Droplet is a compact self-contained module with an integrated RGB LED, a photodiode and a CR1216 lithium coin battery -- which provides roughly one day of power in the gadget's current early prototype status.

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Bluetooth low energy runs off a small coin cell battery, lasts about a year, and allows a small tag to send data to a phone.

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Viiiiva runs on a coin-cell battery for over a year.

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RFLKT's sleek profile and lowly 2 ounce weight, make it virtually unnoticeable when mounted, while the coin cell operated battery has a life of over one year and requires no charging.

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Clutch a coin in a sweaty hand and the metals connect through the salty solution to make a mini-battery.

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