Some CMOS batteries are designed to be recharged such as the cylindrical types often found on 386/early 486 motherboards.
You can't just install a lithium CR2032 non-rechargeable battery in its place.

That ^^ is spot on. Judging by the corrosion and the CR2032 holder patchwork, I say the motherboard originally had a barrel battery (and the circuitry to charge it). You can check for that with your DMM by powering up your board without a battery in place, there must be no voltage present on the holder contacts. If there is, this means your non-rechargeable battery is being charged, and you're lucky it died gracefully without spilling its guts on the board 😁

Usually there is a diode that can be removed in order to prevent charging, you'll need to poke around to find exactly which one is it. Anyway I recommend desoldering the battery holder and thoroughly neutralize the green goo (use vinegar, water, isopropyl alcohol), else in time it will eat your board.

Correct you both are. Looks like I have work to do this weekend. I powered up the board on the bench and the battery terminal was pushing out 4.7v. As you said, I got really lucky that things did not get explosive. I will get the terminal out and clean the board up. As far as Diodes go, I can see two on the board near the battery and neither show continuity with the terminal. Is that a thing? Or should follow some traces to determine which one?

There is also a series resistor to limit the charging current, so the circuit could be as follows: (+5v)-----|>|------|===|-----(battery+)
Where the first thing is a diode and the second a resistor. You can remove either/both.

There are also rechargable coin cells available. I have never tried them, but they have been recommended here several times.

Personally I'd suggest getting the battery off the board altogether. Three AAA rechargeables in a separate battery holder are overkill in terms of capacity, but cheap, easy and deliver the correct voltage. Just solder in two pins where the CR-2032-holder is currently located and put the batteries somewhere else (preferably below the motherboard so if they ever do leak, the board is safe).

I added a non-rechargeable Lithium 3.6V AA battery to my 286 the other day. Just put a small diode in series with it (polarity!) to stop the mainboard from charging it, and you're done.

I documented it on video:

https://youtu.be/seCoC2AcHqs

You can velcro the whole contraption inside your case. Maybe even put it in a small 3D printed enclosure, to keep it safe from shorting out or to protect against possible leakage, which I doubt is a problem with the lithium batteries.

I desoldered a single pin of D4 on the left side of the board after tracing the negative line that way... The cmos recharge voltage dropped from 4.7v to ~1.4v. Am I heading in the right direction? Should the cmos voltage drop to 0?

quicknick wrote:

There is also a series resistor to limit the charging current, so the circuit could be as follows: (+5v)-----|>|------|===|-----(battery+)
Where the first thing is a diode and the second a resistor. You can remove either/both.

I believe I have found the resistor and the diode. I removed both and the battery terminals now read -2.7v when the board is on. Is this what I should be seeing?

EDIT: pretty sure it is not. I checked another board which gets ~0v when powered on.... More research needed.

I've done this mod on two or three motherboards, and as far as I remember the voltage was pretty close to zero after removing the charging diode. Reading a negative voltage doesn't seem right to me, but maybe there is a -5/-12V trace nearby and you're picking leakage from that. You can also check if there's a charging current at the battery terminals using your DMM on the (milli)ampere range. Before modding, my boards had a charging current of around 4 or 8 mA, cannot remember exactly. Was 0 uA after removing the diode.