I just installed a battery backup for my Raspberry Pi to prevent file system corruption and data loss in case of a power failure. I choose the UPS Pico from PImodules. This module also has a Real Time clock on board which is something the Pi is natively missing. On the web page of PImodules there is a detailed description how to install and configure the module.
First tests were successful. It recognizes a power loss and if the power does not restore within a few minutes it will automatically run a clean shutdown of the Pi. After power is restored again it boots up automatically again.
As the Pi with the power module did no longer fit into the case I was using before, I also bought a new case (UPS Pico Case B+) with enough space to host the power modul including the battery.
So far I am very satisfied.