I wanted to see if I could keep my mini-ITX running 24x7 so I started to investigate what was available in the way of uninterruptible power supplies. It became apparent that just about everything on the market was aimed at 240V AC output and, since my mini-ITX had an external PSU that produced around 13V or so, it didn't make sense to have a system charging a 12V battery which then supplied an up-converter - only to be converted back down again to virtually the same voltage. It had to be more efficient to use the battery output to drive the internal supply in the mini-ITX.

I made a fruitless search for a 12V UPS and so decided to build one. I based it around a solar charge controller and a 12V sealed lead-acid battery. The solar charge controller is designed to take its input from a solar panel but can handle around 15-16V from a standard PSU - in this case a laptop switched mode supply.

After an unsuccessful attempt with one supply which failed after trying to deliver around 120W to charge the battery for 30min or so I was more careful in my choice. I found one rated at 150W which claimed to be current regulated at 8.5A. The picture here shows it without its case.

Despite its fan it ran a bit hot when charging the battery so I decided to upgrade its cooling by adding extra heatsinks and a bigger fan and building it into a bigger box.

There is also a diode and a 12V regulated output that I built into it so that it could drive a separate "mains power present" signal. More explanation of that later

Here are a couple more pictures showing what it looks like in its new case. The larger fan and extra heatsinks made a dramatic difference and it runs fairly cool even on full load (i.e. when the battery has been discharged).

I built the complete UPS into a box from the Really Useful Company which turned out to be ideal for the job. You can just see the battery which is rated at 34Ah - so plenty of capacity to run the mini-ITX all day if I wanted to!

These are some shots of the ITX - actually a spare one I picked up from e-bay. The main one I use is similar but in a black case rather than this quirky salmon colour.
The ITX is fitted with an M1-ATX PSU which is designed for use in cars so has a wide input voltage tolerance and can take an "ignition" signal. The ignition signal is normally used for starting and stopping the unit when the car is running. I fed it from the 12V regulated signal that I added to the laptop PSU. I also configured the M1-ATX to run the ITX for 3 hours during a mains outage before performing a graceful shutdown.