Here's a familiar scenario: You're walking somewhere, and you realize you're going to be late. You reach for your phone to call, but your battery is as dead as Osama. You need some juice, stat. But with no outlet or charger nearby, you're up the ol' creek again.

I've just tested a device made for moments like these: Tremont Electric's nPower PEG backup charger. It purports to capture the energy your body gives off just by just walking or cycling, funneling volts into your dead cellphone battery, and bringing the device back to life or extending talk-time into the great beyond.

It draws its power from kinetic energy – carry it with you as you walk, run, or bike, and the device will keep the battery within continually topped off.PEG stands for Personal Energy Generator. The 10.5-inch plastic cylinder weighs just under a pound, and contains a 2,000 mAh lithium battery. It draws its power from kinetic energy – carry it with you as you walk, run, or bike, and the device will keep the battery within continually topped off. Alternatively, you can fully charge it in about 3.5 hours using the included mini USB cable.

Once filled, it holds a charge for around three months. Red and green LED lights alert you to the PEG's charge level.

Kinetic power does not come cheap – the nPower PEG costs around $200. While it includes one interchangeable iGo.com mini USB tip for use with most non-Apple phones, other tips average an extra $10 each. With literally dozens of other recharging backup batteries available for as little as a tenth of the cost, the audience for the PEG seems to be limited to wilderness hikers, outdoor enthusiasts and "Be Prepared" survivalists. It makes sense, then, that the PEG just made its retail debut last week at REI.

Of course, I discovered the PEG's output varied based on the particular activity I was performing. Starting with a depleted PEG, I carried it on a ten-minute run. This yielded about a minute of talk time on a pre-3G phone. But it took over 25 minutes of brisk walking to get just a minute on a 3G smartphone. I didn't test how much time it takes to power a short call on a 4G phone because I have a strict no-marathons policy.

The PEG can only harness kinetic energy as it moves up and down when positioned vertically. So to work properly, you have to use the carbineer-like loop built into the top to fasten it to something. This works in a backpack, a briefcase or a pocket – though the latter will surely prompt your friends to ask, "Is that a battery in your pocket, or are you just charged to see me?"

The PEG is still useable if you aren't moving about. Keeping it in an upright position, you can manually move it up and down heartily for a few minutes.Of course, keeping the device in a bag has its limitations, too. The manual warns against positioning the PEG close to items that are attracted to magnets, as this will limit its amount of movement and thus reduce the amount of charge it can collect. What's not mentioned is what the magnetic attraction will do to nearby credit cards (and their magnetic strips), or the data on your laptop's hard drive.

All you non-calorie burners will be happy to know the PEG is still useable if you aren't moving about. Keeping it in an upright position, you can manually move it up and down heartily for a few minutes. This is best done in private, as it is perhaps too reminiscent of the act of male self-love. But still, it's just as effective when you need to generate some battery power.

WIRED Recharges cellphones with low or dead batteries using the kinetic energy of your bodily movements. 14 ounces and not too bulky, so you can carry it anywhere. Great for emergencies.

TIRED Only 5V DC, 500mA of output, so it will not charge a laptop or iPad. Innate magnetic activity may affect nearby credit cards, hard drives, and artificial limbs. Expensive compared to other available rechargers. You know why everyone laughs at the Shake Weight, right?