what happens in a battery that creates an electric charge at the terminals ?

Answer 1

Individual atoms in the battery are made up of a nucleus (protons and neutrons) surrounded by electrons. The chemical reactions in batteries cause these atoms to gain or lose electrons so that electrons become free to be conducted to another atom, or conversely the atom that has lost an electron becomes open to accepting an electron because it has a hole where an electron used to be. This flow of electrons (or holes) are what is called electrical current. The potential between atoms is measured in volts. The flow of electrons is measured in amperes (or amps).

Answer 2

chemical reaction between metal and acid.