First rule of thumb when testing battery, cranking, charging, in fact anything electrical on vehicle systems.........start with a fully charged or as near as battery, hence the engine running bit, obviously at the roadside the only way to get some semblance of a charge in it and would be approx. 15 mins at a high idle..............you can't do a cranking test with a flat, or very low battery !
This system was used, had to be used, back in the day but time has moved on, thankfully.
The resistive drop test is OK but doesn't tell a full story and on rare occasions when instructions haven't been followed, can cause a bit of fun with a bang from the battery.
My Midtronics battery tester does the following from connection to a battery, asks if it is on the car or off, asks if the test is being done on a remote jump start post or battery terminals, warns if the croc clips are making a poor connection, shows the ambient temperature, asks the type, regular, spiral etc, asks the capacity and if SAE, DIN, JIS etc, then if it's happy performs the test.
It will show as battery good, needs replacing, or simply recharging, if the volts are too low at the start it will tell me to charge then retest, if on the vehicle it will ask for the engine to be started, via a jump pack/leads, run at a high idle then say when to shut down, switch on headlights to remove the surface charge then when removed re-test.
The test results show volts at start of test, expected capacity and actual available capacity, state of charge and state of health of the battery, important bit these two, because you can have a battery that show say 12.6v so presumed to be good but the capacity has dropped from what it should be say 800A to 380A.
Whilst charging on the vehicle it will do a charging test, volts, look for diode ripple and so on.
It will also do a cranking test showing cranking speed, amps being pulled, volt drop etc. and from this to an experienced eye the condition of the starter motor can be determined to a degree, such as the odd one where the armature bearings are worn allowing it to rub on the magnet, modern sintered magnets or on the stator pole pieces.
Costs a bit but worth it's weight in gold.
But like any test equipment from a DVM upwards, only as good a the user.