A battery is a device that uses an electrochemical oxidation-reduction (redox) reaction to convert chemical energy stored in its active components directly into electric energy. Electrons are transferred from one material to another via an electric circuit in this type of reaction. A battery can be made up of one or many electrochemical cells (as in Volta's original pile). An electrolyte separates two electrodes in an electrochemical cell. A chemical reaction occurs between the electrode and the electrolyte when the anode and cathode of a battery are connected to a circuit. This reaction transfers electrons across the circuit and back to the cathode, where they undergo another chemical reaction. Primary batteries are disposable batteries that must be disposed of after usage. Rechargeable batteries are referred to as secondary batteries.