Is 12v constant from hitch actually necessary?

Everyone is going to have different needs. My truck has no 12V wire on the system. I have never come across a need for it. My trailer is plugged in when at home and it is plugged in when I get to the campground. We never camp more than 25 miles from home, so I have no need to run the fridge on gas or anything else on battery power for that matter.

If your truck already has one, I wouldnt remove it or pull a fuse to disable it. Eventually I will probably add one to my truck but so far in the 4 years that I have owned this truck I have not had a need for it.

Dayle1 wrote:


How do you overnight w/o shore power and still KNOW you have sufficient battery power for the break away circuit the next morning?

Because I always idle my truck for abt 15 min before pulling a load. And then I'm in no hurry so it has plenty of time to charge the RV battery to enough to operate the brakes. As long as it's not completely dead in the morning.

RustyJC wrote:

Dayle1 wrote:
Any trailer with a residential refer should have a dedicated 12v system isolated from the primary battery or a shutoff circuit to prevent draining the primary battery.

Why? The +12VDC wire (pin #4 in the Pollak connector) has no problem keeping the trailer's battery bank charged while we're on the road. If I were running with no +12VDC feed from the truck as some suggest, then that could be a problem insofar as the breakaway system is concerned, but as I've said repeatedly, I would never run without the +12VDC feed being connected. It's there for an important, valid, safety-related reason.

Rusty

How do you overnight w/o shore power and still KNOW you have sufficient battery power for the break-away circuit the next morning?

I agree, the important, valid, safety-related reason is power for the break-away circuit to operate the trailer brakes. Last time I looked at the DOT regulations for commercial cargo trailers, they were required to have a dedicated battery for the brakes, nothing else could be connected to that battery (interior lights or other 12V items must be powered via the tow vehicle harness). The circuit also includes an LED light that shows when the brakes have been activated. An LEO could pull the break-away pin and the light needs to stay on for minimum of 15 minutes. Failure could result in a ticket.

Why would you want that pin hot?
Some cargo trailers I have towed have an interior light that is only powered from that pin.
There are adapters that will plug into your 7 pin on the truck and give you a "cigarette lighter" outlet which can be used for all kinds of things.
When camped and the RV is plugged in the converter will keep the truck starting battery charged (the wire works both ways). This way, after the truck sits for several days with various radios, cell phone chargers, etc. plugged in it will still start.
If camped without electricity and then towing to a new campsite also without electricity I would certainly want my tow vehicle providing any charge it can.
If it really bothers you that it's hot install a relay. I've had 4 Chevys over the last 13 years that were all wired this way. I never had any problems.

The refrigerator on gas draws +12VDC from the battery for its control circuit. Don't believe me? Disconnect the battery and try to run the fridge. There are other +12VDC parasitic loads that are there whenever the battery is connected to the trailer - the propane detector for one. The truck and trailer are wired with a +12VDC feed for the trailer battery(ies) for a reason. Why disconnect it? If you're so worried about drawing the truck batteries down, install a battery isolation relay to break the feed whenever the truck isn't running.

Rusty

enblethen wrote:
Yes, it charges the battery while on the road.

Maybe. More correct, it prevents any significant discharging for loads in the trailer (like the refrigerator) will running down the road.

Most (all?) modern vehicle charging systems are controlled by the PCM. They are quite accurate. The problem is 20-30' of wire to the trailer battery will cause a some voltage drop, likely enough to not fully charge the battery.

If you are starting with a fully charged battery AND have NO LOADS (i.e. refrigerator off or on gas), then I would nor worry about B+ at the 7 pin connector.

The whole kit is just $37

https://www.etrailer.com/Trailer-Breakaway-Kit/Pro-Series/PS50-85-320.html

Dayle1 wrote:
Any trailer with a residential refer should have a dedicated 12v system isolated from the primary battery or a shutoff circuit to prevent draining the primary battery.

Why? The +12VDC wire (pin #4 in the Pollak connector) has no problem keeping the trailer's battery bank charged while we're on the road. If I were running with no +12VDC feed from the truck as some suggest, then that could be a problem insofar as the breakaway system is concerned, but as I've said repeatedly, I would never run without the +12VDC feed being connected. It's there for an important, valid, safety-related reason.

Rusty

RustyJC wrote:


You can say it until the cows come home. You don't know how much charge is left in the battery at the end of a 10-12 hour travel day, especially in my rig that operates a residential fridge off an inverter while we're traveling. You think I should disconnect MY trailer batteries from the tow vehicle? SMH.

Rusty

Any trailer with a residential refer should have a dedicated 12v system isolated from the primary battery or a shutoff circuit to prevent draining the primary battery. Otherwise it is a bigger safety risk than anything else mentioned. For units with an Rv refer, converter output is much higher than normal load and a good battery should be fully charged when leaving a CG. If someone is towing with a defective battery then a 12v feed from the tow vehicle will not make the rig safe.

Don't think or tested and failed? As I said comment was not about your RV.