The battery icon on the panel only changes when I restart or log-out then log back in. For example, my battery is low at 15%, however, the icon indicates its at 79%, which was the percentage when I first booted into the OS.
This is a known bug, and is being tracked and worked on: https://bugs.launchpad.net/ubuntu/+source/indicator-power/+bug/1315434
Me too had the same issue, Power Indicator didn't update once its booted. Found answer in G+. And it works fine for me. Source here.
Downgrade upower to get it working again.
sudo apt-get install upower=0.9.23-2ubuntu1
After downgrade, Do this to prevent the package being automatically upgraded.
sudo apt-mark hold upower