There are currently three types of batteries commonly used for laptops: Nickel Cadmium, Nickel Metal Hydride, and Lithium Ion.
Nickel Cadmium (Ni-Cd) batteries were the standard technology for years, but today they are out of date and new laptops don't use them anymore. They are heavy and very prone to the "memory effect". When recharging a NiCd battery that has not been fully discharged, it "remembers" the old charge and continues there the next time you use it. The memory effect is caused by crystallization of the battery's substances and can permanently reduce your battery's lifetime, even make it useless. To avoid it, you should completely discharge the battery and then fully recharge it again at least once every few weeks. As this battery contains cadmium, a toxic material, it should always be recycled or disposed of properly.
NiCad batteries, and to a some degree NiMH batteries, suffer from what's called the memory effect. Memory Effect means that if a battery is repeatedly only partially discharged before recharging, the battery will forget that it can further discharge. The best way to prevent this situation is to fully charge and discharge your battery on a regular basis.
Nickel Metal Hydride (Ni-MH) batteries are the cadmium-free replacement for NiCad. They are less affected by the memory effect than NiCd and thus require less maintenance and conditioning. However, they have problems at very high or low room temperatures. And even though they use less hazardous materials (i.e., they do not contain heavy metals), they cannot be fully recycled yet. Another main difference between NiCad and NiMH is that NiMH battery offers higher energy density than NiCads. In other words, the capacity of a NiMH is approximately twice the capacity of its NiCad counterpart. What this means for you is increased run-time from the battery with no additional bulk or weight.
Lithium Ion (Li-ion) are the new standard for portable power. Li-ion batteries produce the same energy as NiMH but weighs approximately 20%-35% less. They do not suffer significantly from the memory effect unlike their NiMH and Ni-Cd counterparts. Their substances are non-hazardous to the 0. Because lithium ignites very easily, they require special handling. Unfortunately, few consumer recycling programs have been established for Li-ion batteries at this point in time.
Smart batteries are not really a different type of battery, but they do deserve special mention. Smart batteries have internal circuit boards with chips which allow them to communicate with the laptop and monitor battery performance, output voltage and temperature. Smart batteries will generally run 15% longer due to their increased efficiency and also give the computer much more accurate "fuel gauge" capabilities to determine how much battery run time is left before the next recharge is required.
Even if the battery case looks the same, you cannot just upgrade to another battery technology unless your laptop has been pre-configured from the manufacturer to accept more than one type of battery type, since the recharging process is different for each of the three types of batteries.
A battery that is not used for a long time will slowly discharge itself. Even with the best of care, a battery needs to be replaced after 500 to 1000 recharges. But still it is not recommended to run a laptop without the battery while on ac power -- the battery often serves as a big capacitor to protect against voltage peaks from your ac outlet.
As the manufacturers change the shapes of their batteries every few months, you might have problems to find a new battery for your laptop in a few years from now. This is somewhat of a concern only if you anticipate using the same laptop several years from now. If in doubt, buy a spare battery now - before it's out of stock.
New batteries come in a discharged condition and must be fully charged before use. It is recommended that you fully charge and discharge the new battery two to four times to allow it to reach its maximum rated capacity. It is generally recommend that you perform an overnight charge (approximately twelve hours) for this. Note: It is normal for a battery to become warm to the touch during charging and discharging. When charging the battery for the first time, the device may indicate that charging is complete after just 10 or 15 minutes. This is a normal with rechargeable batteries. New batteries are hard for the device to charge; they have never been fully charged and are not broken in. Sometimes the device's charger will stop charging a new battery before it is fully charged. If this happens, remove the battery from the device and then reinsert it. The charge cycle should begin again. This may happen several times during the first battery charge. Don't worry; it's perfectly normal. Keep the battery healthy by fully charging and then fully discharging it at least once every two to three weeks. Exceptions to the rule are Li-Ion batteries which do not suffer from the memory effect.
Batteries should be stored in a discharged state since they can self-discharge and may become inactive after a long storage period. They should not be stored for any length of time while connected to the laptop. High humidity and temperatures can cause the battery to deteriorate, so these should be avoided during storage.
Do not remove and carry a battery pack in your pocket, purse, or other container where metal objects (such as car keys or paper clips) could short-circuit the battery terminals. The resulting excessive current flow can cause extremely high temperatures and may result in damage to the battery pack or cause fire or burns.