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Filmmakers know that five minutes — even though it doesn’t sound lengthy — can be a long amount of screen time. And now users of YoGen’s new hand-cranked cellphone charger will discover that truism as well.

The $40 YoGen, from Easy Energy, Inc., is one of a crop of cellphone rechargers designed to provide emergency power when a wall outlet isn’t available. Supplied with multiple tips to fit a range of devices, including the iPhone, the YoGen is smaller than a pack of cards. To charge a device, you pull continually on a nylon self-retracting string, as if you were trying to start a recalcitrant outboard motor.

YoGen executives say that the amount of power supplied over a given period of time is the same as if you had plugged in a conventional charging cord. But don’t expect to see the battery meter start climbing quickly. After several minutes of tugging I couldn’t get the iPhone’s green battery meter bar to budge. When I tried it the next morning after allowing the battery meter to fall so far into the red that the phone wouldn’t turn on, several minutes of recharging failed to bring the phone back to life.

Which means that users need to be prepared to pull and pull. The company says that five minutes of charging will bring 10 minutes of talk time, which should be enough for emergencies; but I’d be surprised if your arm didn’t hurt after that amount of time; mine did, and I’m in pretty good shape.

Easy Energy recommends the YoGen for places where an outlet is not available: on a camping trip, or when power is lost, such as in an emergency. In those instances, the charger could be a great help, assuming you can get a signal in the wild. Just don’t whip out the YoGen in a car, give it a few yanks, and expect to be back in business. In those instances, a standard car charger cable will do you and your shoulder much better.