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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
You could use one of the old Nokia phone batteries... I've got a BL-5B here and it fits quite nicely within the footprint of the Espruino.
Also you're spot on with the Nokia display mentioned above - power draw from that is tiny. If you update it every second, Espruino will still end up drawing around 12mA (it's unlikely to be able to enter deep sleep at the moment because it can only wake itself up once a second from the RTC and it won't think it can enter sleep mode and wake back up in time).
So, updating every second with the Nokia battery you'd still comfortably be looking at over a day of usage.
If you had it updating every two seconds it should enter deep sleep and you'll be looking at well over a week I think.
Only other thing to say is that while Espruino uses its own RTC, it uses a built-in RC oscillator which isn't very accurate. There are some pads near the chip where you can solder on a wired 32kHz watch crystal though - and I've got some JavaScript code which will enable this crystal - but sadly I haven't managed to get it to switch the RTC over to use it yet! If someone could have a play and get that working it'd be awesome though :)