That could be done to squeeze the fullest amount possible from a battery, but doing so requires a different chemistry than lead acid. One that isn't so damaging to the cell structure after complete discharge, like, Ni-cad; NiMh; molten salt types(industrial), and even the old alkaline NiFe.that's a great explanation!
but i wanted to know how you do it with a circuit.
do you compare the voltage levels using comparator??
It is necessary to have a voltage regulator for battery operated circuits right?
Thanks!
If you use a lead acid battery, you will want to keep discharge within a small defined window. 99.9% of circuits are just fine with this range as the supply.
If you want to have operation during 'recharge' then adding a regulator would be good. The voltage boost during recharge might be to much for some circuits.
Do only what you must do to make a circuit work well, and work safely. Needless complexity leads to failure(by various definitions) in most cases.