One of the benefits of the Raspberry Pi is its low power consumption and cheap price. Because of that, it makes a great wireless backup server. However, adding an expensive battery backup would kind of defeat the whole purpose of using a Pi, so DIY enthusiast George decided to make his own.
The battery backup here uses an inexpensive custom circuit that automatically powers down the Pi when it loses power. It uses a couple of 9V batteries as a regulator to supply the voltage to the Pi to keep it alive.
It's a pretty simple and cheap solution, and it's worth doing if you're using your Pi as backup server. Head over to Repair Hub for the full guide to make this for yourself.