For some mice like the Logitech M185 there are no battery charge statistics, only a percentage (which I assume is extracted from the mouse via HID++ by upower). Ideally xfce4-power-manager would display that percentage.
Created attachment 5275 Display Power Percentage Here's patch that displays the percentage, which works for my Logitech M185 Unifying wireless mouse.
Created attachment 5276 UPower dump for the M185 mouse UPower dump for more context
This may overlap with bug #6652
Created attachment 5277 XFCE4 Power Manager with Percentage screenshot
Thanks for the patch, pushed to master in: http://git.xfce.org/xfce/xfce4-power-manager/commit/?id=788ca362f2ce261da75e10a0637bf2b057257771