Today a new startup, Coin, launched with an intriguing product: A battery-powered, credit card-sized device that stores all your credit card information in one place. Perhaps intentionally, it looks a bit like the heavy, exclusive black Centurion card from American Express, but with a small LED display. You simply tap a button on Coin and cycle through the cards you’ve stored on it. When it shows the one you want, you swipe it like a regular card. It sounds like a great idea… until you realize that the price of replacing a handful of plastic credit cards with a single one is that your ability to pay for anything becomes entirely dependent on the battery life of your smartphone. That’s for security reasons: If it loses contact with your phone for a user-specified period of time, from one to 10 minutes, Coin deactivates itself. Coin CEO Kanishk Parashar says he wants to solve this problem, someday, but it’s not clear when or how that will happen. Thus, Coin has the same problem that all digital wallets have: In those odd moments that you find yourself with a dead phone battery, it ceases to function. That might not happen so often, but think of the times it might, and how they’re precisely those moments you would most want your credit cards to function: Late at night, when you need to pay a cab, tow truck, restaurant bill, or whatever else you need to get you back to a place where you can charge your phone again.