Smart battery charger and battery back up ideas

[jameske]

Greetings,

A project of mine in the design stage requires a battery back up of some sort. After comtemplating what to do I thought I'd come and ask everyone on the forum here! The circuit requires 12V DC at 600-700mA, and what I'm trying to do is make a battery backup for the circuit so that when the supply drops out the battery kicks in and when the supply comes on again the battery drops out. Now when the battery 'needs a charge' a charger circuit will kick in keeping the battery charged and ready when the supply drops out.

So my question is, what would the suggested type of battery be (Lead-acid... etc) while still keeping cost relatively low. And also, what sort of circuit idea could be used for the switching cicuit - to switch to the battery when the supply drops and then switch to the supply when it comes back? I thought that maybe a relay circuit could be used with a capacitor to fill in the debouncing and delay of it switching (illustrated in the pictures), but then again its just a thought.
Acording to the battery type what charger circuit would be recomended? If lead-acid battery I found a charger circuit here!

Thanks again for your support and suggestions,

James