Capacity – the amount of electrical energy available from a battery or cell of a given voltage measured in amp hours or milliamp hours. A milliamp hour is 1000th of an amp hour.
1 Ah = 1000 mAh
What creates a battery’s capacity?
Batteries are made up of cells (or sometimes one cell) which are themselves constructed from a cathode and an anode immersed in electrolyte (see Battery Glossary – Cell). The materials used for these three components define the voltage of the cell while the surface area of the cathode and anode that are exposed to the electrolyte define the capacity. So the larger the cathode and the anode, the more capacity a battery has.
Note – The type of surface area is important, not just the physical amount of material. Most engine starting batteries use anodes and cathodes made of lead alloys which are manufactured as grids, not solid plates as their name often implies. The grid structure increases the surface area of lead alloy exposed to the electrolyte which means the plates (and hence the battery) can be smaller.
In recent years Lithium batteries have been at the forefront of refining this concept further. With each plate having a thickness less than that of a human hair the amount of cathode and anode exposed to electrolyte can be increased allowing physically small batteries to carry relatively large capacities.
What does the capacity mean?
Many people mistake the Amp Hour rating to mean a 5 Amp Hour battery can power a 5 Amp device for one hour. This is wrong. See Ah, Ampere Hour or Amp Hour for a full explanation on what Amp Hour ratings actually mean and how to calculate the capacity for the battery you need.
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