What Should the Voltage Be At Trailer Battery Charge Line Circuit on 7-Way

Updated 11/30/2021 | Published 11/29/2021

Question:

I was told that when towing my camper trailer the automobile would charge the camper batteries through the 7 pin plug. The voltage at the seven pin plug maxes out right at 12.25 V although measuring the voltage at the cigarette lighter inside the automobile shows as much as 14.25 V when initially charging the automobile battery. The 12.25 V is insufficient to cause charging of the RV batteries. The plug was wired by an automobile dealership. I suspect he wired the red wire incorrectly. Where should the red wire be connected?

asked by: Dan C

Expert Reply:

In regards to the battery charge line circuit, the exact voltage will vary from vehicle to vehicle and the state of the battery but it should be anywhere from 12 to 14 volts. That said, you'll lose voltage the more resistance you have between the power source and since the trailer connector is at the rear and the battery is at the front, you're likely going to be on the lower end of the somewhere closer to 12 volts. This will still work great to keep your trailer batteries topped off but the circuit in general isn't really designed to charge a large battery or battery bank. For this, you'll need an actual charge separate of the circuit like the Redarc In-Vehicle BCDC Battery Charger part # 331-BCDC1225D.

expert reply by: Conner L