last week I went wheeling with a friend and I got stuck pretty bad. I started using my winch and notice that the winch consumes more power than what the alt can charge. I already knew this so I decide to get an aux battery. I already have the battery but I dont know how to connect them. I know that if I connect the +'ves and -'ves both battery will drain at the same time. I want just the aux to drain when winching living the other to run the car. I have research and it looks like a good option are some voltage sensitive relay that automatically switch between batteries. however, I don't know if this relays work even when the motor is running.

has any body done this in his truck ? what are your advices? thanks