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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
It won't hurt to try at all (as long as you don't put the battery in backwards :) ).
I've literally just got some Lithium batteries (1/2 AA cell) this week. Having tried it out, you can connect to the 'Bat' pin quite happily. At low power draws (~20mA or so?) the voltage drop across the regulator is very low (0.02v when I measured at 10mA). Since the Pico will operate down to 1.7v you'd be able to flatten a CR2450 without much trouble.
Powering via the 3.3v rail would also be fine and would avoid the regulator's voltage drop, but you then couldn't use USB or you'd end up putting 3.3v on the battery and charging it