A battery is a source of electric power consisting of one or more
electrochemical cells with external connections for powering electrical
devices. When a battery is supplying power, its positive terminal is the
cathode and its negative terminal is the anode. The terminal marked negative
is the source of electrons that will flow through an external electric circuit
to the positive terminal. When a battery is connected to an external electric
load, a redox reaction converts high-energy reactants to lower-energy
products, and the free-energy difference is delivered to the external circuit
as electrical energy.