It takes some time after the change in state for it to actually reflect in the Unity Panel.
upower -m also has the same delay to update.
I have tried reinstalling upower and gnome-power-manager according to this answer using the following commands:
sudo apt-get purge upower gnome-power-manager sudo apt-get install upower gnome-power-manager
Does anyone know to solve this and make it responsive?
Laptop Model: Acer Aspire E 15 (E5-573-5108)