So I stumbled on an interesting HEI phenomenon

While working at a Ford dealership, customers would come in with same problem, "starts and drives but runs like crap". To tell them they needed only a new battery would often lead to argument, especially when they heard what the battery costs there.
Starters, lights, etc.. will operate on 11 volts. The difference in how they are operating, starter turning slightly slower, lights not quite as bright, is difficult to recognize. The electronics are very sensitive to low voltage.
I'm glad it was the service advisors job to try explaining these things to customers.
By the way.. I've seen batteries that were so bad, put a meter on it and see 12.2, open the door and the dome light alone would drop the meter reading to 2.9 in a heartbeat.