When choosing a laptop battery, you should first make sure that it is compatible with your computer, and then consider the materials it's made out of. It's also best to look at the power rating and service life, since these have a large impact on its performance and lifespan. Additionally, you may want to consider whether you want a brand name or generic replacement, as well as whether you want a new battery or a refurbished one. Whichever one you choose, you should also look at any warranties offered, and make sure that it complies to safety standards.
The first thing to look for when buying a new laptop battery is compatibility with your computer. Most are made to work with specific models of computers, and have this information listed in their product descriptions. To make sure that one will work with your computer, you'll need to know the make and model number of your laptop; if you don't know this offhand or don't have the necessary paperwork, check the inside of your laptop's battery compartment while it is off and unplugged. You may also find the part number of the battery, located on its casing.
You should also consider the material that the battery is made out of: Nickel Cadmium (NiCad), Nickel Metal Hydride (NiMH), Lithium Ion (LiON), or Lithium Ion Polymers (Li-Poly). NiCad batteries are largely obsolete, since they are very heavy and don't last very long. NiMH batteries last longer than NiCads, but significantly shorter than LiONs and Li-Polys. They also weigh more, and have the "memory effect," which causes them to lose the ability to fully recharge.
Generally speaking, LiON and Li-Poly batteries are the best choice, as long as they're compatible with your laptop, since they are light, last longer than NiMHs, and are about a third lighter. They also charge more quickly, and are also more environmentally friendly. Regardless of which type of laptop battery you end up getting, you should make sure that the cells are manufactured in such a way that they meet international standards of laptop battery safety, like those designated by Underwriters Laboratories (UL).
The capacity, or power rating is another important consideration. This is the amount of power that the battery can produce, and is usually written in milliamperes (mAh). The greater the capacity, the longer the battery can run before it needs to be recharged, so look for the highest mAh number. Most new ones are rated to at least 5,000 mAh.
Like other technological products, laptop batteries have a designated service life — the length of time that they're expected to work optimally. This is usually written in terms of cycles, with one cycle consisting of completely running the battery out, and then completely charging it again. Most are are able to go through between 300 and 1,500 charging cycles, and can be used after the maximum cycle count is reached, but may not hold a charge as well. Some computers come with a cycle counter built in, but there is free cycle counting software is also available online for download.
Branded vs. Unbranded
You may additionally want to consider whether you want to buy a brand name laptop battery, like the one your laptop came with, or a generic replacement one. Many brand companies and companies that make generic batteries get their components from the same manufacturers, so it may not make a huge difference, but some brand name versions meet more rigorous safety standards than generic ones, and some do use different, higher quality components. Also, some brand name versions come with better warranties than generic ones. Branded batteries do tend to be more expensive than generic ones though, so if you compare a brand name laptop battery and a generic one and find them to be largely similar, then you may want to go with the generic one.
New vs. Refurbished
Besides the issue of branding, you should also think about whether you want a new laptop battery or a refurbished one. Though some refurbished ones are very good and are often much cheaper than new ones, it's a little risky to buy refurbished, since you don't really know how old the battery is, and it may die suddenly. Also, many refurbished batteries don't come with very good warranties, so it may be worth it to just get a new one, especially if you use your laptop a lot.