I went to Lowes and bought a back-up battery pack/jumper cables, etc. Stanley, if I remember correctly, and cost around $50. It has USB ports and plug-in ports. Sits on the front floor and I plug my gps, phone and whatever into it while traveling. Nice, because you can listen to Pandora on your iPhone, too, eliminating the need to restore the dashboard radio.
Of course, it also gives you the back-up needed in case your car battery needs a jump.
Supposedly there is such a battery back-up system that is about the size of a phone, but I haven't heard much good about those.
Here's about the same thing at Walmart, with compressor.
http://www.walmart.com/ip/38243753?wmlspartner=wlpa&adid=22222222227027518957&wl0=&wl1=g&wl2=c&wl3=62688801888&wl4=&wl5=pla&wl6=89025291648&veh=semThis Post was from: https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=162911