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It's a wind-up
A typical smartphone lasts about a day before its battery dies. This case can help it last a lot longer, however.
CrankCase features a handle on the back. Wind it up, and you'll juice up your phone. The handle is connected to a rare earth magnet dynamo generator which creates energy as you crank.
100 seconds of cranking will give you an hour's standby time, 12 minutes of audio play and five minutes of talk time. Five minutes will give you three hours of standby time, 36 minutes of audio play and 15 minutes of talk time.
It has an auxiliary USB port, so you can charge other devices too. 100 seconds of cranking will give your smartwatch five hours of standby time, while five minutes gives 24 hours of standby time. For a tablet, 100 seconds of cranking gives 30 minutes of standby, while five minutes gives 90 minutes of standby.
The handle can also double as a kickstand when it's folded out.
The case protects your phone from bumps and scrapes. At the moment, it's only available for the iPhone 5, 5s and 6, but the firm will make models for other smartphones soon.
Read more: Mu System is the world's thinnest international charger
It also makes the CrankCase, which is for any USB device. It's a standalone charger, rather than a case for your smartphone.
You can snaffle the CrankBox for $29 (£20) and the CrankCase for $39 (£26) if you're quick. They've raised $35,000 of a $50,000 target. If successful, the products should ship in October.