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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.
parasquid
When speaking about Lithium ion batteries (and most others except lead acid probably) that is true. They are most comfortable between 20-80% of charge. A full charge can usually not even be reached by you because manufacturers of the devices makes sure that when the percentage reads 100% its not actually 100% internally (that could be a myth too, after all apple has a feature where they charge your iPhone up to 80% during the night and then to 100% shortly before your alarm rings in the morning to avoid having it at 100% charge for too long). I don't know of that is the case with the bangle, so it might be a good idea to unplug it at around 80%.
I don't know enough about the science behind it to explain it, but AFAIK the duration doesn't matter either. Fully charging or flattening it in both cases isn't ideal for LiIon batteries and should be avoided.