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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
@Marty_McFly you're asking about the bare MDBT42 module, not the breakout board?
If so just be aware that @Robin's quote of "2.5 - 16v voltage input" isn't right. Putting much over 3.6v on a bare module will destroy it, however the module can function down to 2v or so.
But you can connect non rechargeable lithium cells directly since they're between 2 and 3v, and @Robin's suggestions on actual battery sound good.
In terms of the LEDs, if you're looking at seven segment displays, you can 'scan' them out, so you're only leaving the LEDs on for a fraction of the time. If you do that you should be able to get the peak power draw down significantly (in fact you'd have to, because you'll struggle to draw more than 50mA or so from a Lithium cell reliably).