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Modern electronic systems solve so many difficult problems that they often seem like magic. Nonetheless, these systems all have thesame basic limitation: they need a source of electrical power! Most of the time this is a straightforward challenge for the electronicdesigner, because there are many power-delivery solutions. Yet sometimes a device has no direct power source, and running wiresor replacing batteries is impractical. Even when long-life batteries are usable, they eventually need to be replaced, which requires aservice call.
