Grumps explained :
Hi All
As per subject really. Is this at all possible/recommended?
According to the 12S pinout, pins 4 and 3 are constant power and ground.
Difficult to imagine as I would think that the local chavs would've put a
paper clip in there by now.

If it is wired to modern standards, then pin 4 should be fed via a
relay that only energises when the engine is running/ the main battery
is receiving a charge. Like wise pin 6 fridge supply, which is only
live under the same circumstances.

Pin 4 is intended to charge the battery in the trailer.

My ciggy lighter socket is live all the time and I use a small
maintenance charger which plugs in there. I have the charger mounted on
the garage roof beam, with its 12v plug hanging ready to plug straight
in as I stop.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk