dennisE wrote on Yesterday, 12:05:

How do I check the voltage at pin 24 of the HD146818? I understand that I connect pin 24 on one side of the tester, and the other side (battery or power supply)?

You need to measure between pin 24 and GND (ground plane). For GND you can pick pin 12 (opposite corner of 24) or perhaps one of the black wires in any spare PSU connectors. Using the pins directly will give you more accurate result but with decent PSU wires even using a plug far away from the chip the difference shouldn't be more than 0.1V anyway.

dennisE wrote on Yesterday, 12:05:

If I put in an external battery, for example, 3.6V, will it be enough to work?

It should be enough, but some older mobos required 6V external pack, and had more than 1 diode that caused voltage drop. These are rare though. The 3xAA pack will give you 4.5V which is a perfect value, high enough to ensure good chip power supply, low enough to not be drained if the mobo has just 1 diode (unlike 6V pack would be). Cheap, comes with wires, preferably look for one that already has the pin-compatible connectors.
Note you can't just connect a battery in place of the original rechargeable NiCD without modding the power supply path there. If you just do a 1:1 swap the mobo will force current through the new battery, trying to recharge it, and it will eventually cause the battery to fail - bloat and/or leak. That's another reason why I don't like doing CR2032 mods to these old mobos, it's not as simple as it looks.

Deunan wrote on Yesterday, 12:48:

If you just do a 1:1 swap the mobo will force current through the new battery, trying to recharge it, and it will eventually cause the battery to fail - bloat and/or leak.

And that's the reason why I claim, that his CR2032 battery "modification" is inappropriately done and I gave him a link and a picture to see how it should be done... but the last question about the chip voltage measuring explained everything.

I noticed the battery is a 3.6V LIR2032. It's rechargeable! What do you think? I checked the voltage at pin 24 and it's 3V with the PC off. If I turn it on, it goes to 0V! Is this normal?

Rechargeable is good, it'll probably work OK. There is no charge control or termination on the mobo, it'll just supply about 4.5V but through a resistor to make the current small. The question is if it's small enough for the coin battery, but I would give it a go. With 3V on the chip it should keep the settings, and time, just fine. But the 0V is not normal at all, who did the mod? Maybe they did remove some diodes and now there is no charging but also no power from PSU. The same resistor I mentioned will drop the voltage very low when the chip is accessed by CPU, it might be even connected to the ALS245 bus driver, which is TTL. That will draw a lot of power through the I/O pins, sagging the battery-only power rail.

I think before messing with this any further you need to know exactly what the battery mod does here.