Speaking of keeping your phone charged and multi-use devices, I've carried one of these 12v batteries for years. Add the cigarette lighter plug charger of your choice and you can charge anything you want. Many times.

Its sealed lead acid which is specifically authorized for carry-on in the TSA rules. Getting to the multi-use facet, its quite heavy for its size and has a nice long shoulder carry strap.

Plan ahead if you intend to carry it on as almost every TSA agent feels compelled to swipe this thing and run it though their fancy explosives detector, but it passes every time.