Using100/30 mppt solar charger as dc-dc charger in caravan

My solar panels aren’t in use during transit so will it charge the battery if I take a positive feed from the caravan fusebox using a splitter in the ‘car’ fuse and running it to the pv+ with a negative from the caravan battery to pv- ?

No.

The Mppt will only start to charge a battery if the input voltage is 5V above the battery voltage (that it’s charging).

Also, the MPPT’s are designed to have a current limited input source, so taking input from another battery can exceed the maximum input current under certain conditions.

Better to use an automatic charging relay, or a proper dc/dc converter designed for the job.

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Ok so a dc charger it is then but can I still take the pos feed from the caravan fusebox ?

If that is the feed from the car alternator / battery, then yes.

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