RV draining Truck Batteries

[tech740]

Most of our trips I take the RV to campsite a day or so before the rest of the family get there. My schedule has recently changed at work and I am thinking that I can take the trailer to work, then leave in the morning and save myself about 45 minutes to an hour on my trip. I work 1830-0630. I checked the other day with a voltage meter and the positive lug on my 7 blade connector is always hot. I have a residential fridge and inverter in the camper. My question is: If I park in the lot at work for 12.5 hours wouldn't the trailer be pulling from my truck batteries also? Or does the converter only try to recharge the RV batteries on shore power? I really don't want to go to leave work and have dead batteries in the truck. I am guessing that I can just unplug the umbilical in the lot and reconnect when I leave. Just trying to eliminate steps.

Thanks

[chuckster57]

If you have a residential fridge then the inverter has to stay on to provide power to the fridge. With a constant charge line, your trailer will pull from the truck battery. You could pull the umbilical, or turn off the inverter. I would look into multiple batteries in the trailer and unplug the umbilical.

[NH_Bulldog]

I agree, if the circuit is hot all the time, the truck battery is part of the trailer battery circuit and will deplete the truck battery. It is unlikely but still possible that it could discharge enough to make starting the truck a hit or miss after 12.5 hours. The question becomes how many batteries (and size/capacity) on the RV, and can they run the trailer themselves for that long? How many batteries on the truck, and is it diesel or gas? Depending on the above, you MIGHT be okay keeping the truck connected and still start it at the end of the day, but I would disconnect the truck.

[LewisB]

I'm not doubting any of the responses...this is a learning experience for me.

I thought all trailer/fifth wheel setups were designed so that the trailer could NOT drain the batteries on the tow vehicle when the system was parked! Apparently, that is not true. Is there a way to tell if your rig is susceptible to this issue (without purposely letting it happen)?

If I'm reading the responses correctly, using a volt meter on the 7 pin connector of the tow vehicle when disconnected, insuring that Black pin #4 is at 0 volts when the tow vehicle is shut off would determine if the system is "constant charge" or not? I didn't know any of the manufacturers made them that way. Or are we talking about user-installed aftermarket setups?

Thanks for any clarification available. (I've never checked my rig for this!):eek:

[flybouy]

Brad, I'm guessing you read something that you have misunderstood or incorrectly recalled. I am not aware of any travel trailer that has the circuitry or equipment to disconnect the tow vehicle battery. Most rucks however do. Most modern trucks from the "bid 3" have a relay that will latch when the truck is running to power the 12v battery circuit to the trailer. Some newer trucks will only energize that relay when the truck is running AND in gear.

If your truck has that circuit energized constantly then I would pull the umbilical and place a note on the steering wheel with "Plug in the trailer" printed on it so you don't forget.

[NH_Bulldog]

Brad, I'm guessing you read something that you have misunderstood or incorrectly recalled. I am not aware of any travel trailer that has the circuitry or equipment to disconnect the tow vehicle battery. Most rucks however do. Most modern trucks from the "bid 3" have a relay that will latch when the truck is running to power the 12v battery circuit to the trailer. Some newer trucks will only energize that relay when the truck is running AND in gear.

If your truck has that circuit energized constantly then I would pull the umbilical and place a note on the steering wheel with "Plug in the trailer" printed on it so you don't forget.

My truck has the lockout when not actually running. In fact, I had to “turn on” the feature since it was available but not active from the factory.

[LewisB]

Thanks Marshall - your explanation was exactly what I was thinking - and better worded.:) With modern trucks, when the key is off, the power is disconnected from the umbilical pin #4 and therefore disconnected from the trailer. So with a "normal" setup, the OP's refrigerator should not drain his tow vehicle batteries when the vehicle is off but still connected to the trailer, correct?

So when the OP originally posted "I checked the other day with a voltage meter and the positive lug on my 7 blade connector is always hot", I read that to mean he has a "non-standard" 7 blade connector, correct?

To tech740, when you tested this, was your truck disconnected from the trailer? If not, with the truck shut off but the trailer still connected, you could have been actually reading the trailer battery voltage on Pin #4.

Sorry - I don't intend to be a pain. This posting sort of threw me for a loop.
Thanks,

[flybouy]

Thanks Marshall - your explanation was exactly what I was thinking - and better worded.:) With modern trucks, when the key is off, the power is disconnected from the umbilical pin #4 and therefore disconnected from the trailer. So with a "normal" setup, the OP's refrigerator should not drain his tow vehicle batteries when the vehicle is off but still connected to the trailer, correct? That's correct. However, the danger in that is depleting the trailer's battery below 50% capacity and doing permenant damage to the battery. I'd suggest to going to 2 6vdc "golf cart batteries" to do this.

So when the OP originally posted "I checked the other day with a voltage meter and the positive lug on my 7 blade connector is always hot", I read that to mean he has a "non-standard" 7 blade connector, correct? I don't know what his "always hot" comment meant. It wouldn't be the 7 pin being unconventional but haw it's powered. If he checked it with the engine off and the ignition switch off (not an acc listening to the radio) then it could be a malfunction or someone circumvented the feature.

To tech740, when you tested this, was your truck disconnected from the trailer? If not, with the truck shut off but the trailer still connected, you could have been actually reading the trailer battery voltage on Pin #4.
Not if he was reading the 7 pin on the back of the truck. By default the trailer plug would have to be pulled out to do that.r
Sorry - I don't intend to be a pain. This posting sort of threw me for a loop.
Thanks, Not a pain. The smart thing to do is ask for clarification if it's not clear to you.

Read my answers in red above.

[chuckster57]

With modern trucks, when the key is off, the power is disconnected from the umbilical pin #4 and therefore disconnected from the trailer.

NOT always true!! Just tested a newer Chevy and it was live all the time. When we deliver a trailer, we always check for charge line and brake control,I use a tester that plugs into the TV 7 way and I can test for brake lights turn signals, running lights, brake control and charge line. it might surprise you how many newer vehicles have an active charge line ALL THE TIME...Just sayin :cool:

[flybouy]

I would defer to Chuck's experience on this. The electronics on my F250 stay on (radio, blower motor, wipers, etc) after I turn the truck off until I remove the key. If I turn the truck off and leave the key in It will stay on for about 5 min. Then it will shut off for about 10 seconds, then come back on for another 5 min. If my headlights are on auto then they will stay on for 30 or 60 seconds and then go off but if I immediately exit the truck and hit the lock on the remote the lights immediately go off. So best to consult the truck's manual to see what is supposed to happen.

[chuckster57]

It has been my experience that Ford uses a relay to activate the charge line and Chevy keeps it live all the time. Lots of SUVs I have tested are the same way.

[tech740]

Thanks guys. I have two 12 volt batteries on the trailer and I know they will make it over night so I can unplug the umbilical in the parking lot. When I tested my truck it had been in the garage for about an hour so I know it is always hot at the positive pin. Sitting for 12 hours is the longest it will go without power so I really don't plan to switch to deep cycle batteries. I will have to check what the voltage is after 12 hours of sitting and use that as my deciding factor. Truck is in my signature but it is a 2019 ram 3500, so at least Ram and sounds like Chevy is hot all the time. If someone checks a new Ford we can know the big three.

Thanks for the discussion, helped me to be sure I understand how the system works.