Let’s face it, the main reason external batteries aren’t as much of a runaway success as they could be is because no one wants to carry yet another bulky item in their pockets. But phones will keep running out of juice, and you’ll keep turning off Bluetooth, WiFi, brightness and whatnot to get that extra mile out of your phone. What about the Gokuusu portable battery? It’s 4.5mm thin and about credit-card sized, which means that it could fit into a wallet with ease. Sure, the 1,000 mAh isn’t going to bring a phone back to 100%, but in emergency situations, it’ll be more than welcome. The connection is done through a retractable USB cable and a set of tiny adaptors for whatever phone you carry. Better yet, it also has 4GB of storage. At 2,980 Yen ($30), it wouldn’t make much sense not to get one if you’re the type of guy who’s constantly getting home with your mobile running on empty.