That sounds like a bad test to me. Batteries are checked (properly) under load. Disconnect them and check them with a load tester It's the only way to accurately test batteries. What you use for load can vary, but I get the best and easiest results from one of these.
A good 12v 7.5ah battery will go to 9 volts and hold steady there. A bad one will start at less than 9 and go down or instantly drop to zero. For reference.
