The Lithium Ion battery is the most common laptop battery and ships with the latest laptops. Batteries wear down due to usage and the natural aging process. The hotter the temperature the faster they lose their charge. So, a laptop battery in Florida will not last as long as one in Northern California.

Capacity also decreases with each charging/discharging cycle. You can expect to get 600 to 800 charge/discharge cycles over a three year period.

Leaving the battery inside the machine when plugged into the wall outlet means it constantly goes through the charge/discharge cycle. This tends to age the battery quicker than if it were removed from the laptop. The ideal would be to keep it out of the laptop at 60 degrees, when it will not be used for a long period of time.