[Formulabruce]

Just because you have 12 volts, does NOT mean your
Battery is Good. You can have partial plate function in your battery while the rest of the plate is basically open, or semi shorted with corrosion. A battery 7 years old is ready to be replaced. Newer batteries do NOT have the warranty period as long as older batteries due to them putting in more plates, which are closer together. This gives more Amps, but will short out in a shorter period of time. Electric fans really make no difference if wired with big enough wire and your alternator can handle it. Typically fans will draw 20-30 Amps and most alternator are not fully "excited" and charging till around 1600 rpm. Obviously, as mentioned, clean contacts are a must. I have 2, 10 year old Delco batteries that measure 12.6 volts and only use them for testing circuits. or my power probe tool. Neither would ever start a car again.

[i82much]

I ordered a load tester and the battery tests good. The positive terminal and battery post both have some powdery corrosion on them, so I think I will try just cleaning them up to see if it solves the issue.

Just in case, I finally broke down and bought one of those lithium jump starters too. Probably a good idea regardless!

[Joel Koontz]

Years ago I had a 455 in a 66 2+2 Convertible and it had a set of 400 HO heads on it. Very high compression. IIRC it had 220 PSI of cranking compression. This was before aftermarket starters were common and it had a factory Pontiac starter on it. Even with everything in great shape it cranked over hard.

One day it would not crank over so I checked the battery terminals. They were not loose, but not as tight as they could have been. I pulled the battery terminals off. They looked pretty good so I reinstalled, tightened thoroughly and tried again. It still would not crank over.

I then pulled the terminals off again and cleaned the posts and terminals. When I reconnected, it cranked over and started.

Even though the posts and terminals did not appear to be corroded, they apparently were. This taught me to always clean the posts/terminals if having starting issues.

It seems that having the terminals even slightly loose contributes to this "invisible corrosion" so I think it is good practice to clean terminals/posts before tightening any time you find terminals that are loose.

[Greg Reid]

Once again, the volt meter told you where the problem was. As for load testing, you can do a decent job of checking that with a volt meter also. If the battery drops down a volt or two to 10 or 11 volts when the starter is engaged, it can't handle a load and is bad.
First issue that needed to be solved, day one, was the voltage drop between the post and the terminal. That alone was enough to cause your issue.