I don't see why the test is compared with a capacitor. You have an electrolytic (galvanic) cell. One electrode is the metal parts of the radiator itself. The coolant is the electrolyte and the voltmeter probe that dips into coolant is the other electrode.

There are other variables, like the metal the rad and the probe are made from and the coolant composition that could affect voltage.

There is a ton of stuff on the internet about using this method. 0.3V is the magic number, it seems. You should be below that (I think this assumes aluminum rad)

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5