Gateway laptop battery problems...?

Purchased computer Dec. 08, have had to replace battery twice, just installed latest one, but read that computer should work without a battery, so I'm wondering if something else is wrong that is causing my batteries to die? The first was replaced by Gateway, this time it was out of warranty, so $140.00 later, I don't want to have this one die. I seldom use the computer using just the battery, normally it's plugged in, could this be the problem? Thank you for any information you can give me.