Get a gadget juicer.
Besides the inconvenience of having your smartphone lose power when you're navigating through the middle of East Jabib, there are more pressing horrors: You're unable to search for the best place to eat off I-76 and so must play Quickie Mart hot dog roulette; your whiny kids take to kicking the back of your seat, since they can't play their ultra-violent yet somehow pacifying video games; and your beloved can't scan Facebook, which means finally getting to talk about having a fourth kid. It gets deep, and fast.
Thus the need for a dual-USB charger, like this one from Mediabridge that pops in a cig lighter and lets you charge smartphones as well as amp-hungry tablets (there are iOS- and Android-specific versions, but both mostly work for any USB device – they just take longer to charge). Another option that won't drain your battery is a solar charger from Wenger, the redoubtable makers of the Swiss Army knife. They come in three sizes and each feature polycrystalline solar panels and lithium-ion batteries connected by USB. They also come with a five-pronged USB cable that lets you charge up to four gadgets at once.