Check your hire car’s radio. If it has an a USB socket for playing music via your iPhone, it will also charge the phone. Not much. But enough.
The ideal is for you to have an adaptor that plugs into the cigaratte lighter but – no, actually, the ideal is for batteries to last longer but if wishes were horses we’d all be in the supermarket meat trade. If you don’t have an adaptor and you do have a low battery, the radio trick will work.
I just did this in France, driving about four hours with my iPhone on the red-line 5% battery level. At the end of it, when I peeled myself out of the car and slithered across the pavement like all my muscles had been erased, the iPhone was still at the 5% redline.
But it had worked as my GPS SatNav all the way.
So it’s not like you’re going to get a lot of power out of this radio USB connector. But you’ll get enough.