Hey 
I am curious to know how old the computer is. To go from 100 down to 30 in only a half hour is crazy. I think even Aravisian mentioned this, if your battery is old, that means its not holding a charge for as long, a common thing among batteries.
But the thing that I think is odd, is you say that the battery doesn't drain as fast when on Windows. That almost gets me thinking about processes or applications running in the background on Linux.
I like to use the Stacer app for viewing running apps and processes. When you launch Stacer, click on the processes tab. Yes its normal to see Stacer using a lot of CPU while its open, don't mind that.
Look for anything bellow Stacer in that list, that is using a lot of CPU, post a screenshot if you feel up to it. One thing I have found, is internet browsers like Chrome and Firefox use a surprising amount of CPU, this is because they run background processes.
The simple fact is, if your running a computer off a battery, well, the more CPU that is used by apps, the quicker your battery will drain.