Battery Technology

A battery stores energy chemically and releases it as electricity through electrochemical reactions. Each cell has two electrodes — a positive and a negative — separated by an electrolyte that allows ions to move between them. When the cell is connected to a circuit, a chemical reaction at the electrodes drives electrons through the external load, producing current. The cell’s voltage is set by the materials used, and several cells are connected in series within a battery to reach a higher voltage.

Batteries fall into two broad classes. Primary cells, such as alkaline and zinc-carbon (Leclanche) types, are used once and discarded when their chemical reactants are exhausted. Secondary, or rechargeable, cells can be restored by passing current through them in the reverse direction; these include lead-acid batteries used in vehicles, nickel-cadmium (NiCd), nickel-metal-hydride, and lithium-ion and lithium-polymer cells widely used in portable electronics. Lithium-based chemistries are popular because they store a large amount of energy for their weight.

Each chemistry has its own characteristic voltage, capacity, and rules for safe charging and discharging. Older nickel-cadmium cells could show a so-called memory effect that reduced usable capacity if repeatedly partly discharged. Capacity is commonly rated in ampere-hours, indicating how much charge a battery can deliver. Fuel cells are a related technology that generates electricity continuously from a supplied fuel, such as hydrogen, rather than from a fixed internal store of reactants.

Frequently asked questions

What is the difference between primary and rechargeable batteries?

Primary cells are used once and then discarded, while rechargeable (secondary) cells can be restored many times by passing current through them in the reverse direction.

What is the battery memory effect?

It is a reduction in usable capacity historically seen in nickel-cadmium cells that were repeatedly recharged after only partial discharge, so they appeared to hold less charge.

How is battery capacity measured?

Capacity is usually given in ampere-hours, which indicates how much charge the battery can deliver, for example how many hours it can supply a given current.