RE: Adjustable Power Supply HELP

I'm trying to build in UPS protection into a circuit I'm working on and I was wondering if anybody had done this already and had some circuit tips for me. I want to charge batteries all the time and then switch to them when mains is disconnected. For error checking I want to check battery level. I guess I will use AD for this and if it starts to drop when charging then I will automatically assume the batteries are going bad. But I also want to know when I'm on the batteries and when I'm on the charge...