Batteries

A battery is a practical, everyday source of electrical energy built from one or more galvanic cells joined in series. At its core it works on exactly the same principle as any galvanic cell — a spontaneous redox reaction supplies the electrical energy — but to be useful in real devices, a battery must satisfy some practical requirements: it should be light, compact, and its output voltage should not change appreciably while it is being used.

Batteries in common use fall into two broad classes:

• Primary batteries — the cell reaction runs only once. Once the reactants are consumed, the battery goes dead and cannot be recharged or reused. …