CMOS battery actually dead! Symptoms to help others

[Whoozit]

Been building PCs for 30 years and finally got my first properly dead CMOS battery. That's the one on the PC motherboard, which powers the BIOS memory for startup.

The PC in question had two system drives, one an HD and the other an SSD copy as an upgrade. I left both connected as the HD was partitioned and had a data partition, just in case anything was missing. SSD install was fine, selected as the default startup drive, PC faster. Job done.

A couple of months after the swap, the system started booting from the old HD system drive and was dog slow. Not easy to diagnose why remotely (it went to Uni with my daughter) but eventually I spotted in Disk Manager the names of the drives (renamed as a matter of course, Old System and System is a giveaway). Weird. I reset it all, and when I had physical access again I disconnected the HD.

A couple of days ago... "PC won't boot". Hmmm. No boot device found? Check the BIOS settings, they seem correct and see the SSD. Still no boot device found on startup and the PC is memory checking every boot. Weird. Try swapping around power and data cables just in case something's broken... nope.

OK, let's try the old system drive. Oooooh.... that isn't found either, so it probably isn't a corrupt OS! Just as well, I don't have a big enough USB stick onto which to load a windows repair OS.

I remove the CMOS battery, give it five minutes, replace with a fresh one and when the BIOS loader asks what I want to do, revert to the default BIOS.

Bingo.

Posted for posterity in case it helps.

[surveyor]

Useful info..

I had a nightmare with various issues mostly revolving BIOS not saving. Changing the BIOS battery changed nothing.

Ended up deciding the board was dead.

Used one of the spare batteries in a car key. Dead. Checked the battery. Dead....

At some point need to rebuild a system on that board, as I suspect it was just a dead battery...

[LuS1fer]

I set up my mother's old computer and it kept losing the settings and a quick Google pointed to the motherboard battery.
Replacing it solved the issue.

[psi310398]

By the same token, for those with HiFi streamers, I packed away some seriously dear Meridian streamer/HDD units during a lengthy refurb and when they came out of storage some units simply wouldn’t fire up.

The user forums were full of tales of fried power supplies but that didn’t seem to be the problem (using my multimeter). The penny then dropped that these were really nothing more than gussied up computers and, when I opened them up, a simple BIOS battery replacement got them up and running again.