I upgraded from lion to mountain lion on my Macbook air 2012. This is how I fixed my mountain lion battery issues on a 2012 macbook air. As soon as I installed mountain lion I instantly noticed that the battery life was terrible. Instead of 8-9 hours I was getting 2-3 hours. Under the same load.
I looked round the apple forums, and didn’t find anything.
I found a post somewhere [sorry, can’t remember where] that suggested I delete the desktop preferences plist file and restart the Dock.