Electronic – Battery in parallel with main power source

batteriespower supplyvoltage

I have a system that is powered by a main voltage supply and I want to connect it to a battery for backup, in case of power outage.

I need to know what would happen if I connect the system with the battery in parallel with the source and both the battery and the source give 5 V. Where from will the load take its power? From the source or the battery? And how can I isolate the battery from the system if the source is working? And will the battery power the system when the source stops providing?

Best Answer

Paralleling power sources is a Bad Idea™. The simplest solution is to use two diodes to separate them.

Suppose the main power source is 12 V and the battery 9 V. Then the 12 V source will drop, say, 0.5 V (Schottky diode), so the voltage at the cathode will be 11.5 V, which is higher than the battery's 9 V, so that diode won't conduct current. When the main source is off the battery will supply power via its diode.

Note that the highest voltage has priority, that's why I chose 12 V and 9 V as example. If the main power supply would be for instance 8 V then the battery would supply the power all the time.