Battery will not hold a charge and car will not start

Normally the more observations you can add, the easier it is to narrow down the list of suspects faster, but in this case some are conflicting. The place I would start is with a charging system test, but not a free one at an auto parts store. They do not have the ability to test one of the important things.

This problem is aggravated since GM redesigned their generators for the 1987 model year. They develop a real lot of voltage spikes that can damage the generator's internal diodes and voltage regulator, and interfere with computer sensor signals. The battery is the key component in damping and absorbing those spikes, but as they age and the lead flakes off the plates, they lose their ability to do that. It is not uncommon to go through four to six replacement generators in the life of a GM vehicle. To reduce the high number of repeat failures, always replace the battery when you replace the generator, unless it is less than about two years old. You are covered in that regard, but your generator could have been damaged long before the battery was replaced.

Post the results of the charging system test, then I will interpret the numbers for you. A few testers provide a printout. Most only display the numbers, then the mechanic has to write them on the repair order. Be sure to ask for those numbers in writing in some form. Specifically, I want to see the results for "full-load output current", "charging voltage", and very important, "ripple voltage". Most testers that do not make printouts only show ripple voltage on a relative scale from "low" to "high", and not a specific number. That is okay. We just want to see that it is low.

Also, ask your mechanic to measure the battery's voltage, and write it down, before the testing is started. That might present a clue that the battery is not being fully charged while you are driving.

GM cables tend to corrode inside the insulation. That can make it fun to find a bad connection. My SOP is to run a jumper cable from the battery positive to the cars positive feed either at the starter or jumper lug on the fuse panel. Then the negative from the battery to a ground on the engine. That bypasses the OEM battery cables. Now see if the battery will power/start the car. If it does you know you have a bad connection. Then disconnect the negative and test again. If it is still okay you have a problem in the positive side, if it stops working it is on the ground side.

Then you check all of the connection points for corrosion or a bad cable.

For the small electrical drain, you need a volt meter set to millivolts
https://www.2carpros.com/articles/how-to-use-a-voltmeter
and a voltage drop chart like those at
http://info.powerprobe.com/fusechartsdownload

Then use the chart that matches the fuse type and measure the voltage across the fuse. Look on the chart to find the current flow across that fuse.
HumbleMechanic has a good video on the process.

I can't make an analysis from, "everything tests fine". No doctor would read, "everything's fine" on your lab report. He wants the numbers, and so do I. It is much too common for an inexperienced mechanic to overlook a clue from those readings. I just had one yesterday where the person said, "I found this reading, so I know it's okay". It was not okay.

For your problem, the additional information you just posted includes the glaring clues. Loose or corroded connections will prevent the battery from fully charging. That can lead to slow cranking next time you want to start the engine, and that is typically incorrectly interpreted as there's a drain on the battery. Also, I can't emphasize enough the huge problem GM owners have with their generators, and that is going to be even worse with less-than-perfect battery cables and connections. We are real likely to cause multiple electrical failures on any car if we disconnect a battery cable while the engine is running. I did that every year to show my students how that will cause severe damage. That can also be what is happening with loose battery cables. Once you solve all the cable problems, I would not be surprised if you still have a generator problem. I hope not, but a damaged generator was perfectly fine one minute before the failure occurred. If you still have less-than-proper operation of everything electrical once the cables are fixed, have the charging system tested again. That is not an expensive test. It takes longer to connect the three cables to the vehicle than it does to take the readings. I did this on a friend's van a few nights ago. One twist of the knob, and in less than two seconds we had the diagnosis. (The only symptom and complaint was an excessive whine on AM radio). The cause was one failed diode in the generator that no one ever looked at.

Steve W. Is right about corroded cables. If yours need to be replaced, I would entertain a notion to install a top-post battery with the appropriate cables, only because it's much easier to do any testing in the future, and it's easier to connect jumper cables. The engineers at GM claim side-post batteries don't develop corrosion, but we can see that is not true. Any battery that is about to fail within the next six months will have corrosion around the terminals, regardless of the terminal style. With top-post batteries, it's just easier to see and know that it needs to be addressed. You'll need to be aware that the hoods on some vehicles can hit the posts when switching to a top-post battery, but there are a lot of different sizes available for various car models today. There is a battery that will fit your application.

Also, I know it's more expensive, but given all the grief you've gone through already, please consider replacing the entire battery cables, all the way to the starter and to the engine block / body. You can buy universal cable ends that bolt to the original cable, but those are never meant to be a permanent solution. They are only intended to get you back on the road until you have time to do the proper repair. I'll admit I've used these ends on my vehicles, but I only have me to blame if I end up walking home. I would never risk angering a customer by using these bolt-on cable ends. Also, a lot of cables corrode under the insulation right next to the starter. This is real common on Ford products. Intermittent cranking problems will not be solved unless the entire cable is replaced.