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@Gordon
Hi,
Sorry about this - the battery power is measured using a 'potential divider' which is two resistors, but most resistors are only ~10% accurate. The battery voltage changes by about 10% between 90% charged and 10% charged, so while I try and set values that work with most watches, on some it's just not good enough.
There's a thread on this at http://forum.espruino.com/conversations/371254 and you can upload something that will fix it for your watch pretty easily if it bugs you.
I'd been hoping that I wouldn't have to do this, but it may be that I'll need to store a per-watch calibration value for the battery in the settings file... It's definitely something that can be fixed in a firmware update