Some Thinkpad users can use the tp-smapi kernel module to set charge thresholds. For example, I can set the battery to stop charging at 90% and only start charging again when it drops below 75%. However, when the battery is idle, below 99% and running on AC power, power-manager incorrectly declares the battery as "charging" when it is not. A simple check would be to verify /sys/class/power_supply/BAT1/status. It is "Unknown" when the battery is idle and "Charging" or "Discharging" otherwise. Alternatively, one could check /sys/class/power_supply/BAT1/current_now. If it is 0, the battery is idle.
I discovered that the power manager ALWAYS seems to declare the battery as charging when it is unsure. If /sys/class/power_supply/$BATTERY/status contains "Unknown" or is missing altogether, the power manager will always state it as charging. That should not happen, IMO.
This seems to be a bug within Upower, not Xfce. https://bugs.freedesktop.org/show_bug.cgi?id=70321