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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
About lifetimes: Google 'Nimh discharge curve' and look at the graphs.
The voltage does tend to change as they discharge and charge, but only slightly (until they run out, then it drops off very quickly)... You can still use it to get a pretty good idea how much charge is left though.
As far as I know different makes of batteries are all subtly different, so if you can't find a datasheet you can always experiment - running the battery down slowly and using Espruino to measure what happens to its voltage over time.