Imagine you have a small device which is normally plugged into a wall, such as an alarm clock. You want to put a battery back-up into it, so if the wall power should ever fail (unplugged, outtage, etc), your device still functions. But, at the same time, you want the battery to be rechargeable. Ideally, the device will recharge the battery while plugged in, but switch to battery power when necessary.
So, I don't know too much about how a battery charger works, but here's what I was thinking:
Wall power leads into a typical power supply, yielding 9Vdc. Those9Vdc supply a battery charger which is always charging a rechargeable 9V. The terminals of the 9V go through a 5V regulator to power the rest of the circuit. There is no direct route from the 9Vdc power supply and the 5V regulator.
Does this sound like it would work?
Is there a more common-practice topology for this sort of design?
Anyone have links on how to design a battery charger?