Secondary cells: - Those which can be recharged e.g. - Lead strong     battery, NiCad cell

(1) Lead strong battery: - A 12 v lead strong battery is generally used which consists of 6 cells of 2v each. Each cell consists of a lead anode or a grid of lead filled with finally divided spongy lead. And a grid of lead packed with lead dioxide (P₆O₂) as cathode. Electrodes are separated by thin wooden or fibre glass sheet and suspended in dilute sulphuric acid. (38% by mass or having a density of 1.3 g cm^-3, act as electrolyte). All compartments or cells are in series with each other.

At Anode →Pb → pb²⁺ + 2e^-

Or

Pb + So₄^2- → PbSo₄ + 2e^-

At cathode → Pbo₂ + 4H⁺ + So₄^2- → PbSo₄ + 2H₂o + 2e^-

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Pb + Pbo₂ + 4H⁺ + 2So₄^2- → 2PbSo₄ + 2H₂o

The PbSo₄ produced, sticks to the respective electrodes and density of H₂So₄ fall to 1.2 g cm^-3. when density is below 1.2 g cm^-3, the cell needs recharging using external source of current. Then reaction will be reversed. Then reactant changed to product and product into reactants.

At cathode → PbSo₄ + 2e^- → Pb + So₄^2-

At anode → PbSo₄ + 2H₂o → Pbo₂ + So₄^2- + 4H⁺ + 2e^-

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2PbSo₄ + 2H₂o → Pb + Pbo₂ + 4H⁺ + 2e

Water was to be added periodically because during recharging some water is lost due to its electrolysis to H₂ and O₂

In maintenance free batteries, an alloy of Ca & Pb is used which prevents electrolysis of water

(2) Nickel-cadmium battery (NiCad battery):- More expensive but has longer like due to less corrosive nature of electrolyte.

Anode → Cd + 2OH^- → Cd (OH)² + 2 e^-

Cathode → NiO₂ + 2H₂O + 2e^- → Ni(OH)₂ + 2OH^-

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Cd + NiO₂ + 2H₂O + 2H₂O → Cd (OH)₂ + Ni(OH)₂

Electrolyte used in KOH or NaOH solution

During recharging

Ca(OH)₂ + 2e^- → Cd + 2OH^-

Ni(OH)₂ + 2(OH)^- → NiO₂ + 2H₂O + 2e^-                

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Cd(OH)₂ + Ni(OH)₂ → Cd + NiO₂ + 2H₂O 

As no gases are produced during discharging or recharging so battery can be sealed used in computers, laptop, calculators etc.