OK, some theory. Automobile batteries have short plate separation distances. Deep discharge batteries have large plate separation distances. As the electrolyte gets depleted whiskers develop and short the plates.
The Effective Series Resistance determines whether you have enough current to start the starter. This is bundled into the spec "Cold cranking amps".
I've jumped vehicles that would not jump because of a dead battery. For a few reasons it always makes sense to connect to the frame of the jumped vehicle last and remove the frame connection first. This lowers the series ressitance. The battery cabke from neg to the frame isn't counted.
I did, once, and it's not recommended. I loosened the battery cable on the vehicle and jumped the vehicle with the battery mostly out of the circuit. I was then able to tighten the cable.
Measuring the specific gravity and seeing if all the cells are the same is one first test. It's not the same as a battery test.