[SOLVED] A way to know if the CMOS battery is installed without opening the case?

Soadyheid · 06-05-2013, 11:00 AM

Quote:

Can a pc even boot without one?

Yup! It'll boot (Well.. POST) but the first thing you need to do is jump into the BIOS and reset the date/time. I believe you can run into problems trying to access files which technically don't exist as the PC thinks it's 1990 or something.

(Please disregard the last statement if you are Dr Who as you can obviously jump forward to retrieve the files from the future!

)

Play Bonny!

Ser Olmy · 06-05-2013, 11:20 AM

Quote:

Originally Posted by Habitual

Can a pc even boot without one?

I've seen at least two cases where a missing or low-voltage battery caused ACPI errors during boot. The systems in question were Windows workstations, and when the CMOS battery went flat the Windows kernel would crash with an ACPI-related BSOD during boot.

So I know that at least some PCs and some OSes won't boot if the CMOS battery is defective or missing.

onebuck · 06-05-2013, 02:13 PM

Hi,

You should be able to get into BIOS to setup the machine without a CMOS battery being in or low value of battery level. Be aware that once you remove power that the BIOS information will likely be lost. Replace the battery, not that expensive if the CMOS battery is at fault or even if you suspect it's low due to age.

cascade9 · 06-06-2013, 04:40 AM

Quote:

Originally Posted by Ser Olmy

I've seen at least two cases where a missing or low-voltage battery caused ACPI errors during boot. The systems in question were Windows workstations, and when the CMOS battery went flat the Windows kernel would crash with an ACPI-related BSOD during boot.

So I know that at least some PCs and some OSes won't boot if the CMOS battery is defective or missing.

Windows failing to boot with a BSoD is quite different to the computer failing to boot IMO.

The computer will boot fine, just you've lost a BIOS setting that windows 'neds' to run. I've seen similar situations, like SATA drives being used in IDE mode, but the defualt BIOS settings is to ACHI/Native SATA, and windows being unable to boot.

Ser Olmy · 06-06-2013, 07:54 AM

Quote:

Originally Posted by cascade9

Windows failing to boot with a BSoD is quite different to the computer failing to boot IMO.

Absolutely, but in this case even the Windows setup program BSOD'ed just after the CPU was switched to Protected Mode, before any storage drivers were loaded. The faulting driver was acpi.sys.

It could certainly be a bug in the Windows ACPI driver, but it nevertheless indicates that at least on some systems, the CMOS battery does more than just keep the BIOS settings alive and the RTC going when power is disconnected.

onebuck · 06-06-2013, 09:03 AM

Hi,

Quote:

Originally Posted by Ser Olmy

Absolutely, but in this case even the Windows setup program BSOD'ed just after the CPU was switched to Protected Mode, before any storage drivers were loaded. The faulting driver was acpi.sys.

It could certainly be a bug in the Windows ACPI driver, but it nevertheless indicates that at least on some systems, the CMOS battery does more than just keep the BIOS settings alive and the RTC going when power is disconnected.

CMOS battery does nothing more than provide the means for the CMOS to keep the BIOS state at a set value whenever power is removed with a known good battery or from source for the ATX PSU with AC power plug in place. After the power is removed the CMOS states should remain the same as intended/set for BIOS information. Plus provides source for 'RTC' to maintain time/date to a preset value. See: https://en.wikipedia.org/wiki/Nonvolatile_BIOS_memory & https://en.wikipedia.org/wiki/BIOS

If the CMOS battery provides a low source then erratic setting for BIOS can/will be provided if the power plug is removed. For ATX PSU the source can be from battery or ATX PSU to the CMOS circuitry. Provided through a blocking diode, capacitor circuitry to the CMOS so as not to conflict with CMOS battery.

Excerpt from CMOS battery

Quote:

The memory and real-time clock are generally powered by a CR2032 lithium coin cell. These cells last two to ten years, depending on the type of motherboard, ambient temperature and the length of time that the system is powered off, while other common cell types can last significantly longer or shorter periods, such as the CR2016 which will generally last about 40% less than CR2032. Higher temperatures and longer power-off time will shorten cell life. When replacing the cell, the system time and CMOS BIOS settings may revert to default values. This may be avoided by replacing the cell with the power supply master switch on. On ATX motherboards, this will supply 5V power to the motherboard even if it is apparently "switched off", and keep the CMOS memory energized. In general one should not work on a computer that is powered.
Some computer designs have used non-button cell batteries, such as the cylindrical "1/2 AA" used in the Power Mac G4 as well as some older IBM PC compatibles, or a 3-cell NiCd CMOS battery that looks like a "barrel" (common in Amigas and older IBM PC compatibles), which serves the same purpose.

HTH!