The battery is there to start the (ICE) car. Its voltage under ~500 of amps of load is the relevant voltage. The voltage when the car is off is almost always irrelevant (it provides no guarantee about the ability to crank) and the voltage when the car is running is doubly irrelevant (alternator). What you want to do is watch and record the voltage dip while cranking, and this alone will give you a sense of your battery health.