Unplugging from tow vehicle

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Unplugging from tow vehicle

Postby Crabapple » Sun Aug 21, 2016 8:09 pm

I seem to think I read somewhere three yrs ago when I got my little camper that one should unplug the trailer from the tow vehicle when camped to avoid also running down the vehicle battery if the trailer battery gets low.
This true or bogus?
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Re: Unplugging from tow vehicle

Postby Philip » Sun Aug 21, 2016 8:43 pm

It depends if the tow vehicle plug is wired for 12 volt constant power to the plug. If it is unplug the trailer.

I never hook a 12 volt constant to a trailer plug. I have seen to many vehicle plug wiring melting from water/ debris in the back side plug.

I put the 12 volt constant on a switched relay. It only operates if a switch is thrown or wired into a circuit that is only hot when the engine is running.
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Re: Unplugging from tow vehicle

Postby troubleScottie » Sun Aug 21, 2016 8:55 pm

The trailer draws power from whatever battery / batteries that are connected to it.

Assuming that your have the appropriate connector eg 7 pin connector or bypassed the towing connector with your own power/ground cabling, this means the trailer is connected to the towing vehicle (TV) battery.

Depending on how it is wired determines if the trailer is drawing power from the TV.


The simplest connection is a direct connection with no switches/solenoids/relays between the TV battery and the trailer. Any power consumption in the trailer may draw on the TV battery. Disconnecting the TV from the trailer will break this connection. Your manual disconnecting acts a switch. Not particularly eloquent but it would work. You do of course have to remember to do this. You should have at least one fuse near the TV battery.

The next most likely scenario is there is a solenoid (isolating solenoid) between the TV battery and the trailer. The isolating solenoid does just that, isolates the TV from the trailer. The solenoid is designed to turn on (close and complete the connection) when the car is running OR a switch is thrown. Thus provide power to the trailer. If the solenoid turns off (opens), no power can be drawn from the TV battery. Obviously when the car is running, any extra power generated in the TV is sent to the trailer. The nice thing is that you never have to remember, the solenoid opens when the car is turned off.

One could have a simple switch in stead of the solenoid. (this switch will have to rated for high amperage flow eg 40-100A). Probably not the best plan.

Beyond this there numerous schemes to allow the TV battery and trailer battery to be connected or not connected using relays or inverters or whatever.
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Re: Unplugging from tow vehicle

Postby Dale M. » Mon Aug 22, 2016 8:53 am

Others have nailed it.... My Chevy PU is constant (always on) power to plug, when I wired my Jeep I put it on relay (solenoid) so if ignition is off, power to trailer is off.... Have contemplated adding relay to PU so I can have luxury of not having to disconnect for short overnight stops..

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Re: Unplugging from tow vehicle

Postby Camp4Life » Mon Aug 22, 2016 9:19 am

Here's a picture of a 7-pin plug and what pin is what:

Image

If you want to be sure about if the connection is hot or not while parked, a cheap multimeter will tell you this. Just connect the multimeter leads to the pins for the +12v and Ground while your vehicle is off. If you get nothing, then your TV is wired to only send power to the trailer when the engine is running. If you're getting any voltage (~12v), then your TV is always sending power to the trailer and you should ensure you ALWAYS unplug it.

My uncle's old truck always sent the 12 volts. His solution was easy. He hung a sign from chains in the doorway of his trailer that said "UNPLUG THE TRUCK!", so as soon as you open the camper door, there's the reminder (it was removable). Effective and low-cost solution :lol:
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Re: Unplugging from tow vehicle

Postby Crabapple » Mon Aug 22, 2016 9:41 am

You guys are awesome! Thanks :)
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Re: Unplugging from tow vehicle

Postby Crabapple » Thu Aug 25, 2016 11:57 am

Yep reading 12.96V with engine off. Although my devices are supposed to cut out b4 draining either battery too much, I think I will continue to unplug the trailer while camped!
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Re: Unplugging from tow vehicle

Postby Camp4Life » Thu Aug 25, 2016 12:01 pm

Crabapple wrote:Yep reading 12.96V with engine off. Although my devices are supposed to cut out b4 draining either battery too much, I think I will continue to unplug the trailer while camped!


