Charging time will depend upon the battery voltage that is to be charged. Lesser the battery voltage more will be the time required to charge the same.

Under normal circumstances, it is very difficult to retrieve the drained battery.

If the battery charger is supplying rated current then the battery is in discharge mode. In other case if battery charger supplying very low current then the battery is fully charged.

CCCV means constant current constant voltage. It means output voltage will be constant till load draws rated current. Beyond this, if load current increases then output voltage will start reducing. It means now current will be constant at the level of current limit and voltage will start reducing.

While choosing the battery charger for charging any battery the thumb rule is to have charging current equal to 10% of battery Ah. Ex: If a battery is of 100Ah the charging current should be at least 10 A. However it is suggested to check the charging characteristics of the battery that is used.

To calculate the battery backup time first we should know total load current and Ah of battery. Then dividing Ah of battery by total load current we will get backup time.
Ex: – If load current is 10 A and ah of battery is 120 h then(120/10) =12 hours.