Battery
Definition
A device that stores energy chemically and releases it as electricity on demand — from phone batteries to the massive packs in electric cars.
Unit: volts (V); ampere-hours (Ah) for capacity
Technical definition
An electrochemical cell (or pack of cells) converting stored chemical energy into electrical energy via redox reactions. Key ratings are voltage (V), capacity in ampere-hours (Ah), and energy in watt-hours (Wh); rechargeable types reverse the reaction with an external current.
Example
A typical phone battery stores about 15 watt-hours — enough to run a 5-watt screen for three hours. An electric car pack stores around 60–100 kilowatt-hours, thousands of times more, which is why it weighs several hundred kilograms.
Batteries do not store 'electricity' like a tank stores water — they store chemical energy that becomes electricity only during discharge. And a battery's voltage stays roughly constant while its charge drains; what runs out is the stored chemical energy (capacity), not the voltage, until the very end.