1. Two electrodes and a paste
Inside a battery are two different materials, called electrodes, with a paste or liquid between them called the electrolyte. Each electrode has its own voltage, and the difference between the two is the voltage the battery gives you.
2. The reaction runs out
As a battery works, the chemicals are used up and the voltage slowly falls. A fresh CR2032 gives 3 volts, and as it drains it drops towards 2 volts, which is where most devices stop working.
3. Why most batteries can’t be recharged
Recharging would mean running the reaction backwards, and in an alkaline or lithium cell it doesn’t cleanly reverse. Gas builds up instead, which is why charging an alkaline battery makes it leak or burst.
4. What the letters tell you
The letters of an IEC code name the chemistry of the cell: CR for lithium, LR for alkaline, SR for silver oxide and PR for zinc-air. The numbers after them give the size, and the size decides what it fits.
5. Heat and cold change the speed
Heat speeds the reaction up, so a battery left in a hot car drains faster than one on a shelf. Cold slows it down, so the same battery gives less power on a freezing morning.