Whatever random battery I stick in there.
That's a pretty common battery/ tester failure when the battery is suffering reduced capacity and high resting voltage, the tester doesn't actually load it down that much, then extrapolates from how much it did drop. Zap it 2x in a row , or hit is with a real 300A carbon pile tester, and it will fail.
The high resting voltage then fools the alternator , which undercharges the battery, weakening it further in a vicious cycle.