You question is more suited for the Electrical Design Stackexchenge site - Because you can always rely on the Gurus there, like Oli, to help design easy, up to date circuits.
I think that this question will be asked by many more Raspberrians to come so this is actually a very good place to answer your question.
My answer will go more into engineering your own circuit so that you can have full control of what you want it to do.
The circuit should handle.
- Initially deciding what kind of circuit you need. Powered by USB 5V? or maybe by a 12 volt source? You can also power it from a lower power source like 3.3V/1.5V but is very inefficient in converting power. This decision also fundamentally contributes to what kind of voltage regulator you are going to use. if any.
- Keeping the battery charged up during normal operation (also each type of battery requires have charging characteristics, Lead Acid, Ni-Cd, Li-Ion, etc)
- The circuit needs to sense when primary power (USB +5V) stops providing power or similar.
- The backup circuit to charge your type of battery and an embedded circuit to possibly route power back into the main circuit when the main power is off.
- Optional. Build a trigger into the circuit that connects to the Raspberry PI's I/O system to send you and email,text message, make a phone call, trigger an alarm or turn of your kitchen lights.
Searching around the internet most UPS circuits and
schatics will include a transformer to reduce 110V/220V down to DC 12 Volts.
- Here is a very simple circuit used with Lead-Acid batteries (They are easy to charge and they keep charge for a very long time) Do not attempt to charge any other kind of battery with circuit. they will blow it up!
- R1 - 39 ohms 1/2W
- D1, D3, D4 - 1N4001 or similar diode
- D2 - 13V zener rated 1W
- C1 - 220uF electrolytic capacitor rated 25V
- C2 - 10uF electrolytic capacitor rated 10V
- IC - 7805 or similar 5V regulator
- BAT - 12V lead acid battery rated 1.2Ah minimum
- DC Input - 12 Volt DC
- I found this circuit while rummaging through the internet. This guy pretty much rips apart a notebook and recycles a few parts to make a decent system - Just remember to regulate the output voltage to 5V
- Final circuit - one of the easiest and most probably the one that many people would prefer to use is a USB charged UPS circuit using some small lithium batteries. This was answered at Engineering SE, please refer to the answer for details .
Do not feel like building your own circuits? There is this MUPS available for purchase that does pretty much what the final circuit design outlines.