Hello everyone, i want to have a batterie in my caravan. At the moment there is only the power of the towing vehical on the input to the 12V "network". I want to have also a battery. My plan is to switch of the battery when the caravan is pluged into the car. This should be done with a 40A relay. My question is: is it nessesery to do it also with the ground wire, as in the attached drawing? Or does just one relay on the "powerline" just fit? Is there a danger for the electronic in the car or the battery? This is no advanced electronics question and i hope my english is good enough! Hope to get help!