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I recently split up the ground between the two batteries in my van to install an isolated Orion Tr 12/12 30 and replace the automatic dual relais. So everything powered by the second batterie has his own ground. Since then I measure steady 1.3V Potential between the two grounds. Should I leave it like that? How does this happen? Should I replace the isolated version with non isolated or can I just connect the two grounds again?
If the two grounds are truly separated, and there is NO interconnection between them, any potential between them is irrelevant and meaningless.
There is no reason that any two separate battery systems will be at the same potential relative to each other. You have floating "negatives." It doesn't matter if they are +1.3V or -10.5V or +50.7V relative to the other battery system, only that the positive be 12V above the negative in the same system.
The reason that mobile DC power systems frequently (but not always!) have negatives connected across different systems is it can be hard to be SURE that the systems really have no interconnections that can lead to troublesome stray currents in places they are not supposed to.
Thank you for the detailed answer! So I should connect the two grounds in order to prevent stray currents ?
