The result is a material that has an energy density of about 0.005 watt-hours per kilogram.

ECONOMIST: Carbon-composite batteries

And by covering the fibres with carbon nanotubes, at least 20 watt-hours per kilogram is expected by the end of next year.

ECONOMIST: Carbon-composite batteries

Not much, admittedly, when compared to the 128 watt-hours per kilogram of a lithium-ion battery, like that found in many electric cars.

ECONOMIST: Carbon-composite batteries

The Turbo battery has 342 Watt-hours of real capacity, meaning that all of this energy can actually be used by the rider instead of some theoretically possible calculated number.

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Virtually nothing at national levels is stored as electrons, not even at your iPad level where watt-hours are stored in batteries in an intermediate form as ionized chemical molecules.

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The two-watt Atom chip inside the Series 5 is rated for about nine hours of battery life, whereas the Core i5-powered Series 7 is expected to last between four and five hours on a charge. (The Series 5 is lighter, too, at 1.65 pounds, compared with 1.89 for the Series 7.) And, being a lower-powered device, the Series 5 has no vents.

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