/diy/ - Do It Yourself

I have a circuit which I want to run from either mains or batteries. Basically, I want it to be powered by a power supply and charge the batteries when it's plugged into mains, and then run off the battery when it's disconnected.

What's the best way to do this? Is pic related a good idea, or can you just charge the batteries under load and basically use the charger as a power supply? I'm thinking I'll need a buck-boost converter or something to regulate the output from the battery anyway, so I'm not worried about the higher voltage from the charger.