Let's look at some facts here...
#1. Laptop batteries are not cheap to replace.
#2. Over a short period of time (1-2 years) the charge a laptop battery holds diminishes.
#3. Many of us use our laptop while keeping it plugged in.

Well guess what? That last one, #3... that right there... it adds to #2... which causes us to revert to #1
But here's a little trick. If you are keeping your laptop plugged into a power source while using it, chances are it can run without having the battery attached. If you remove the battery, you will lessen the time it's "in use" which will lengthen the time before it starts to stop holding a decent charge.
Just don't forget to be prepared and make sure that battery is charged for when you'd like to hit the road with your lappy!