Walmart, like most major retailers, is replacing the labels on its shelves with small electronic screens, so it can change prices and offer promotions quickly and cheaply.
But the switch from paper to screens has created another problem, one that affects all connected devices: they need power.
The number of smart devices, such as speakers and lights in homes, or sensors and screens in factories, shops and warehouses, is projected to surpass 42bn by 2025, each requiring its own battery or source of power.
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