This scheme is called float and boost charging.(different from float cum boost charger)When the input AC supply fails the DC load is fed by the battery.After the AC supply resumes the battery needs to be charged at a faster rate and in that case the boost charging is used.The float charger is used to fed the DC load and the boost charger is used to charge the battery but if the AC supply fails during the period of boost charging the continuity of DC power will not be available and to take care of this situation 80 % cell tap is provided along with blocking diodes.
There is a contactor which is open during boost charging and separates the load bus and the battery bus to prevent the load being subjected to high boost charging voltage.Now if the AC supply fails the contactor closes again but there will be a time delay (in millisec) for the contactor to close.So momentarily in that period the load is supplied by 80 % cell tap with diodes.Once the contactor closes the diode is again reverse biased and the whole of the
battery bank is available for the load.
