In reply to Run_Ross_Run:
Someone on here recommended a RavPower one to me a couple of years ago, 16000 mAh, it has been really good and will do my smart phone a couple of times and my little iPod a couple of times too and still have some power left in it. Probably more than you need for an over night trip, but I've used it on long bike rides when I've been navigating a lot using my phone as well. This looks like the closest model in the current line up https://www.amazon.co.uk/Portable-RAVPower-16750mAh-Upgraded-External-Black...
I've got a much smaller and lighter 3000 mAh one too that I'll chuck in my pack winter climbing and the like as I now use my phone in part for navigating and as a camera, so want to make sure if I got benighted or similar that I'd have some spare juice if necessary. They now do them at 5000 for the same cost as mine is a few years old too https://www.amazon.co.uk/Poweradd-Portable-Technology-Lightning-Included-Bl...