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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@Gordon
Yes, absolutely - and there's a standard BLE characteristic for reporting it too.
CR2032s have quite a good discharge curve (pretty steady for 90% of capacity, then drops off suddenly for final 10%), so it's pretty easy to tell from the voltage when they need replacing.