Q.Among the following cells: I) Leclanché cell
II) Nickel–Cadmium cell
III) Lead storage battery
IV) Mercury cell Primary cells are

A battery is a packaged galvanic cell built to deliver steady direct current, split into primary batteries (non-rechargeable, discarded once discharged) and secondary batteries (rechargeable, since their cell reaction can be driven backward by an external potential greater than their own emf). The Leclanché (dry) cell — Zn anode, graphite/MnO2 cathode, NH4Cl/ZnCl2 paste electrolyte, ~1.5 V — is the classic primary cell; its emf drifts downward over its working life because its overall reaction, $\text{Zn} + 2\text{NH}_4^{+} + 2\text{MnO}_2 \rightarrow \text{Zn}^{2+} + \text{Mn}_2\text{O}_3 + \text{H}_2\text{O} + 2\text{NH}_3$, involves ions whose concentrations genuinely shift as discharge proceeds. The mercury button cell — Zn(Hg) anode, HgO/graphite cathode, KOH/ZnO electrolyte, ~1.35 V — is also primary, but its overall reaction, $\text{Zn} + \text{HgO} \rightarrow \text{ZnO} + \text{Hg}$, involves no ions at all, which is exactly why its voltage stays remarkably flat throughout discharge. …