Why do PCs still require a CMOS battery, given that they're already plugged in?

We are providing lots of power to the PC by plugging it into AC mains electricity, so why does it still require a CMOS battery?

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    So, you are saying that a battery does not give you "electricity"? – Andreas Rejbrand Apr 30 '15 at 11:11
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    The way you phrased your question you are implying that pc's ran on something else before, like fuel. – Marcel Burkhard Apr 30 '15 at 18:23
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    @Marcel I believe Babbage's design would have been powered by steam. Granted it wasn't built, and wouldn't have been a PC, but it wouldn't have used electricity. – cpast Apr 30 '15 at 19:10
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    @cpast and I thought I was offtopic. – Marcel Burkhard Apr 30 '15 at 19:23
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    Why does a laptop need a battery, or your phone for that matter? Because they need to perform some operation when they don't have a more continuous power source. Same reason. – jpmc26 May 1 '15 at 6:45

The CMOS battery is not there to provide power to the computer when it is in operation, it's there to maintain a small amount of power to the CMOS when the computer is powered off and unplugged. The primary function of this is to keep the clock running even when the computer is turned off. Without the CMOS battery, every time you turned on the computer you would need to reset the clock.

On older systems the CMOS battery also provided the small amount of charge required to maintain the nonvolatile BIOS memory, which remembered BIOS settings between reboots. On modern systems this information is typically stored in flash memory and does not require a charge to be maintained.

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    I learned this during my first internship. We had a computer so old that it no longer kept the time between reboots. Live and learn. – Calculus Knight Apr 30 '15 at 12:48
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    Since when are BIOS settings stored in flash memory? I don't remember any modern motherboard where I couldn't reset its settings by pulling the battery out... – Ruslan Apr 30 '15 at 13:23
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    @user20574 AT PSUs really switched the power off. They had a real switch and sent the 230V through it. When the computer was off, it consumed zero electricity. ATX PSUs do no longer have these properties. – Alexander Apr 30 '15 at 13:32
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    @Alexander Not entirely true. Most of my ATX PSUs do have a real switch on the back. They no longer run 230V to a switch on the front of the computer. So the power button on the front of the computer doesn't turn to PSU off completely, you have to use the one on the back to do that. – kasperd Apr 30 '15 at 14:10
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    @kasperd Most retail ATX PSUs do, most OEM ATX PSUs don't. IIRC that switch isn't part of the ATX specs (just like the 110/220V switch some older ATX PSUs have). – Alexander Apr 30 '15 at 14:12

The CMOS battery, or RTC battery, provides power for the real-time clock so that your computer can continue to keep time when it is unplugged.

  • The name "CMOS battery" is a misnomer on modern computers. Older computers used the battery to maintain BIOS settings, which were stored in a CMOS SRAM chip that would lose its contents if disconnected from power. This setup does have the advantage of making it easy to clear the BIOS settings, either by removing the battery or by inserting or removing a jumper. Newer systems store firmware (BIOS or UEFI) settings in flash memory or EEPROM, which does not require power to maintain data. See: Wikipedia article on nonvolatile BIOS memory and How can a CMOS battery store data?

  • This battery was, and still is, used to maintain the real-time clock. As this is the sole purpose of the battery today, it is sometimes called the RTC battery. This ensures that your computer can continue to keep time even when unplugged. The battery is usually a readily-available CR2032 button cell, and is typically usable for 2 to 10 years. If your computer loses track of time when unplugged for more than a few hours, it is time to replace the battery. See: How long does CMOS battery (3V) on a motherboard last?

CMOS battery on Pico ITX motherboard
CMOS battery on Pico ITX motherboard – Image source

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    That's one huge battery. – Ayesh K May 4 '15 at 7:38
  • @AyeshK But very thin. – user11153 May 4 '15 at 8:36
  • How can BIOS settings be reset then if the battery only maintains the RTC? – Peter Mortensen May 4 '15 at 11:26
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    @AyeshK It's more like that's one tiny mainboard. – iFreilicht May 4 '15 at 12:27

Think of it like your car battery. When you unplug the battery, your radio loses all of its presets and the clock resets. Orginally, the CMOS battery held a similar function, maintaining the memory that held the BIOS settings and keeping the Real-Time clock running when AC power was unavailable.

However, with modern computers the CMOS battery plays a lesser role as most BIOS firmware is smart enough to automatically detect the correct settings and those settings are stored such that they don't need power to persist. The CMOS battery is still required to maintain the RTC.

More information is available in Nonvolatile BIOS memory.

  • I have a mid-00s laptop with a dead CMOS battery--unplug it and it's going to try to boot from physical drive #1. That's not the boot drive. – Loren Pechtel Apr 30 '15 at 21:22
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    Wouldn't it be nice if cars had CMOS batteries so that we wouldn't lose presets whenever the main battery had to be replaced? – jp2code May 1 '15 at 15:45
  • @LorenPechtel I wouldn't consider that a modern computer first of all. Secondly, laptops are, in many ways, a special breed that don't follow convention. Third, a laptop with more than one (permanent) drive is very unusual, i would consider that an acceptable default – smokes2345 May 1 '15 at 17:12
  • @smokes2345 The laptop is engineered for multiple drives, the BIOS should understand that. (And the repair manual really should give some indication of where that battery is hiding!!) – Loren Pechtel May 1 '15 at 19:46
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    [flippant]The world should be a better place, generally.[/flippant] – Agi Hammerthief May 2 '15 at 7:57

protected by Community May 3 '15 at 15:14

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