The most traditional of all energy storage devices for power systems is electrochemical energy storage (EES), which can be classified into three categories: primary batteries, secondary batteries and fuel cells. The common feature of these devices is primarily that stored chemical energy is converted to electrical energy. The main attraction of the process is that its efficiency is not Carnot-limited, unlike thermal processes. Primary and secondary batteries utilise the chemical components built into them, whereas fuel cells have chemically bound energy supplied from the outside in the form of synthetic fuel (hydrogen, methanol or hydrazine). Unlike secondary batteries, primary batteries cannot be recharged when the built-in active chemicals have been used, and therefore strictly they cannot be considered as genuine energy storage. The term 'batteries', therefore, will only be applied for secondary batteries in this chapter. Batteries and fuel cells comprise two electrode systems and an electrolyte, placed together in a special container and connected to an external source or load. These two electrodes, Fitted on both sides of an electrolyte and exchanging ions with the electrolyte and electrons with the external circuit, are called the anode ( - ) and cathode ( + ) respectively.