Rechargeable Batteries

We've all heard a number of different tips on getting the best life out of your rechargeable batteries, regardless of if they are in your cell phone, laptop or any other device. What does it really mean when they say that charging wears away the battery? Why do I want to let the battery drain completely before recharging it? These two concepts in particular may seem counterintuitive, but when you consider the following analogy the confusion clears up. Think of a rechargeable battery as a knife. Using that knife slowly wears away at the blade and dulls it to the point where it's not useful. At this point you want to sharpen it, but doing so actually wears away a bit of the blade. You're left with a sharp, useful blade that is slightly smaller than before. Charging your battery is sharpening a knife; it eats away at the battery ever so little, but it makes it useful again.
When you are using your laptop, you are draining the battery, or dulling the knife. If it's always plugged in, all that you are using is the top 10% of the battery; that's like only using the tip of the knife for everything, and only resharpening the tip. The knife will wear away unevenly, and become shorter in the process. This is why draining the battery is recommended. Fulling draining the battery before recharging wears away at the battery evenly, and maintains better performance overall.