s simply an analogy which helps convey some of the ideas involved.
Edit by Tobbe: mAh stands for Milli Ampere Hours, “milli” meaning “one thousand”. So 2000 mAh can really be written 2 Ah.
To see how battery life works, let’s look at a very simple system of a battery with a bulb (the “load”). When you connect the wires to make a loop with both ends of the battery, and the bulb forming part of the loop, current will flow from one side of the battery (positive) to the other (negative) and through the bulb. The bulb will light.
The battery is measured in a number of ways; it has a voltage, it has a maximum current capability (measured in milliamps [mA]) and it has a life rating, measured in milliamp-hours [mAh]. So – a standard AA battery has 1.5 volts, can deliver about 300mA maximum, and has a life rating of about 1300mAh (which means it can deliver 1mA of current for about 1300 hours – or if it could deliver 1300mA, it could only do it for an hour).
All this is confusing stuff – so how can we describe it in terms most people “get”?
Let’s change the battery for a resevoir of water with a weighted piston on top; the wires for pipes, and the “load” can be a fan which turns at a rate decided by the flow of water. The weight pushes down on the piston, which forces the water through the system turning the fan, and then flows back to the resevoir but not in a place where it can be used.
The rate the water flows at is determined by the size of the outlet from the resevoir; measured in litres per minute, it’s a similar concept to “current” in the electrical system. The voltage is analogous to the weight pressing on the piston: a heavier weight will make the water flow faster (a higher voltage will create higher current, a brighter bulb) which makes the fan go faster. The “life rating” of the resevoir is how many litres the resevoir holds when it’s full.
So it is with batteries; the voltage keeps pushing the current around the circuit until it runs out of electricity to push. If you now change the “load” to be a mobile phone, the situation changes a little bit because the phone changes how much current it uses all the time; it’s varying and flowing faster and slower depending on what the phone is doing. That’s like someone messing with the fan, slowing it down and speeding it up all the time so that the water’s flow rate changes. This makes it very difficult to predict the length of time the water will last; but watching it you can get a general idea. This is like batteries too – you can get a general idea but you can’t ever be certain.
So – the idea is either: Make the resevoir REALLY BIG, with lots of litres (lots of mAh) and not worry about how much water the load needs: or MAKE THE LOAD USE IT SLOWER while doing the same job.
What HTC have done with their phones is drastically improve the latter; they have made their phones use less current which means that a battery (which will actually fit in the phone package), lasts longer. That’s why the HTC One only needs 2300mAh – because it uses it more slowly. HTC couldn’t find a battery maker who could guarantee the reliability of a higher capacity battery in the physical size they needed – so they made the phone use as little as they could; and it’s a good result generally – extremely heavy use will of course make more current flow and shorten the total battery life; extremely light use will cause hardly any current to flow and therefore extend the battery life. This effect could be achieved in the water system by having lots of pipes come out of the resevoir with a load each; and little taps which could start and stop the flow to each load.