Nickel Cadmium (NiCad) Batteries
The oldest of the three technologies, nickel cadmium is rarely used today. It has a shorter life and is sensitive to improper charging and discharging. After being charged, NiCad batteries hold the charge very well. However, their life can be severely shortened if they are not fully discharged before recharging, or if they are overcharged.
Nickel Metal Hydride (NiMH) Batteries
NiMH batteries have a longer life than NiCad (about 50 percent longer), and are less sensitive to improper charging and discharging. They are more expensive than NiCad and don't hold a charge as well when not used. They usually cannot be recharged as many times. They are, however, used in most portable systems, especially those at the lower end of the market.
Lithium Ion (Li-Ion) Batteries
Li-Ion batteries cannot be overcharged, hold a charge well when not in use, and are longer lived than the other two types of batteries. They are also proficient at handling the heavy-duty power requirements of today's higher-end portables. Unfortunately, Li-Ion batteries can be used only in systems specifically designed for them.
Never install a Li-Ion battery in a system designed for a NiCad or NiMH battery. Doing so could result in a fire.
Because they are the most expensive of the three battery technologies, Li-Ion batteries are usually found only in high-end systems.
Sometimes, buying a system with a Li-Ion battery does not mean you will realize a longer battery life. Some manufacturers take the opportunity to make the battery smaller because it is more powerful, thereby saving some space inside the computer while delivering the same performance as a NiCad or NiMH.
Battery technology has trailed behind nearly all the other advancements of the portable system. A battery life of two hours is considered very good even when a system's power-saving features are utilized. Some manufacturers are designing systems that hold two batteries to try to overcome this limitation.
A fourth type of battery technology-the Lithium Polymer-has been in development for several years, but it has not yet appeared on the market. Lithium Polymer batteries can be formed into thin, flat sheets and installed behind the LCD panel. They provide approximately 40 percent more battery life while adding far less weight to the system.
All battery types function best if they are completely discharged before recharging. Even Li-Ion batteries perform better and last longer if they are discharged before being recharged. Another tip is to store charged batteries in the refrigerator. This helps them maintain their charges longer.