The memory battery (aka motherboard, CMOS,
real-time clock (RTC), clock battery) is generally a
CR2032 lithium coin cell. This cell battery has an estimated life of three years when
power supply unit (PSU) is unplugged or when the PSU power switch is turned off. This battery type, unlike the
lithium-ion battery, is not rechargeable and trying to do so may result in an explosion. Motherboards have circuitry preventing batteries from being charged and discharged when a motherboard is powered on. Other common battery cell types can last significantly longer or shorter periods, such as the smaller
CR2016 which will generally last about 40% less time than CR2032. Higher temperatures and longer power-off time will shorten battery cell life. When replacing the battery cell, the system time and CMOS
BIOS settings may revert to default values. Unwanted BIOS reset may be avoided by replacing the battery cell with the PSU power switch turned on and plugged into
an electric wall socket. On
ATX motherboards, the PSU will supply 5V standby power to the motherboard to keep CMOS memory energized while the system is off. Some computer designs have used non-button cell batteries, such as the cylindrical "
1/2 AA" used in the
Power Mac G4 as well as some older
IBM PC compatibles, or a 3-cell
nickel–cadmium (Ni–Cd) CMOS battery that looks like a "
barrel" (common in
Amiga and older IBM PC compatibles), which serves the same purpose. These motherboards often have a four pin straight header, with pin 2 missing, for connecting to an external 3.6v battery, such as the Tadiran TL-5242/W, when their soldered-on batteries run out. Ni–Cd batteries have a tendency to
leak devastatingly after a period of disuse, damaging components and traces on the circuit board near the battery. ==See also==