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Today, laptops with non-removable batteries are becoming more and more common due to a pressure to make laptops lighterweight and thinner.

Historically, computers have had a CMOS battery to operate the real-time clock and perhaps retain BIOS settings. I know that most laptops with removable battery still have the CMOS battery because they do not lose clock settings when temporarily taking out the removable battery.

However, modern non-removable battery laptops could in theory remove completely the additional CMOS battery. Is this done in these cases?

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    Many of your assumptions are incorrect... most modern laptops (NOT so much with desktops) do not have a removable CMOS battery at all, they have a fixed (soldered) RTC battery and "CMOS" (or more accurately EFI) settings are stored in non-volatile flash RAM. Could they use a single battery for both, sure, have we seen it, not to my knowledge.
    – acejavelin
    Commented Aug 17, 2017 at 19:05

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The chances are big that pc's and laptops (even those with non-removable batteries) will still have a CMOS battery.

The reason is for the technology of the removable battery. They're rechargeable batteries, and its in the nature of a rechargeable battery that it will slowly run out of battery, regardless of it being used. A CMOS battery however, does not.

It would be very weird, if your device is not being used for a month and its non-removable battery runs out, the BIOS would lose its settings.

One could argue that nowadays you have flash medium that you can write to that do not need to be powered in order for their memory to be retained, but the downside to those is that their state deteriorates with each read and write, and believe it or not, a BIOS is still being read and write a lot to.

Technology does change, and they may find new ways to more efficiently store power in rechargeable batteries that will retain their power for much longer or find a way to have a BIOS using flash memory in such way that it will not matter on wear that this changes in the future, but I don't see that happening anytime soon. The use of a CMOS battery is not something that costs a lot of money, so the gain to use different approaches would likely cost more.

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  • The other issue is that modern operating systems expect the RTC to keep working. That needs a power source to keep ticking, so there has to be something present to keep it powered even if the main battery dies.
    – Austin Hemmelgarn
    Commented Aug 17, 2017 at 19:07
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Although the CMOS battery is less of a factor in modern PC's I still don't think it will completely go away. The BIOS settings are now stored in non-volatile memory, instead of CMOS RAM. This negates the need for the CMOS battery to remember your settings for BIOS.

The CMOS battery does still function for keeping track of time when your computer is turned off. If you did not have the CMOS battery to keep track of RTC, you would have to reset the system time each time you reboot your computer.

From my experience, when the RTC is out of whack it seems that all hell breaks lose when you try to boot your PC. Maybe it's just my bad luck.

Some people may try to make a point that your computer will adjust the time when you reboot by synchronizing with an NTP (Network Time Protocol) server. While that sounds like a fool proof idea, you're not always guaranteed to have Internet access or some sort of NTP on your LAN.

On the non-removable battery note, the battery is always removable, it's just less accessible.

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