That's a good idea. Good thing you checked! You could wire up a relay that will turn off the power when your engine is off, but honestly, as long as you remember to do it, it's much easier to just unplug it :FNP
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Re: Unplugging from tow vehicle

Postby Nobody » Thu Aug 25, 2016 6:15 pm

Many (if not most) modern tow vehicles, especially those with factory tow packages, have a 'switched' 12v wire from the battery/alternator to the 7-pin connector. It's easy to check this & if yours doesn't, a relay/solenoid is relatively easy to install that'll automatically shut off power to your TD when the tow vehicle ignition is off. Or, just get into the habit of disconnecting the tow vehicle from the trailer when you stop for the night...
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Re: Unplugging from tow vehicle

Postby lrrowe » Thu Aug 25, 2016 7:26 pm

I might be a little confused on this one. If you do not have anything on the trailer draining or drawing power, then why bother with unplugging, that is if you are not removing the trailer from the TV.

If all your trailer lights (call them off grid lights i.e., reading lights, pumps, USB usage and so forth)'are drawing 12v from the TD/CT's house batteries, then again, there is no drain on the TV's battery.
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Re: Unplugging from tow vehicle

Postby GuitarPhotog » Thu Aug 25, 2016 7:54 pm

lrrowe wrote:I might be a little confused on this one. If you do not have anything on the trailer draining or drawing power, then why bother with unplugging, that is if you are not removing the trailer from the TV.

If all your trailer lights (call them off grid lights i.e., reading lights, pumps, USB usage and so forth)'are drawing 12v from the TD/CT's house batteries, then again, there is no drain on the TV's battery.


If the two batteries are connected in parallel (the usual method), you will discharge BOTH batteries simultaneously. Just because one is closer to the load doesn't cause the trailer battery to be used preferentially.

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Re: Unplugging from tow vehicle

Postby H.A. » Thu Aug 25, 2016 8:35 pm

lrrowe wrote:I might be a little confused on this one. If you do not have anything on the trailer draining or drawing power, then why bother with unplugging, that is if you are not removing the trailer from the TV.


For the above scenario, short term.
Nothing detrimental occurs leaving both batteries connected.
Just dont park the car & leave them plugged together days on end.
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Re: Unplugging from tow vehicle

Postby Dale M. » Fri Aug 26, 2016 8:04 am

The longer you keep them connected the more time Murphy has to work on situation.... I'm in favor of unplugging or adding disconnect relay on TV...

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Re: Unplugging from tow vehicle

Postby aggie79 » Fri Aug 26, 2016 8:46 am

H.A. wrote:
lrrowe wrote:I might be a little confused on this one. If you do not have anything on the trailer draining or drawing power, then why bother with unplugging, that is if you are not removing the trailer from the TV.


For the above scenario, short term.
Nothing detrimental occurs leaving both batteries connected.
Just dont park the car & leave them plugged together days on end.


The above is true only if both batteries are charged equally. If one or the other is lower voltage, they will equalize with no intervening disconnect.


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Re: Unplugging from tow vehicle

Postby Nobody » Fri Aug 26, 2016 8:54 am

If your TV/Trailer electrical connector has a 'live' wire (non-switched, or no shut-off relay/solenoid) from your TV battery, use of 12v items (appliances. lights, etc) in the trailer will use 'power' from both batteries (TV & trailer) simultaneously... On my first truck camper (1960s) I had no battery in the camper & the truck battery was wired directly to the 12v system in the camper. We've camped for days without depleting the truck battery but I was always careful of power use & if we were going to be parked for more'n a few days I'd start & run the truck engine for a while every few days to keep the battery charged. I still do that with our current truck camper even tho it has a 'house' battery, to keep that battery charged if I'm parked in one spot for more'n a few days without electrical hook-ups.
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