AutoX I would assume that measuring for electrolysis / chemical voltage is normally done with the engine off? In fact I would assume the car battery could be disconnected.
I've always seen/done it with the battery connected. My published articles don't specifically say connected or not. But if they don't specifically say to disconnect the battery, I think it's safe to say they want it connected.
But I see what you are saying about disconnecting the battery to eliminate the grounding and electrical system issues and focusing on the coolant itself.
I see value doing it with the engine off and on. That's how the original poster found he had a bad ignition switch.
