After having an issue with blockchain file corruption due to a poor shutdown a few weeks ago I have now set up my Core node (0.14.1) on a Raspberry Pi 3 with the full blockchain stored on an external HDD and use a USB Power Bank to act as an uninterrupted power supply. Thought the UPS aspect might be of interest to some people…
The key thing is that the power bank must offer both pass-through charging and be able to switch from outputting mains power to battery power without any interruption at all. This last requirement is the tricky bit when picking a power bank to use as it's not a feature advertised by manufacturers. I get the feeling it's more 'lucky coincidence' for my needs in the one I have than any purposeful attempt to provide uninterrupted power by the manufacturer as they are normally used to charge devices with their own internal batteries, so a brief delay in power as it switches from mains to battery wouldn't be an issue for most use cases.
It's taken a few failed attempts to get the right power bank but it works fine with the one I now have and I can pull the mains supply without the Pi noticing. My router goes down if there is a power cut (happens a bit too often for my liking) so my node is off-line until mains is restored but it at least reduces the risk of corruption of a blockchain file or the Pi's SD card.
Powerbank is a TeckNet PowerZen 2nd Gen 16750mAh 2-Port (3.4A Output). There are likely others that would work if they have pass though and uninterrupted power provision. If you know of any please share as I have found that different capacity versions (i.e. different mAh ratings) of a manufacturer's power bank model can work differently in terms of pass through. Also some technologies that detect the maximum input a connected device can take in order to speed charging can also interrupt the supply briefly.
The set-up (apologies for dust – needs moving to a more permanent home now!): http://imgur.com/a/pQc8K