`color{green}text(Definition) :` Any battery (actually it may have one or more than one cell connected in series) or cell that we use as a source of electrical energy is basically a galvanic cell where the chemical energy of the redox reaction is converted into electrical energy.

`color{green}text(Characteristics of a Battery) :` For a battery to be of practical use it should be reasonably light, compact and its voltage should not vary appreciably during its use.

There are mainly two types of batteries :

`color{green}text(Definition) :` In the primary batteries, the reaction occurs only once and after use over a period of time, battery becomes dead and cannot be reused again.

`color{green}text(Examples) :` (i) Dry cell (known as Leclanche cell after its discoverer) which is used commonly in our transistors and clocks.

`color{green}text(Composition and Working) :`

`=>` `color{green}("Anode")` : a zinc container.

`=>` `color{green}("Cathode")` : A carbon (graphite) rod surrounded by powdered manganese dioxide and carbon (Fig.3.8).

`=>` The space between the electrodes is filled by a moist paste of ammonium chloride `(NH_4Cl)` and zinc chloride `(ZnCl_2)`.

`=>` The electrode reactions are complex, but they can be written approximately as follows :

`color{green}("Anode")` : `color{red}(Zn (s) → Zn^(2+) + 2 e^(-))`

`color{green}("Cathode")` : `color{red}(MnO_2+NH_4^(+) + e^(-) → Mn O ( OH) + NH_3)`

`=>` In the reaction at cathode, manganese is reduced from the `+ 4` oxidation state to the `+3` state.

`=>` Ammonia produced in the reaction forms a complex with `Zn^(2+)` to give `[Zn (NH_3)_4]^(2+)`.

`=>` The cell has a potential of nearly `1.5 V`.

(ii) Mercury cell, (Fig. 3.9) suitable for low current devices like hearing aids, watches, etc.

`color{green}text(Composition and Working) :`

`=>` `color{green}("Anode")` : Zinc–mercury amalgam.

`=>` `color{green}("Cathode")` : A paste of `HgO` and carbon.

`=>` `color{green}("Electrolyte")` : Paste of `KOH` and `ZnO`.

`=>` The electrode reactions for the cell are given below :

`color{green}("Anode")` : `color{red}(Zn (Hg) +2 OH^(-) → ZnO(s) +H_2O + 2 e^(-))`

`color{green}("Cathode")` : `color{red}(HgO +H_2O +2 e^(-) → Hg (l) +2OH^(-))`

The overall reaction is represented by :

`color{red}(Zn (Hg) +HgO (s) → ZnO(s) +Hg(l))`

`=>` The cell potential is approximately `1.35 V` and remains constant during its life as the overall reaction does not involve any ion in solution whose concentration can change during its life time.

`color{green}text(Definition) :` A secondary cell after use can be recharged by passing current through it in the opposite direction so that it can be used again.

`=>` A good secondary cell can undergo a large number of discharging and charging cycles.

`color{green}text(Examples) :` (i) The lead storage battery is commonly used in automobiles and invertors.

`color{green}text(Composition and Working) :`

`=>` `color{green}("Anode")` : Lead.

`=>` `color{green}("Cathode")` : A grid of lead packed with lead dioxide `(PbO_2 )`.

`=>` `color{green}("Electrolyte")` : A `38%` solution of sulphuric acid.

`=>` The cell reactions when the battery is in use are given below :

`color{green}("Anode ")`: `color{red}(Pb (s) + SO_4^(2-) (aq) → PbSO_4(s) + 2e^(-))`

`color{green}("Cathode")` : `color{red}(PbO_2 (s) + SO_4^(2-) (aq) + 4 H^(+) (aq) + 2e^(-) → PbSO_4 (s) + 2H_2O(l))`

Therefore, overall cell reaction consisting of cathode and anode reactions is :

`color{red}(Pb(s) +PbO_2 (s) +2H_2SO_4 (aq) → 2PbSO_4 (s) +2H_2O (l))`.

`=>` On charging the battery the reaction is reversed and `PbSO_4(s)` on anode and cathode is converted into `Pb` and `PbO_2` respectively.

(ii) Nickel-cadmium cell.

`=>` It has longer life than the lead storage cell but more expensive to manufacture.

`=>` We shall not go into details of working of the cell and the electrode reactions during charging and discharging.

`=>` The overall reaction during discharge is :

`color{red}(Cd (s) + 2Ni (OH)_3 (s) → CdO (s) + 2Ni (OH)_2 (s) + H_2O(l))`