I could imagine the CMOS Bios Battery circuits are quite similar on all recent mainboards.

What does it look like and how does it work?

Background:

My mainboard started draining the bios battery (an ASRock H55M). I inserted a fresh battery with 3.3v. It only measured 2.9V when I removed it, after the PC had been turned off for about 8 hours. While booting, the BIOS is showing the error message "CMOS Setting Wrong" in combination with "CMOS Date/Time Not Set". My suspicion is, that the battery voltage - while being drained by the mainboard - likely dropped below the 2.9V and caused the error.

So started wondering what could have failed and what this circuit would look like.