Battery charging- Float mode and boost mode
A 12 volt "Smart" charger will deliver its full rated current until the battery's voltage reaches about 14.4 volts, then will reduce the charging current to maintain 14.4 volts.  After some time, the charger will reduce the charging current to allow the voltage to drop to a float charge level.

If you try charging a 12 volt battery with a 24 volt charger, the charger will deliver its full rated current for ever, as the battery voltage will never get up to the 28.8 volts the charger is trying for.  The battery will eventually boil dry and be destroyed.