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There are three types of computer batteries found on a computer. The first type of battery is the backup battery, which is commonly referred to as the CMOS battery. This battery is what holds your computer's computer settings, such as the time and date. This allows your computer to remember basic computer settings and allows you to boot the computer without entering those settings each time you start the computer.

Next, the bridge battery is only found in portable computers and is used as a temporary backup for the main battery. This allows you to remove the main battery and replace it with a good battery without having to turn off the computer.

Li-ion:-Short for Lithium-ion, Li-ion is a fragile technology requiring protector circuit, the Li-ion is used where very high energy density is needed and cost is secondary. Li-ion batters are generally more expensive but have never experienced memory issues that have affected other battery technologies.

Pioneering work for the lithium battery began in 1912 by G. N. Lewis but it was not until the early 1970's that the first non-rechargeable lithium batteries became commercially available. Attempts to develop rechargeable lithium batteries followed in the eighties, but failed due to safety concerns.

Lithium is the lightest of all metals, has the greatest electrochemical potential, and provides the largest energy content. Rechargeable batteries using lithium metal as an electrode are capable of providing both high voltage and excellent capacity, resulting in an extraordinary energy density.

After much research during the eighties, it was found that occasional shorts from lithium dendrites could cause thermal run-away. The cell temperature quickly approaches the melting temperature of lithium, which results in violent reactions. A large quantity of rechargeable lithium batteries sent to Japan had to be recalled in 1991 after a battery in a cellular phone exploded and inflicted burns to a man's face.

Because of the inherent instability of lithium metal, especially during charging, research shifted to a non-metallic lithium battery using lithium ions from chemicals such as Lithium-Cobalt Dioxide (LiCoO2 ). Although slightly lower in energy density than with lithium metal, the Li-ion is safe, provided certain precautions are met when charging and discharging. In 1991, Sony commercialized the Li-ion and is presently the largest supplier of this type of battery.

Nicad-Trademarked name for a Nickel-Cadmium battery owned by SAFT America Inc. Nicad is a type of rechargeable battery, made mostly of of nickel and cadmium. A primary problem with nickel-cadmium batteries was that they needed to be completely drained of power before they could be recharged.
Known as a memory loss or memory effect, if the battery was not completely drained, once the battery reaches its last low level, the battery would go dead, assuming that no power was left.

NiMH:-Short for Nickel-Metal Hydride and also abbreviated as Ni-MH, NiMH is a type of rechargeable battery used primarily in portable computers.

Research of the NiMH system started in the seventies as a means for hydrogen storage for a Nickel Hydrogen battery. The metal hydride alloys were unstable in the cell environment and the desired performance characteristics could not be achieved. As a result, the development of the NiMH slowed down. New hydride alloys were developed in the 1980's that were stable enough for use in a cell. Since the late eighties, the NiMH has steadily improved, mainly in terms of energy density. Design engineers have indicated that the NiMH has a potential of yet higher energy densities.