This is a design circuit for basic interruptible Power Supply Circuit consist of regular power supply adapter and battery connection. This basic system is a “hot” battery connection, meaning that there is no switching mechanism in connecting and disconnecting the battery, the battery is always connected! This hot connection is very simple to implement and very robust because there would be no switching delay, the output voltage will be 100% continue if a power down happens, until the battery loose its capacity. This is the figure of the circuit;
When the power line is normal, the current from the main adapter (transformer-rectifier diode bridge-filter capacitor) flow to the load through D3, and the battery is charged via R1 and D1. The diode D2 prevent the current at D3 output to be shorted by the empty battery in the normal power line condition. If the power down happens then the battery will supply the current to the load through D 2. Use a 15V AC output transformer for 12 volt lead acid battery. LED1 will indicate if the power line is normal. LED D1 will turn off if the power line is down.