Comment 78 for bug 629258

I believe gnome-power-manager hasn't been able to provide a time estimate since the rewrite for upower if your battery does not supply a charge/discharge rate in /sys/class/power_supply/BAT*/current_now. My battery always shows a 0 there, so I suffer from this problem.

Before upower, it used to make a projection based on measurements of the charge level over time. That would take some work to re-implement.

The behavior of saying 'estimating' instead of supplying a percentage is specific to the indicator applet code (the notification area icon doesn't behave that way), and it's pretty easy to fix.

In fact, I just fixed it in this PPA: https://launchpad.net/~brian-rogers/+archive/power