So yesterday I turned off my PC through Windows, then the PSU behind it, then the UPS and finally the multiple socket power outlet.
I did this because today the electricity company guys were coming to do some work, so to be safe I made sure I turned everything important off.
It wasn't the first time I did this, but this time I think it lasted at least 6 hours with everything turned off. I don't remember how much time I left it off in previous occasions.
This time, however, it seems my BIOS settings reset completely after I turned my PC back on, and I wasn't really expecting that as I had left everything turned off before and there were no problems. Just now I tried leaving everything off for about 5 minutes and the BIOS didn't reset. It's a GA-K8NXP SLI nForce 4 from 2006.
Is it the battery that's getting weaker or did something else wrong could have happened (possible short circuit when I turned PC back on)?
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I just left the computer's PSU turned off again this night and the BIOS reset once more. Normal?
Your CMOS/BIOS battery is dying. Open the case and on the motherboard you'll find a watch battery, pull it out and go buy a new one...