Up till about 1989 when the cars just had a feed when the key was off because to keep the clock running, station preset, ECM memory. If you had less then about 9.6 to 10.5 volts you did not have a drain.
So at the dealership we would roll the Sun Bat 9 or was it a 6 or a 13. I don't remember. Though I have a BST 5. We would disconnect a bat cable and put the analog meter in series. When we saw bat voltage we knew something was on or there was a short to ground. It was usually a glove box, hood light, or trunk light staying on.
Once the cars went to more complex computers, added BCM's, etc, the voltage readings were to close to bat voltage to use the old analog volt meters for a battery drain quick test.
So we then had to use digital multi meters.
However using a old time analog voltmeter to measure a parasitic drain was essentially using the volt meter hooked up in series as if it was an amp meter. Combination of the meter was rugged enough and the parasitic load current flow was low enough that it was possible to check to see if there was a drain causing the battery to go dead.
Also shops back then rarely had an analog amp meter that would read milliamps so they improvised.
So if I had an old time analog volt meter, and pre 1981 auto and suspected a parasitic drain I would not hesitate to still test for it that way.
Problem with using a test light for a parasitic drain is that unless you have a test light made back in the 1960's or before is that the newer bulbs draw less current and there can be enough of a drain to make the battery go dead but not see the filament glow.
However a true voltage drop test is using a voltmeter that will read voltage below .1 volt. And it is measuring the voltage drop in parallel not in series as above when removing a battery cable.
Two weeks ago my Jeep would not crank. I measured a series of voltage drops with my multi meter. I put one lead on the negative bat post and the other lead on the bat cable clamp, then from neg clamp to neg cable. I did the same with the positive side. Initially I was getting voltage drops of .1 v.
That told me the contacts were all good without having to remove anything to verify there was no corrosion.
Until I measured the voltage drop across the pos cable terminal end and the pos cable. That v drop was .6 volts.
I then took the metal strap off and removed the positive cable from the terminal and there I saw build up of crud between the terminal and the cable.
Point is without that excessive voltage drop there was no visual corrosion on the cable terminal or in the exposed bit of wire of the positive battery cable. Even the bat posts to cable terminals looked clean.
