Ok,this is getting frustrating, but perhaps there's a simple answer.

Have an 88 780 with V-6. I was having an occasional no start situation, so I had a new set of brushes put into the alternator. Now the system was showing 13.5V on the mechanic's meter. But after leaving the mechanic's garage, the voltmeter in the dash was dropping down to around 12V with no lights on, and into the red zone when the headlights were turned on.

I know that bad grounds are the source of most of the electrical issues that old bricks have. Cleaned the ground points on the passenger (battery side) inside fender well. This didn't change the situation very much, so I waited until the next not cold or rainy day to work on the other side. (Hey, it's Winter in Michigan, and I don't have a heated garage)

The ground point for the driver's side is tucked in beside the air filter box, so I removed the box, then removed the ground point and cleaned it up, filed it down to fresh metal, sanded the metal on the inside fender and put everything together with dielectric grease.

I tested the voltmeter with nothing connected to this ground point, and the voltmeter went to straight up at midpoint (desired point). Turned the headlights on, and of course nothing lit up, because there was no ground in the circuit. Began connecting the grounds one at a time, and then checking the voltmeter with the lights on. Very slight drop when the headlights came on, until I connected the ground for the windshield washer pump, and that took the voltmeter down into the danger zone. So I've left that ground unconnected, because I need headlights much more than windshield washer fluid.

Now in doing this testing, I had turned the car on and off about 30 times to test each ground point with and without headlights turned on. The car started without a hiccup each time. Then I finally put the aircleaner assembly back in place and went to start it up one more time. Nothing...

Brought my 240 over to jump the car, and it fired up immediately. Took the 780 for a 15 mile highway drive to ensure the battery was getting a good charge. Voltmeter stayed in the midpoint, and only dropped slightly when I turned on the headlights.

Brought the 780 home. Parked it in the driveway and turned it off. Nothing...
Connected the jumper cables from the 240, and it fired right up again.

I'm thinking that I've got a bad cell in the battery, but it seems too coincidental that the battery could die during all the other work I did.

Anyone else have any diagnostic conclusions?