two types of charging systems on old style Mercs, one had a rectifier, one has a regulator. They are pretty basic, the rectifier is used on the low amp type, its basically runs wide open, it doesn't produce enough amperage to screw anything up, but makes a lot of voltage and output is directly related to rpm. The regulator type is similar, but operates in an on/off type situation, you'll see high charging rates, then no charging till the voltage gets in the 12 volt range, then high charging. So to answer your question, sounds like its operating normal. My johnrude does the same thing, it will scare you when you're offshore and look down to notice the voltage around 12 volts idling, you rev it up and it will come up.
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