To elaborate: The remaining battery charge itself does NOT suddenly drop. The battery is not defective. It's just the OS that - because of the bug - suddenly reports a wrong value of the actual remaining charge which causes other functions - like for example the low battery warning and power save mode - to kick in. Rebooting will cure the wrong reading and after a reboot you'll see the reading being (almost completely, minus the power needed to restart the phone) back to what it was before. Also you can continue using the phone without rebooting until the phone shuts itself off.
I think the reason why the battery percentage will drop suddenly is that the lithium battery just works like that. At full charge, it has higher voltage and stress withstand but when the charge is getting low the battery cannot offer the same punch of power as at higher charge states. Therefore, the battery voltage will drop suddenly at a power demanding situation (the battery cannot hold the voltage because it has not enough charge left) and N9 will recognize it and will alarm. (The Li-On or Li-Po battery will work fine on voltages between approx. 3.2 to 4.2 V, but below that it has no charge left. The voltage must be higher than 3 V because otherwise the battery will die or at least be damaged. If it is stressed at low voltages the battery protection circuit will cut the power comsumption and the phone will be shut down.)
If you don't use any wirelesss connections or stress the CPU the battery will go down evenly.
It would be nice to know how the battery will work after the calibration: does it have the sudden percentage drop or not?
Also, if you just put the lock screen on after the sudden drop the battery percentage will rise a bit after a while.
I had crashes to 4% from 300 to 400 mAh charge level last week; it had been rising slow but steady during the year.
I did Makeclick's solution of letting it really run dry and then charging the phone when it was shut down. It was harder than one would assume as the phone has the tendency to try to boot up when it is connected to a charger instead of going to recharge mode. 'Shutdown -h now' as devel-su seemed to work best if it woke up unnecessarily.
I didn't pay attention to restarting part of the trick, but during three full recharge cycles I saw "the dip" move down 100 to 150 mA between every recharge, and after the third time the battery (or bme) seems to be calibrated spot on.
and save it in /home/user/.local/share/applications folder as bmeup.desktop file
and that's it - now You'll have bmeUp icon on Your homescreen. tap it and bme will be restarted and battery percentage set correctly.
This way the icon is a simple green one - You are welcome to create one and share it here as I'm not very into icon creating and designing process. Icon You would like to have as bmeUp icon should be uploaded to Your N9, and full path to it written in "Icon=" line in bme.desktop
