What I feared has come upon me:
The battery charging circuit in this machine is kaput.
I rebuilt the battery - using 3 cells instead of the original 6, since I can still get more than the original 2800 mAh using 3500 mAh cells (the originals were 1400 each, 3 parallel pairs of 2 each in series to get 10.8 V) - but the unit still exhibits the exact same functionality: the power meter stays at 80% all the time and the battery will not charge. I have the bottom of the battery case off so I can test the voltage and it does absolutely nothing. No charging at all. It says it's charging it, but in reality it's not even trying.
I found a forum post where someone has an identical problem (complete with high pitched whine): https://forum.thinkpads.com/viewtopic.php?t=124457
Considering that this guy bought a whole new battery and it still didn't fix it, I think maybe the DC/DC board is shot. I wonder what's wrong with it. I found one here: https://www.ebay.com/itm/IBM-05K3195-THIHKPAD … Z867V:rk:1:pf:0 but I don't really want to pay $50 for it plus shipping.
It appears to have a lot of electrolytic capacitors on it. I wonder if changing all of those would fix it.
I also have to replace or somehow repair the CD drive at some point. There's a combo CD/floppy unit on eBay for $20. Geez. Maybe I should go for the CD/floppy unit, and deal with the DC/DC board while I'm replacing that.
I really did not want to have to tear this unit apart, ever.
Anyone have a different one they want to trade me with? 🤣
World's foremost 486 enjoyer.