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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.
@Robin
Sun 2019.12.22
What will the load be? LED's, temp sensor, GPS? Will impact the best choice.
Google is a fantastic resource to discover answers like these. . . .
Range of watch batteries using Google: 'rechargeable watch battery' see image at page bottom
A typical standard for power consumption and delivery is the CR2032
A fully charged Li-Ion CR2032 may damage a MDBT42Q module however
Datasheet for the smallest CR1220
For the breakout board where voltage is a bit more forgiving, consider a micro copter drone battery. Will need a separate charging board for constant current charging.
Abount the same size as the breakout board 852540 Lipo
It appears the drone battery route has already been discovered a while back