At just 0.28-inches thick, Powerocks claims the Tarot is the thinnest backup battery you can buy. And while it's still considerably thicker than a credit card, it's certainly thin enough to slip into a heftier wallet. Like with any external battery the Tarot's got a USB port for charging/recharging and it's got enough capacity to fully recharge a single smartphone. So as long as you're not heading out into the jungle for a week, for just $35 it should easily serve all of your emergency power needs. More here.