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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
As @fdufnews says, the charger above can only produce max 3.3v, so it wouldn't be able to charge LiPi or Li-ion batteries as-is.
You could actually use the Pico's onboard FET to charge one battery, or could wire up external FETs/transistors to charge multiple. You'd also have to add a potential divider to check the voltage, since it would be too high for Espruino's 3.3v ADC.
Having said that, lithium batteries can burst into flames when overcharged, so you'd want to be very sure you knew what you were doing with the software to charge them. The charger above is nice and safe with NiMH/NiCd as you could do pretty much whatever you wanted with the outputs and you could never make the batteries explode :)
For LiPos, you could just use a charger IC like the MAX1555. It's safe, and comes on cheap pre-made charger boards (you'd need one per battery). You could still attach an Espruino that could maybe discharge the batteries if you wanted, and could also look at the 'charge in progress' output of the chip to detect how much charge had gone into the battery.