why does mercury cell give a constant voltage throughout its life?

The cell potential of a Mercury cell remains constant throughout it's life as the overall reaction doesn't invovle any ion in the solution whose concentration may change.
 

Mercury cells use KOH(Potassium Hydroxide) or NaOH(Sodium Hydroxide) as their electrolyte, and cells using NaOH have nearly constant voltage at low discharge currents, making them ideal for hearing aids, calculators, and electronic watches. Potassium hydroxide cells, in turn, provided constant voltage at higher currents, making them suitable for applications requiring current surges, e.g. photographic cameras with flash, and watches with a backlight. Potassium hydroxide cells also have better performance at lower temperatures. Mercury cells have very long shelf life, up to 10 years.

Electrolyte is not consumed in the cell process of mercury and in the overall reaction there is no involvement of any ion in the solution whose concentration may or may not change.

kk

Aunda ni Yaar aapa nu

no ion in overall reaction
 

Hg cells use KOH or NaOH as their electrolyte and cells using NaOH have constant voltage at low discharge current.KOH in turn provides constant voltage at higher current