CMOS or Complementary Metal-Oxide Semiconductor refers to certain transistor circuits located on the motherboards of computers. CMOS memory is used for storing important system information such as date and time, drive parameters, and memory configuration. CMOS is also referred to as real-time clock (RTC) or non-volatile RAM (NVRAM), and is powered by a battery known as the CMOS battery. Laptop computers may contain an additional battery to power the CMOS known as a resume battery. There are several types of CMOS batteries.
Lithium or coin cell batteries are the most widely used batteries for powering the CMOS due to their long life. Lithium coin cells can last up to five years. These batteries are made of manganese dioxide and lithium and resemble watch batteries. Coded CR2032, coin cell batteries are 20 millimeters in diameter and 3.2 millimeters in height. Lithium CMOS batteries are recyclable, but not rechargeable. Coin cells are available in retail stores, as well as online retailers.
Some computers, including Apple's Macs, utilize 4.5-volt alkaline batteries to power the CMOS. These batteries are not rechargeable or recyclable. The major disadvantage to using alkaline batteries, aside from environmental unfriendliness, is that they tend to corrode faster than lithium batteries, and do not last as long. Alkaline CMOS batteries usually last two to three years.
Some CMOS are powered by 3.6-volt nickel cadmium (NiCd) batteries. NiCd batteries are rechargeable. However, if improperly disposed of, NiCd batteries can be very dangerous to the environment. These batteries will not last as long if recharged before the battery is fully drained. Battery chargers can also damage NiCd batteries by overcharging instead of shutting off after charging is finished. In theory, NiCd batteries have a life of three to five years, but this is greatly reduced by overcharging.
Nickel Metal Hydride
Nickel Metal Hydride (NiMh) batteries are rechargeable. NiMh batteries have a 40 percent higher charge rate and run longer on a charge than NiCd batteries. NiMh batteries do not have to be fully discharged before charging. These batteries can be stored for long periods, but need to be charged after six months in storage. NiMh batteries are more environmentally friendly due to their chemical construction. Whereas NiCd batteries use toxic cadmium for their negative electrodes, NiMh batteries use a hydrogen-absorbing alloy. NiMh batteries used to power the CMOS are 3.6 volts and typically last from one and a half to three years.