I have an analog Volt Meter and a Digital Volt Meter that plugs into the Lighter Socket.

It simply could be that the Meter is not making good contact inside of the socket or the Lighter Socket has some issues. You might check the Fuse that goes on that circuit.

Also I find that the Digital One that I have uses up about 1 Volt to run itself.

If you had a regular Volt/Ohm Meter you could check it against the plug in one.

Is there any sort of pattern as to when it is not charging like it always does not charge at idle speeds?
Also what other electrical stuff is running during the test?

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84 300D, 82 Volvo 244Gl Diesel