a few years ago I got my kids a battery bank that was the size of a thick smartphone that was capable of jumping a car battery - came with jumper clips, and charged with a micro USB adapter. it has a USB port for phone charging, so I got one for myself and started carrying it on trips. Now every car has one under the seat continually charging when we drive the cars.
No shortage of good ones out there that will charge a phone multiple times in the $30ish range
