# Computer battery question **URL:** \ **Category:** Factual Questions\ **Created:** [October 19, 2005, 8:23pm UTC](https://boards.straightdope.com/t/computer-battery-question/326971 "2005-10-19T20:23:32Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Dewey\_Finn](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dewey_finn/32/4222_2.png) [@Dewey\_Finn](https://boards.straightdope.com/u/Dewey_Finn)\ **Post date:** [October 19, 2005, 8:23pm UTC](https://boards.straightdope.com/t/computer-battery-question/326971/1 "2005-10-19T20:23:32Z")
I’ve got a PC where the system clock seems to drift by up to ten minutes during the day. I’ve heard this can be because of a dying CMOS battery. Is that correct? If so, why is that? This PC is on basically all the time, running on AC power, so why does it need the CMOS battery to keep the correct time? Wouldn’t it need the CMOS battery only when it’s powered down? ---
**Author:** ![nivlac](https://avatars.discourse-cdn.com/v4/letter/n/3bc359/32.png) [@nivlac](https://boards.straightdope.com/u/nivlac)\ **Post date:** [October 19, 2005, 10:13pm UTC](https://boards.straightdope.com/t/computer-battery-question/326971/2 "2005-10-19T22:13:35Z")
> [@Dewey Finn](#): > > I’ve got a PC where the system clock seems to drift by up to ten minutes during the day. I’ve heard this can be because of a dying CMOS battery. Is that correct? If so, why is that? This PC is on basically all the time, running on AC power, so why does it need the CMOS battery to keep the correct time? Wouldn’t it need the CMOS battery only when it’s powered down? Like all batteries, the CMOS battery will run down after a while. If you’ve had your computer for over four years, changing out the CMOS battery is not a bad thing. I changed mine when I started having problems related to lost configuration settings. Changed the battery and all is fine. ---
**Author:** ![Dewey\_Finn](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dewey_finn/32/4222_2.png) [@Dewey\_Finn](https://boards.straightdope.com/u/Dewey_Finn)\ **Post date:** [October 20, 2005, 12:44am UTC](https://boards.straightdope.com/t/computer-battery-question/326971/3 "2005-10-20T00:44:00Z")
Yes, but if the PC is plugged in and on all the time, why does it need the CMOS battery to keep the correct time? ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [October 20, 2005, 1:48am UTC](https://boards.straightdope.com/t/computer-battery-question/326971/4 "2005-10-20T01:48:42Z")
The clock drifting while powered on can be a software problem. Certain combinations of OS and applications can cause the soft clock to occasionally miss a tick and it adds up over time. My downstairs server does this when I’m running certain heavy long term jobs. For MS-Windows OSes, there are MS Knowledge Base articles about this but I have never found any to be helpful. [Here’s](http://support.microsoft.com/default.aspx?scid=kb;EN-US;q189706) a typical sample. ---
**Author:** ![spingears](https://avatars.discourse-cdn.com/v4/letter/s/ebca7d/32.png) [@spingears](https://boards.straightdope.com/u/spingears)\ **Post date:** [October 20, 2005, 1:53am UTC](https://boards.straightdope.com/t/computer-battery-question/326971/5 "2005-10-20T01:53:14Z")
> [@Dewey Finn](#): > > I’ve got a PC where the system clock seems to drift by up to ten minutes during the day. I’ve heard this can be because of a dying CMOS battery. Is that correct? If so, why is that? This PC is on basically all the time, running on AC power, so why does it need the CMOS battery to keep the correct time? Wouldn’t it need the CMOS battery only when it’s powered down? WHAT? You have never had a power outage? You are most fortunate among the unfortunate ones! FYI the CMOS battery is to maintain the CMOS settings. Live it up. Remove the CMOS battery and come back after you experience a power outage! ---
**Author:** ![MyNameIsEarl](https://avatars.discourse-cdn.com/v4/letter/m/e36b37/32.png) [@MyNameIsEarl](https://boards.straightdope.com/u/MyNameIsEarl)\ **Post date:** [October 20, 2005, 2:39am UTC](https://boards.straightdope.com/t/computer-battery-question/326971/6 "2005-10-20T02:39:03Z")
> [@spingears](#): > > WHAT? > You have never had a power outage? > You are most fortunate among the unfortunate ones! You see, they have these newfangled things called “uninterruptible power supplies”. They look like an overgrown power strip but have a big lead-acid battery in them, so that when\if the power supply gets cut off, the computer… _keeps on working!_ The computer I’m typing this into right now has been on for over a year now (rebooted, sure; powered off, no)… so don’t be such a smart-ass… As to the OP, almost ALL CMOS batteries are of the CR-2032 variety. You can buy them at Office Depot\Staples for around $1.99 or so. Certain brand names put their own “initials” in the partnumbers (Duracell’s version is DUR-2032, for example). Swapping it out takes around 2 minutes. I’ve seen a dead battery make a PC clock drift, even the server in question had been powered on and tasking for 14 months.