how does a battery charger work?

Answer

In a battery, a chemical reaction takes place that releases electrons and creates a potential difference between the poles. This allows a current to flow when you place the battery in a circuit.

Which chemical reaction that is depends on the type of battery. When the basic products of the chemical reaction are used up, it is over and the battery is ‘dead’.

With a rechargeable battery, the chemical process is reversible: if you send a current (in the opposite direction) through the battery, a reverse chemical reaction occurs, which returns the battery to its original state.

So your ‘charger’ is a power source that starts the chemical reaction in the battery.