Every laptop battery carries a certain "reservoir," or capacity of charge per cycle. Much like your car's gas tank, you fill it up every so often and then use the gas bit by bit. In the case of a laptop battery, the tank usually holds 2,500 to 4000 milliAmp hours. (For an explanation of Amp hours, see our article on Volts and Amps.)
Just like your car, your laptop computer battery lasts longer depending on how you "drive" your laptop. Many laptop users are familiar with the Power Management settings icon in the Control Panel, which allows the user to control use of the laptop battery. This control forces your laptop to conserve its fuel by limiting the operation of the highest energy-consuming functions. Unfortunately, there is a trade-off between computer performance and energy conservation.
The key "consumers" of the laptop battery are the processor, the screen, wireless networking cards, drives such as the hard drive, CD-ROM and DVD-ROM, and other peripherals. The more these functions are used, the faster your laptop battery will run out of juice. (Note that you "use" your screen more when its brightness is set to high; to conserve laptop battery power, dim the screen.) DVD movies are a major culprit when it comes to laptop computer battery drain because both the drive and the screen must be run at full power for as long as two hours.
With the "pedal to the metal," your laptop computer battery uses as much as 2500 milliAmps per hour, meaning that a 3000 mAh laptop battery would last for just over an hour. You can reduce the laptop batteries use by up to 75% with intelligent use of your laptop.
This connection between laptop computer battery life and laptop usage patterns explains why we cannot answer the often-asked question: How long will my laptop run with your new laptop battery? Unfortunately, all we can say is: "it depends". Just as with a car, your mileage will vary.