Tow vehicle battery drain?

[ABFJV]

We sometimes overnight in a rest area or WalMart. I have been unplugging the 7 way connector from the RV to the truck thinking that possibly while using the house battery (1) to power the TH it will also be drawing power from the truck batteries (2) thru connector charge line. Is it necessary to disconnect from the truck? The truck is a 2003 F250 7.3L if that makes a difference.
Thanks

[rockfordroo]

You'll have to check. Turn off your truck. Use a multimeter or test light to check the 12VDC power pin to ground (i.e., the blade at the 1 o'clock position to the blade at the 7 o'clock position).


Some TV's will kill the power with the ignition off, some won't.

[ABFJV]

Thanks rockfordroo, that makes sense to me.

[Herk7769]

With my 2008 GMC Sierra, the battery pin is hot all the time.

[Mr. Dan]

Originally Posted by Herk7769

With my 2008 GMC Sierra, the battery pin is hot all the time.

Same with my ‘14 Sierra.

[TitanMike]

All it takes to make those normally hot 12 v pins dead when the key is off is to install a continuous duty solenoid inline. Find the feed up under the hood (often with a circuit breaker on it) and put the solenoid inline. Connect the control wire to a circuit that is only hot when the key is on. Often there is an unused position on the fuse block that makes a good source of switched power.

Then there is no worry of draining the TV battery on a cold night or forgetting to plug in the trailer when you leave.


PS: These solenoids look like starter solenoids but if you buy one, make sure it's rated for continuous duty.

[rockfordroo]

Here's one:

https://www.amazon.com/Auto-Truck-Re.../dp/B007JPPQH6

You can get them at any auto parts store, too.

I put one in my 2009 Silverado, but wired it to a rocker switch I put in the dash so I could turn it on/off whenever I wanted to.

Didn't bother on my new truck, since I don't really leave the trailer plugged in for long periods, except when stopping for lunch or something.

[Larry-NC]

Originally Posted by rockfordroo

I put one in my 2009 Silverado, but wired it to a rocker switch I put in the dash so I could turn it on/off whenever I wanted to.

If I were to install a solenoid, I'd wire it to the ignition (key switch) circuit. If I used a separate switch, I'm sure I would forget and leave it on.

Larry

[rockfordroo]

Originally Posted by Larry-NC

If I were to install a solenoid, I'd wire it to the ignition (key switch) circuit. If I used a separate switch, I'm sure I would forget and leave it on.

Larry

Sometimes it's nice to be able to turn it on without needing to get out the key and turn on the ignition.

Gives you added flexibility, but yes, if you leave your TT plugged in a lot, you could forget to turn it off.

All depends on how you roll.

[PenJoe]

On our Ford E450 chassis, the power supply to the TT/toad is the center pin. If the TV ignition is on, the power is live, ignition off, no power. You can check this simply with a cheap continuity tester. Virtually any 12 volt light bulb with a + and - wire attached will work.

On some circuits, the center pin can be used for reverse lights.

[Murbella7]

Originally Posted by ABFJV

We sometimes overnight in a rest area or WalMart. I have been unplugging the 7 way connector from the RV to the truck thinking that possibly while using the house battery (1) to power the TH it will also be drawing power from the truck batteries (2) thru connector charge line. Is it necessary to disconnect from the truck? The truck is a 2003 F250 7.3L if that makes a difference.
Thanks

Been there, done that. Yes, if you leave the connector in it will draw power from the truck battery, unless there is a diode in the circuit that prevents it.

[TitanMike]

Originally Posted by rockfordroo

Here's one:

https://www.amazon.com/Auto-Truck-Re.../dp/B007JPPQH6

You can get them at any auto parts store, too.

I put one in my 2009 Silverado, but wired it to a rocker switch I put in the dash so I could turn it on/off whenever I wanted to.

Didn't bother on my new truck, since I don't really leave the trailer plugged in for long periods, except when stopping for lunch or something.

I personally would use one like this:

https://www.walmart.com/ip/NEW-12V-150A-CONTINUOUS-DUTY-SOLENOID-RELAY-FOR-GOLF-CARTS-300A-IN-RUSH-1114208/184081686?wmlspartner=wlpa&selectedSellerId=1134&a did=22222222227048897229&wmlspartner=wmtlabs&wl0=& wl1=g&wl2=c&wl3=146902899258&wl4=aud-261800281900la-259848050095&wl5=9033354&wl6=&wl7=&wl8=&wl9=pla&wl 10=112343853&wl11=online&wl12=184081686&wl13=&veh= sem

They are rated for 150 amp. If you connect a discharged battery to a fully charged battery, there is a pretty large current flow for a moment or two. This large inrush current can often destroy the smaller relay's that are rated for 30-40 amp. The larger the battery bank that is discharged, the higher the inrush current. The only limiting factor is the resistance of the connecting wire. Repeat a few hundred times and the little ones can fail. For the same price, essentially, why not go with the "big dog"?

[ABFJV]

Thanks to everyone for their input. I checked my truck as rockfordroo suggested in his first response and sure enough, ignition off and there is no draw, only with ignition on is there current thru the charge blade on the connector. I checked it 3x's just to be sure.

[babock]

For people with GM products where it is hot all the time, install a BlueSea ACR on the trailer side between the battery and the trailer connector. This will only allow power transfer to occur when one of the batteries is getting charged. This of course would charge your truck battery as well if the trailer converter/solar is operating.

[Jim in Halifax]

Furnace fan will kill my Silverado battery in a short peirod of time

[rockfordroo]

Originally Posted by TitanMike

I personally would use one like this:

https://www.walmart.com/ip/NEW-12V-150A-CONTINUOUS-DUTY-SOLENOID-RELAY-FOR-GOLF-CARTS-300A-IN-RUSH-1114208/184081686?wmlspartner=wlpa&selectedSellerId=1134&a did=22222222227048897229&wmlspartner=wmtlabs&wl0=& wl1=g&wl2=c&wl3=146902899258&wl4=aud-261800281900la-259848050095&wl5=9033354&wl6=&wl7=&wl8=&wl9=pla&wl 10=112343853&wl11=online&wl12=184081686&wl13=&veh= sem

They are rated for 150 amp. If you connect a discharged battery to a fully charged battery, there is a pretty large current flow for a moment or two. This large inrush current can often destroy the smaller relay's that are rated for 30-40 amp. The larger the battery bank that is discharged, the higher the inrush current. The only limiting factor is the resistance of the connecting wire. Repeat a few hundred times and the little ones can fail. For the same price, essentially, why not go with the "big dog"?

I showed a 40amp relay, and my truck had a 30 amp fuse, so I wasn't too concerned. The fuse should blow before any damage to the relay. I highly doubt you can push even 30 amps down the wire unless you've got a dead short.

[otisite2016]

If you are leaving the ignition switch on you are going to drain the battery even while towing

[TitanMike]

Originally Posted by rockfordroo

I showed a 40amp relay, and my truck had a 30 amp fuse, so I wasn't too concerned. The fuse should blow before any damage to the relay. I highly doubt you can push even 30 amps down the wire unless you've got a dead short.

If that factory wire is upgraded and a high current connector used to transfer power to the TT so the battery bank can be charged faster from the TV, you will see far more than 30 amp. If left stock, with a max of #10 wire (which is what the 12V wire is in a 7-wire cord), you're right.

For those who want to charge faster, just install heavy wire as if you were going to use a "Rescue Winch" at the rear hitch. Charging current to the TV battery bank is limited only to the available output of the Alternator that is not consumed by engine systems and accessories. Otherwise, with the factory wiring, you merely have a 'trickle charger".