Unless you're completely killing your smartphone's battery on a daily basis, you don't need to haul around a massive backup battery. A single emergency charge is all most of us need for those days when we talk or stream more than we intended, which makes the Tarot's 1,500 mAh capacity the perfect balance of size vs. power.
At just 7mm 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, it should easily serve all of your emergency power needs.