1. Volts: how hard it pushes
Voltage is how strongly the battery pushes power into a device. It has to match. A 3-volt device needs a 3-volt battery.
2. mAh: how long it lasts
Milliamp-hours (mAh) is how much energy the battery holds. A CR2032 holds about 225 mAh and a CR2025 about 165 mAh, so the CR2032 lasts longer.
3. More mAh is fine, other volts are not
A battery of the same size and voltage with more mAh simply lasts longer. A different voltage can damage the device or stop it working.
4. Why devices stop before it’s empty
Voltage slowly drops as a battery is used. Once it falls below what the device needs, the device stops, even with some energy left.