Is it true that batteries should be fully discharged before recharging? If so, why?

Question

Or is it just an urban myth? Can anybody provide a source?

Answer 1

Batteries have DoD memory, and if they are charged before their complete cycle depth of discharge, they reduce their capacity due to qualking (sulphate hardening due to discharge process), thus reducing the surface rear of the electrodes.

Answer 2

That does not apply to car batteries which are ruined if fully discharged (their active metal will flake off and fall down in the liquid to make a pile of short-circuiting metal flakes at the bottom of the battery).

It applies to nickel cadmium batteries. They have a weakness called memory. If you discharge them half way they will remember and think of half capacity as their maximum. If you want their full capacity to remain you must bring them down to zero charge so they can come up as a full capacity battery again.