OT backup battery question

[micky]

And besides the window, there is the door to the bathroom. I doubt if that is closed much, so the hydrogen dissapates through the whole house, exits through open windows and doors.

[micky]

IIRC, batteries (of all chemistries?) keep 90% of their voltage until they are 90% discharged. I wonder if it is the same on the way back up, that they have 90% of the rated voltage when they are only 10% charged.