Batteries

Batteries are, at their core, packaged galvanic cells engineered to deliver direct current at a steady, usable voltage, and they are genuinely indispensable to modern life — lithium-ion batteries power cell phones, while dry cells power torches and countless other everyday devices. Batteries divide into two broad families, distinguished by whether their internal cell reaction can practically be reversed:

• Primary batteries are non-rechargeable: once their reactants are consumed during discharge, the cell reaction cannot practically be driven backward, and the battery is discarded. The Leclanché (dry) cell and the mercury button cell, covered next, are both primary batteries.
• Secondary batteries are rechargeable: their cell reaction CAN be driven backward by applying an external potential slightly greater than the cell's own emf, regenerating the original reactants and readying the battery for another full discharge cycle. The lead storage battery and the lithium-ion battery, covered after that, are both secondary batteries. …