 
The Charging time of a battery is not an exact science.
It depends on a number of factors
- The residual voltage in the battery
- The Internal Resistance of the Battery
- The charging Current
- The ambient temperature
- Age of the battery etc
As you can see from the image above at 11.4 volts the battery is starting to sulphate which is the sih=ngl most reason for battery failure. So keeping the battery charged at 12.7v levels is the best way to ensure the long life of the battery.
In the above table, it is assumed that the battery is being charged at 1/0 of the battery capacity. Meaning for a 100 Ah battery, a max 10 amp battery charger is being used. For a 30 Ah battery, a max 3 amp battery charger is being used.
In addition, it is to be kept in mind that batteries should not be charged in linear method, meaning the charging current needs to start reducing as the battery gets 80% charged and after that the current drops to as low as .1 amp as it reaches 100%. This ensures that the battery is not getting too hot or degassing or the acid is not boiling.
CTEK Chargers use a patented 8 te process that ensures that batteries are charged in the right manner and brought back to life instead of making them worse.
