CMOS batteries (e.g. CR2032) in PCs

mikeymikec

Lifer
May 19, 2011
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This is my understanding:

CMOS information including date and time is stored with the aid of the CMOS battery. My understanding is that the CMOS battery gets used as the power source for storing CMOS information when there is no other power source (e.g. a laptop's normal battery, its mains supply, or a desktop PC's live mains connection). In my experience, CMOS batteries can last a decade or so in a desktop PC if the PC has a live mains connection, whereas if the owner is in the habit of shutting down the PC then switching off its mains connection, the battery tends to only last about 5-6 years.

Is this correct? I'm encountering a scenario with a customer's PC that challenges my belief, in that it developed the typical issue of losing time/date and giving CMOS errors on boot-up (and sure enough the customer had been disconnecting the mains after shut down), so I replaced the battery, but now the PC is apparently not keeping time after resuming from sleep mode or being shut down (despite the live mains connection).
 

Paperdoc

Platinum Member
Aug 17, 2006
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I believe your understanding is correct, with one small correction. The battery power is required to MAINTAIN the data in the BIOS PROM, not to store it there. Storage and retrieval of that BIOS data still requires an operating mobo with power.Battery power also is used (when other power is not available) to operate the clock and keep it correct when other power is not available.

What you describe is certainly odd. IF the "new" battery is a good one with proper output, BOTH the clock and the BIOS Setup options should be maintained when the system is DISconnected from the wall outlet. NOTE that IF the system is still plugged into a live outlet, even though the system is "Off" (or "sleeping"), there IS power from the PSU to that BIOS chip to maintain its functions, and the battery is not used. I am not sure, but if the system IS plugged into the wall BUT the user has switched OFF the main power switch in the back if the PSU, it may NOT be supplying power to the BIOS chip.

In addition to a problem with the clock, a failed battery also should cause loss of any custom settings in BIOS Setup. Such loss would cause BOTH the clock time AND those BIOS settings to be replaced with factory default values on the next boot. Note that failed battery power shuld result not in a clock being merely slow, but a complete reset to a very early default date / time. So, in this machine are there any non-default settings in BIOS Setup that can be checked to see if they are being lost when power is off? If not, I suggest you deliberately change a couple Setup options so you can test whether they are lost when power is off. If they are, (as well as losing proper time) then the new battery is not doing its job. Either it is too weak, or has a bad connection, or is installed backwards, OR the jumper on the mobo RESET pins is in the wrong position.
 

mikeymikec

Lifer
May 19, 2011
15,669
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Thanks for the response. I'm curious to see whether anyone has anything else to add. My first thought is that the replacement CMOS battery is defective in some way, but I would have thought that the live mains connection ought to offset that. I'm also curious to know what would happen if a system had no CMOS battery, just a live mains connection the entire time, whether it would maintain CMOS settings/time while shut down; though I imagine that there would be some BIOS code that does some kind of integrity check on the battery regardless of whether the CMOS checksum checks out.

Good idea with the custom setting though. As a side note, I'm kicking myself for using one of the customer's stash of batteries though they assure me it was new in all senses of the word. I tend to keep a stash with me too but I've never felt the need to wonder how old my stash is.
 

Steltek

Platinum Member
Mar 29, 2001
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Thanks for the response. I'm curious to see whether anyone has anything else to add. My first thought is that the replacement CMOS battery is defective in some way, but I would have thought that the live mains connection ought to offset that. I'm also curious to know what would happen if a system had no CMOS battery, just a live mains connection the entire time, whether it would maintain CMOS settings/time while shut down; though I imagine that there would be some BIOS code that does some kind of integrity check on the battery regardless of whether the CMOS checksum checks out.

Good idea with the custom setting though. As a side note, I'm kicking myself for using one of the customer's stash of batteries though they assure me it was new in all senses of the word. I tend to keep a stash with me too but I've never felt the need to wonder how old my stash is.
Definitely check the batteries with a multi-meter. I've bought several packs of different major brands last year and this year. Even though they all had distant future expiration dates, more than half of every pack was simply defective with no or very little charge.

I'm suspecting that someone has a major counterfeiting operation going somewhere for them.
 

mindless1

Diamond Member
Aug 11, 2001
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Any chance the clear CMOS jumper is in the wrong position? Otherwise, does seem like a bad battery.
 

Steltek

Platinum Member
Mar 29, 2001
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The only other thing I could think of is that someone has previously used the CMOS jumper while the computer was operating and damaged the CMOS memory on the motherboard.

I have seen this a few times in the past from owners that try this procedure without realizing you aren't supposed to do it with the machine powered up.
 

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