This chapter discusses two related energy conversion devices: batteries and fuel cells. A battery is a device which converts chemical energy to electricity, and one or both of the electrodes of the battery are consumed or deposited in the process. A fuel cell is a device which converts chemical energy to electricity through the oxidation of a fuel. The fuel, but not the electrodes, is consumed in the operation of a fuel cell. Oxidation is the process of losing an electron while reduction is the process of gaining an electron. Both batteries and fuel cells contain three main components: an anode, cathode, and electrolyte. The electrode which electrons flow toward is called the cathode. The electrode which electrons flow away from is called the anode anode. The electrolyte is a material though which ions can flow more easily than electrons.