I had installed Ubuntu 14.04 on my AMD A8 laptop. Initially I got only 2 Hr battery backup. After some research, I come to know that AMD Catalyst driver can increase battery life and keep laptop cool. I downloaded latest beta driver from AMD site. Fortunately Ubuntu 14.04 have all prerequisite require for this driver.
I had unzipped the driver zip file and run amd-driver-installer-13.35.1005-x86.x86_64.run file available in fglrx-13.35.1005 sub-directory of directory catalyst. I followed all the steps mentioned on AMD site. Now I am getting around 4 hr battery backup on same laptop.