Dell laptop battery problem

Although I have a laptop, I tend to use the mains rather than rely on the battery. I noticed that the battery life was getting shorter and shorter. I can only surmise that either the battery was faulty or that the battery was getting charged constantly. Once the battery is fully charged by the mains, shouldn't the mains stop charging the battery and let the mains power the computer? This appears not to be the case. I suggest that all laptops should be designed to solve this problem.