Hello everyone, I'm doing a project and could use some advice or help, I'm trying to designed a 9 V backup battery with a 9 V primary source coming in.
I'm trying to design it so that when the primary source gets disconnected or turns off the backup battery will power the rest of the circuit. I'm having troubles as well with the LED's because the first LED works as it should but the second LED showing that the backup battery has taken over doesn't illuminate properly.
Any help would be greatly appreciated.
Welcome, renardak!
Interesting circuit. Good for you for at least taking a shot at it. As you (and MikeMI) point out, it does have some problems.
You might consider a circuit for the backup supply such that it is "isolated" (i.e., is not in the circuit until called for). This type of arrangement is ordinarily handled by a dedicated chip that monitors the condition of the primary supply (battery in this case) and switches to the backup source as needed, for instance:
https://www.electro-tech-online.com/custompdfs/2013/01/slls917.pdf
As you can see, not necessarily a simple circuit.
Ordinarily, for a battery backed radio or clock (for instance), the primary source of power is "robust". That is to say, powered by mains (or a much larger battery). With the device in a "tickle-charger" arrangement that keeps a battery backup fully charged such that upon the loss of the primary the power source, the battery takes over (until depleted or primary power is restored).
Otherwise, a device, such as noted above, is required.
As to your circuit (other than the noted, minor LED orientation), with a suffiiciently "robust" primary power source, a switch-over relay at the primary battery end and some rewiring of the backup battery and its LED circuit, it should work as you expected.
The relay switch-over circuit is key to a simple (albeit, energy intensive) addition to your concept.