I want to design a circuit in, which when the normal power supply is cut off device switches to battery power automatically what are the possible ways to do that

 

Normally closed relay controlled by the normal power supply.

 

is there any method without using relays, I mean like bi-directional power switch

 

Two schottky diodes, one in series with each supply. BUT the load would be supplied from the battery whenever the battery voltage was higher than the normal supply, even if the supply had not failed.