You know you cannot fix a round plug into a square hole; similarly, you cannot fit any laptop battery into any computer. Laptop batteries are made for a specific purpose and for specific computer models only. Now, if you're looking to buy a replacement laptop battery, there are some things you might want to know and consider, before actually purchasing it.

Laptop battery manufacturers are in the plenty. There is no dearth of them. To make options ever wider, retailers are plentiful too! With such huge number of options, it is very easy to get confused, and even be led, to make the wrong decision. To avoid this problem, you need to know something about your laptop - The make of your laptop and its model. You can also look inside the laptop battery's compartment for the model number along with other useful information. In addition to this information, you might also find the laptop battery's part number that could be located on the battery casing itself.
Things to look for
What would you like your battery to do? Fail you the first time you use it? I guess not. So, make a note of this parameter that is important, as well as others, like the technology that is used in the battery - it should be compatible with the technology used in your computer. Buying a replacement laptop battery is not just a joke - it takes lots of time and effort. Finally, choose the battery that has the longest life. That will be the best replacement laptop battery to choose.
NiCad vs. NiMH vs. Li-Ion
Which one is better? Well, to answer this question head-on, NiCad batteries (nickel cadmium) are the older version of batteries. They were very common at a point in time. They do not have the capacity to hold much power, going by the requirements of today's computer user. Moreover, they can be charged only a couple of times. They are easily ruined if left on the charger after reaching full charge capacity. Another problem that most users faced was the declining charge capacity - the battery would not recharge to its full capacity, if not discharged completely on every use.
NiMH (nickel metal hydride) was the next in series, after NiCad. This series is a little better than NiCad, but features the same problems with charging; however, to a lower extent.
Li-Ion (lithium ion) is the best technology to be brought into existence. These batteries hold the same amount of power as NiMH, but are far lighter and more superior. These batteries, unlike the nickel counterparts, do not have the charging problem and hence, last longer.
Now, if your laptop supports all of these battery types, your choice for a replacement laptop battery will be the Lithium Ion battery.
Well, what next? You have more work to do now. Taking care of your original battery as well as your replacement laptop battery is of utmost importance. This will ensure a longer and more enjoyable life of your battery.