Simple as it may sound, batteries are the biggest differentiator between laptops and desktops. Unplug a desktop from the wall, and it shuts down; do the same with a notebook, and it might last a day (or so HP and Dell claim with their 24- and 19-hour batteries, respectively). Or, more realistically, anywhere from 90 minutes to 9 hours.

Regardless of capacity, all new notebook batteries are made up of lithium-ion cells and are rechargeable via A/C adapters. While some manufacturers make it more difficult than others to remove the laptop battery, most make it simple to pop them in and out. This opens up the possibility for mobile professionals to tote a spare battery (sold separately). However, you are better off getting one battery that lasts a long time than lugging a spare; extra batteries are bulky, and you need to shut down your computer before inserting them.

In choosing or configuring a notebook, you may be presented with different laptop batteries sizes and capacities. We can help you decide which one is right for you.

How Much Battery Life Do You Need?

If you plan to use your notebook away from your desk, you will want a bare minimum of three hours of endurance (with four to five hours being preferable), because even when carrying a notebook around the home or office, being chained to an outlet is extremely inconvenient. If you use your notebook in public places (coffee shops, outdoors, planes, etc), you want a minimum of four and a half hours of endurance (with six or more hours preferred).

Choosing the Right Number of Cells

While there are batteries with two, four, or eight cells, most come in three-, six-, or nine-cell packs. If long battery life is important to you, always choose the the largest number of cells available. Whatever you do, don't settle for a three-cell laptop battery; notebooks and netbooks with three-cell batteries tend to get less than three hours of battery life. Batteries larger than six cells—often referred to as "extended batteries"—may add some extra bulk to your notebook, sticking out the back and adding a few tenths of a pound to the weight. However, if you need long endurance, this extra weight is worth it.

Measuring Battery Capacity

While battery capacity is the most important factor affecting battery life, you can't tell just from the battery specs how long a notebook will last. Some notebooks are more power efficient than others and, even with the same notebook, users themselves can have a big impact on battery life based on the power settings they use and the tasks they perform. For example, a quad-core gaming notebook may get less than two hours of endurance out of high-capacity battery, while a netbook with a lower-capacity battery can last more than nine hours.

At LAPTOP, we test notebook battery life by running a custom script that surfs the Web continuously via Wi-Fi. See our "How We Test" section for more details.

However, it's clear that the more wattage a battery provides (usually measured in either milliamps or killowatt hours), the better. Most vendors do not advertise the amount of wattage a battery provides, but rather the number of cells inside the battery. And more cells almost always equals more juice.

Long-Life/Prismatic Batteries

Over time, a typical notebook battery tends to lose its ability to retain a charge. After a year of use, the typical battery has lost 15 percent of capacity, and it quickly declines from there. A handful of new batteries, such as HP's Enviro batteries (made by battery manufacturer Boston Power), use oval-shaped prismatic cells to maintain their ability to hold a charge over time. The Enviro batteries, for example, are guaranteed to last at least three years. While this new kind of battery is a rare option, it is definitely preferable when available.

Secondary Batteries

Many business notebooks also provide the option to purchase a secondary battery, which kicks in when the main battery runs out of power. Secondary batteries typically slide into the optical drive bay (replacing the optical drive), or attach to the bottom as a battery slice. These secondary batteries add to the cost and sometimes the weight, and slices in particular can make a notebook less comfortable on your lap. However, they can add anywhere from two to six hours of endurance.