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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.
@allObjects
Since both Puck.js an MDBT42Q Breakout Board do not have any battery charging / overcharche protection nor converter device, you have to add that.
Todays solution for the battery is usually a very small 1s LiPo - similar as used in watches - see pictures in conversation about 'Espruino on your watch!'. Since power demands are very low, you get away with the smallest charge controller and converter you can find... or you may not even need a converter, a low-drop voltage regulator - or a diode with sufficient forward voltage drop - can do it.
Less modern solution is using 2 cells of rechargeable NiMH or NiCad batteries. The advantage there is the charging voltage is within the limits of what you want to drive and you need only something limiting the charging. You may even uses mc to control it.
The advantage of managing the charging of the battery with (solar panel) source independently from managing the supply of your mc from the battery gives the freedom to optimize for them power harvesting and storing and the constant generation of power for consumption.