Nonvolatile BIOS memory: Difference between revisions

Revision as of 06:42, 6 October 2021

Revision as of 19:45, 2 February 2022

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Today's [[UEFI]] motherboards use [[NVRAM]] to store configuration data (NVRAM is a part of the UEFI flash ROM), but by many [[OEM]]s' design, the UEFI settings are still lost if the CMOS battery fails.<ref>{{Cite web|title=UEFI NVRAM - OSDev Wiki|url=https://wiki.osdev.org/UEFI_NVRAM|access-date=2020-09-11|website=wiki.osdev.org}}</ref><ref>{{Cite book|chapter-url=https://media.springernature.com/original/springer-static/image/chp%3A10.1007%2F978-1-4842-6106-4_4/MediaObjects/488723_1_En_4_Fig6_HTML.png|doi = 10.1007/978-1-4842-6106-4_4|chapter = Firmware Resiliency: Detection|title = Building Secure Firmware|year = 2020|last1 = Yao|first1 = Jiewen|last2 = Zimmer|first2 = Vincent|pages = 115–162|isbn = 978-1-4842-6105-7}}</ref>

=={{anchor|Rechargeable CMOS battery}}CMOS battery==

{{multiple image|direction=horizontal|image1=Battery-lithium-cr2032.jpg|caption1=[[CR-2032]] button cell, the most common CMOS battery|image2=NiCd VARTA battery leakage on circuit board (IMG 3313).jpg|caption2=[[VARTA]]-manufactured [[Nickel–cadmium battery|Ni–Cd]] barrel battery that has leaked onto the mainboard of a [[synthesizer]] and damaged legs and traces|total_width=440}}

<!-- [[Rechargeable CMOS battery]] redirects to this section; if this section is renamed, please change the incoming redirect. Thanks! -->

[[Image:Battery-lithium-cr2032.jpg|right|thumb|150px|Type CR2032 [[button cell]], the most common CMOS battery.]]

The memory battery (aka motherboard, CMOS, [[real-time clock]] (RTC), clock battery)<ref name=whatiscmos /><ref>[http://en.kioskea.net/faq/625-replacing-the-battery-of-your-motherboard-cmos-battery Ask a question » Replacing the battery of your motherboard (CMOS battery)]</ref> is generally a [[CR2032]] [[lithium battery|lithium]] [[coin cell]]. This cell battery has an estimated life of 3 years when [[Power supply unit (computer)|power supply unit (PSU)]] is unplugged or when the PSU power switch is turned off.<ref>

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</ref> 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 [[AC power plugs and sockets|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 "[[List of battery sizes#1.2F2AA|1/2 AA]]" used in the [[Power Mac G4]] as well as some older [[IBM PC compatible]]s, or a 3-cell NiCd[[Nickel–cadmium battery|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.

==See also==