I've been using a little 100wh mini 12v UPS from talentcell. I own three and am happy with them. It has an important caveat though: both the USB and 12v output

I've been using a little 100wh mini 12v UPS from talentcell. I own three and am happy with them. It has an important caveat though: both the USB and 12v outputs are unregulated, so the device you're feeding into must be able to handle a wider voltage range, or you can buy a fairly cheap ($15 or so) step up/down converter to put in-line with it.