Johnson cut open a 12-volt dry-cell battery and crammed in two more cells in an attempt to get another 4 volts.

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Unlike competing noise-cancelling headphones, the Harman Kardon headphones continue to play when the battery runs dry.

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Heaven help the slalom racer if his goggle battery runs dry.

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But building a giant battery using dry-cell technology would require thousands of individual cells - about the size of a soft drinks can - to be strung together in a massive installation.

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This gives them greater flexibility around which devices they use, whether that's using their tablet after their handset battery runs dry, keeping them connected on their PC when they've left their mobile at home or staying in touch when they are in an area with Wi-Fi but no mobile signal.

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