I use my Pi B in my car to do timelapse photos powered off a cigarette to usb plug however I found that sometimes the plug would fall out and thus power down the pi. I also had the problem that even if I wanted to stop for 5 minutes which meant turning the engine off again the pi would be powered down. TO overcome this I thought I would look into using a usb power bank as a mini ups and here are the results of my testing.

I tested for both pass-through charging i.e. a power bank which allows to be charged while powering a device AND ‘UPS’ which is a usb bank which supports pass-through charging with the added feature of power to the connected device (pi) being uninteruppted when the mains charger was switched off/on.