Batteries provide power by electrochemical reaction, converting stored chemical energy to electrical energy. Batteries are a common power source for many consumer and industrial applications, from wristwatches to computer data centers. There are two types of batteries. Primary (disposable) batteries are designed to be used once, and then thrown away. Primary batteries have current available immediately upon assembly. Secondary (rechargeable) batteries, usually assembled with active materials in the discarded state, must be charged before use.