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Napoleon's dictum no longer applies: These days, an army marches not so much on its stomach as on its batteries. Without them, soldiers can't see in the dark, work their radios, or determine their positions. But even the best storage batteries-accounting for one-fifth of the load in a typical infantryman's 45-kilogram pack- can't last the week or so that field soldiers require. The same problem is coming to afflict the rest of us, as we become ever more dependent on our smartphones and GPS navigation.