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?
Your requirements are pretty simple. You'll need a pair of connectors for the batteries and a pair of diodes. Make a connection setup like this
simulate this circuit – Schematic created using CircuitLab
Now you can connect or disconnect either connector as you wish. As long as one battery is always connected, you should have no problems.
Your diodes should be rated for at least 12 volts (20 or more is better), and at least the current required by the controller, probably at least 1 amp. Also, since the diodes will drop some voltage, Schottky diodes are preferred. Something like a 1N5820 should do you just fine. You can try sources like Jameco, Digikey or Mouser, or any number of other suppliers.
I would suggest a PowerPath circuit topology. You could easily find many ICs for that job.
Linear Technology has very affordable solutions. The most cheap common is the LTC4412, a tiny PowerPath controller. In page 10, Figure 2 and 3 show some solutions that might fit your demands.
Consider the "WALL ADAPTER INPUT" as your main 5V input. If you want always 5V output from a rechargeable battery even when the main 5V power is absence, just put a DC to DC step-up converter after the battery charger and before the Primary mosfet.
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.