Should BIOS settings be lost when changing CMOS battery?

  Batch 10:31 15 Nov 2012

I'd been trying to get an old PC (10 years plus and not used for a v. long time) running and was having problems getting it stable (it would soon crash at various different stages after booting), so was trying a variety of different things (e.g. taking all the cards out and reseating them etc.)

One possibly I thought of was the CMOS battery. So I switched it for a new one. Immediately afterwards I got a message that all the CMOS settings had been lost and when I checked the BIOS everything had been reset to defaults.

I always thought that the BIOS settings were stored in non-volatile flash memory and didn't need any power to maintain them (and the battery was just for the clock). Am I wrong or what?

  northumbria61 10:38 15 Nov 2012

The fact that you haven't used your computer for a very long time is the problem. The CMOS battery is responsible for saving settings other than the date/time and clock. Leaving a PC on standby or sleep mode supplies power to the CMOS battery and therefore it lasts a lot longer than a PC that is left switched off for a long period of time. See here enter link description here

  northumbria61 10:45 15 Nov 2012

There is another article here - you are in fact correct that the BIOS itself is a hardware program stored in non-volatile memory but the BIOS settings are what are preserved by the battery (or power supply). That's a much smaller amount of data. If that data is lost, the BIOS will happily reboot from the default settings when the machine is powered up.

The BIOS settings you now have ie: defaults are probably what you already had unless you have changed/modified or overclocked in the past.

  northumbria61 10:46 15 Nov 2012

Sorry - here's the LINK (most of what I have explained) enter link description here

  Fruit Bat /\0/\ 11:26 15 Nov 2012

Yes hardware only stores the default settings removing all power from CMOS reverts BIOS to default.

Your lucky the machine is only 10 yrs old - older machines had a rechargeable battery soldered onto the motherboard that once it died made impossible to renew and hold any BIOS settings at all.

  Batch 11:28 15 Nov 2012

Thanks northumbria61

That first link seems to have something contradictory in it "CMOS is used as a form of nonvolatile memory. It needs power supplied to it in order to maintain the data that is stored in it." By definition, non-voltaile memory doesn't require power (see

The BIOS settings were fine beforehand as I'd booted beforehand and installed an old version of Windows but it kept crashing quite quickly (BTW, in the end I determined that the problem was being caused by poor memory card contacts).

Have also just seen this:

BTW, the BIOS settings are often not the defaults as the PC manufacturer sets them according to the best needs of the hardware configuration (on the mobo).

I was interested as I have 3 (relatively) ageing PCs that could run into CMOS battery problems in due course. I might just take photos of all the BIOS screens, just in case!

  Fruit Bat /\0/\ 11:42 15 Nov 2012

Removing all power to a CMOS was one way of resetting a BIOS password

this made security a joke on an easily steal-able laptop.

the password was placed in non-volatile memory, so requires a CMOS chip change to get round a forgotten BIOS passord on a laptop.

These chips are soldered to the motherboard on laptops to make it even harder and thus not worth the hassle.

If you have a laptop its worth setting a BIOS password but don't forget what it is

  Aitchbee 14:31 15 Nov 2012

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