I suspected a bad battery in my car, cause by high Internal Resistance. My automatic charge exhibited the same properties yours did. The alternator appeared to work, exhibiting 13.8 V, but I saw it lower on my digital meter. There are times the analog meter is the better choice.
So, I knew under load, the voltage dropped sufficiently enough that the battery was defective. I didn't have the test equipment to test everything, and the two places I visited that had it couldn't connect it. One wanted me to wait four hours ... that wasn't happening.
I took it to a shop that had ASE mechanics, you know, those certified types. Yes, I paid more than I had to, but I did contribute to the economy, as their family has to eat too.
So, I knew under load, the voltage dropped sufficiently enough that the battery was defective. I didn't have the test equipment to test everything, and the two places I visited that had it couldn't connect it. One wanted me to wait four hours ... that wasn't happening.
I took it to a shop that had ASE mechanics, you know, those certified types. Yes, I paid more than I had to, but I did contribute to the economy, as their family has to eat too.