An important factor to consider is how much battery life you'll need on your laptop. No matter how much battery you have, using your built in DVD drive a lot and having a 15" TFT screen will drain it faster. Battery technology--just like any technology--is improving all the time. Yesterday's laptops featured Ni-Cad (Nickel-Cadmium) batteries that hurt the environment, didn't offer much up-time (comparatively), and had the annoying habit of remembering how long you have used them and refusing to work any longer the next time. Then there were NiMH (Nickel-Metal-Hydride) batteries. They offered longer life, better efficiency, and no memory problems. The current generation of laptops uses Lithium Ion batteries. These batteries are a gain in efficiency over NiMH batteries. Future batteries may be Lithium Metal batteries. They offer an extremely low discharge rate (long shelf life when charged) and efficiency improvements over Lithium Ion batteries, but movement towards new battery technology has slowed recently.

Some laptops claim up to five hours worth of up-time on one battery--take this figure with a grain of salt. That's probably how long the battery would last if you put on the blank-screen screen saver and didn't touch it for five hours. Expect more in the range of two or three hours per battery under continual use. If you plan to be on flights, trains, or buses for ten hours at a time, buy some extra batteries and use this feature, make sure you have a power plug handy, or consider buying a PDA (Personal Digital Assistant) instead. Most PDAs typically tout a 9 hour or longer battery life.

At one point, Micron offered a battery plate that fit on the bottom of their laptops, adding size and a few pounds, but got you about 10 hours of battery life. If that's what you need, check around for interesting battery setups. Several manufacturers also offer laptops with expansion slots that will accept a variety of peripherals, including a second battery. The second battery can double your battery life, and make sure you get a battery charger to go with it so that you can charge both batteries at the same time (one in the laptop, one in the charger).

If you need it, get 3 or 4 extra batteries and keep them fully charged for those long flights. You may face additional scrutiny at the security check-points, though, and keep in mind that the batteries can cost up to and over US$200 apiece.
If you're having problems with a laptop crashing and shutting down randomly after using it for several months, chances are that the battery is what is causing the problem- even if it's plugged in. Laptops expect to be able to charge the battery, and if the battery is charging in a faulty manner, your laptop may decide to start shutting itself off. Hopefully, most newer designs take care of this type of problem, but we've seen it many times with older models.

Recommendations
Make sure you get a Lithium Ion battery. NiMH or, even worse, Ni-Cad are old technologies that are best forgotten.

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