jakethompson1 wrote on 2022-07-12, 19:08:
Would the two diodes vs. one diode be a factor in which boards can use a CR2032 as external battery as-is and which can't?
It mostly depends on the chip that handles the RTC/CMOS settings, some of them work just fine with little over 2V, so the voltage drop from the diodes does not affect them. Others require at least 3V, so those usually do not keep time properly and need a higher voltage between the battery terminals (usually 4.5V, or even 6V). Also not all types of diodes used have the same voltage drop, depending on the current flowing through them.
The strangest case I found is a 386 motherboard (ECS 386A) which uses a CHIPS 82C206 for RTC/CMOS. The board has no internal battery (good thing, since it means no leakage) and when I got it there was a 3.7V non-rechargeable lithium battery pack plugged in to the battery connector. It was dead, so I replaced it with 3xAA (4.5V)... which worked fine for a couple of weeks until the batteries died. Same with the replacements I installed. I decided to check with a multimeter and the 82C206 was drawing several mA of current!! So for some reason with that higher voltage the chip was working as if it was powered from the PSU instead of entering low power mode, which only draws a few microamps!
I then tried 2xAA or a CR2032 and it did indeed switch to low power mode with 3V. But it seems that voltage is not high enough, so even though the settings were kept the RTC didn't tick when the machine was off.
The definitive solution was to use a 1/2AA 3.7V lihtium battery (same voltage/type as the original), so I connected it using a holder. That one seems to hit the voltage sweet spot to work in low power mode while having enough voltage to keep the clock working. But in my experience 3.7V non-rechargeable lithiums are somewhat harder to find.