Ok, maxtherabbit, but for "ordinary" ones or not-rechargeable AAs or AAAs (these are "primary", right?) batteries I still have to use your simple modification suggestion - removed D3 diode or resistor (in series with D3). And I still prefer to try out first the non-bulky variant with 3 V coin cell 😉 Tomorrow...

Yes correct

Job's done. Everything seems Ok, CR2032 socket fits tight and tidy:

Time to fire it up... Will it work?

What voltage do you measure at the input pin to the RTC when the system is off?

The system is running fine with this battery modification:

maxtherabbit wrote on 2025-05-08, 01:37:

What voltage do you measure at the input pin to the RTC when the system is off?

Currently intentionally I'm using a button cell with the lowest voltage that I could find lying around - with 2.84 V measured. When system is powered on, there's no more +4.3 V on the battery terminals, but exactly +2.84 V (the actual CR2032 voltage), so the reverse current leakage through D2 diode really is negligibly low and I couldn't even measure it. When the system is off at pin 20 of RTC+CMOS chip I measure 2.48 V, but the chip keeps BIOS settings and its clock is running. So, the voltage drop from the battery caused by Si diode D2 is about 0.36 V, which is not perfect, but for now everything is Ok.

I may change this Si diode D2 (1N4148) for some schottky or even better - a Ge diode, if I found some suitable replacement.

Cool. I'd be curious if it keeps accurate time with that low of a standby voltage

maxtherabbit wrote on 2025-05-08, 13:01:

I'd be curious if it keeps accurate time with that low of a standby voltage

Yeah, I'll leave it disconnected from PSU for a couple of days to check this.

Update: I booted the motherboard today to check the date and time in the BIOS and got an unpleasing surprise. The date is still 08.05.2025 and of course the time is also incorrect, but the settings in CMOS are preserved and valid. It seems like the RTS needs some more "juice" to run fine.

So I really have to replace the Si diode D2 (1N4148) with some schottky or Ge diode. Any suggestions for a suitable diode with low forward voltage drop?