I've seen bad batteries show 12 volts. Turn the key and it drops like a hammer, 2 or 3 volts. Bad battery. A battery that properly stores cranking amps , at 70 degrees, no less than 9.5 volts after 20 seconds. Yes you would need to pull the coil wire to run this test. 20 seconds continuous run isn't going to hurt a good OEM starter. I don't know if 20 seconds or longer would damage a modern mini starter.
Since you've seen your battery drop to the 10s in a short period, I suspect it's going bad.
The general term "bad battery" covers a few different symptoms/conditions and creates different charging system conditions.
