EDIT.........AFTER doing what Steve said above (OldmanMopar)
The FIRST thing you must do is to determine whether the 1...battery is defective, whether the 2...starter is drawing over-amps and pulling the battery down, or whether it is 3...simply voltage drop.
EG Voltage drop measurements will not tell you anything if EITHER of the other conditions exist. IE if the starter is defective and dragging, etc, it will exhibit system voltage drop, as well as low battery voltage
You need to get/ borrow a legit carbon pile load tester, like one of these.
You can test both starter draw as well as get an idea of battery capacity. "Generally" you start with a fully charged battery. You must have the problem acting up, in other words engine hot, then crank, or try to crank the engine for 10 seconds while reading the voltmeter on the tester.
Next you crank in resistance until the voltmeter pulls down and reads the same as the starter pull read. At that point the ammeter on the instrument will tell you the starter draw.
Conversely, you can load test the battery and also get an idea of how it's doing. Again, you MUST start with the battery fully charged.
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