Is tow vehicle's battery and trailer's isolated from each?

[afinepoint]

Is the tow vehicle only wired to the trailer's lights and brakes when attached? Was curious if other loads could get power from the tow vehicles's battery such as the power jack?

I would think the trailer and vehicle's batteries would be kept separate since the potentials are not the same. One battery will be stronger and thus discharge to the other as when car batteries are "jumped". The potential (no pun intended) is there to kill you truck's battery and then be stuck.

[2112]

christopherglenn wrote:

2112 wrote:

Allworth wrote:
By the way, I don't like to have shore power and truck power hooked up at the same time. I don't think it can hurt anything, but I would rather not mix electrical systems. Particularly complex ones.

I blew a converter years ago because of this. I had my F150 TV plugged in and running when I connected shore power. The DC side instantly stopped working. I have made sure not to have both plugged in ever since. New technology or proper fusing may prevent this now but I'm not going to chance it.

I doubt that was the cause, with my truck running I am showing 14 volts on the truck batteries - and 13.2-13.5 on the trailer batteries. The set point on my original converter was 13.7, my inverter-charger is 14. There is so much resistance between the truck and trailer batteries that .5+ volt difference (when running / charging) is normal. And yes, the charge line works, there is a voltage drop when I hit the remote start from the trailer, followed by a slight voltage climb a few seconds later.

All I know is I had 12V in the TT just before I connected (fridge was working)and didn't after. The converter had no output and no fuses were blown. This was the first time I had them both plugged in at the same time. Coincidence? I don't know but won't do it again.
We camped the weekend without it.

[christopherglen]

2112 wrote:

Allworth wrote:
By the way, I don't like to have shore power and truck power hooked up at the same time. I don't think it can hurt anything, but I would rather not mix electrical systems. Particularly complex ones.

I blew a converter years ago because of this. I had my F150 TV plugged in and running when I connected shore power. The DC side instantly stopped working. I have made sure not to have both plugged in ever since. New technology or proper fusing may prevent this now but I'm not going to chance it.

I doubt that was the cause, with my truck running I am showing 14 volts on the truck batteries - and 13.2-13.5 on the trailer batteries. The set point on my original converter was 13.7, my inverter-charger is 14. There is so much resistance between the truck and trailer batteries that .5+ volt difference (when running / charging) is normal. And yes, the charge line works, there is a voltage drop when I hit the remote start from the trailer, followed by a slight voltage climb a few seconds later.

[2112]

Allworth wrote:
By the way, I don't like to have shore power and truck power hooked up at the same time. I don't think it can hurt anything, but I would rather not mix electrical systems. Particularly complex ones.

I blew a converter years ago because of this. I had my F150 TV plugged in and running when I connected shore power. The DC side instantly stopped working. I have made sure not to have both plugged in ever since. New technology or proper fusing may prevent this now but I'm not going to chance it.

[RoyB]

On my 2010 F150 There was a RELAY and FUSE in an unmarked envelope in the glovebox of the truck. The FORD manual told me where these two things were to be installed in the front fuse panel.

GOOGLE says "These are the fuse and the relay that powers the 'constant power' or 'battery charging' circuit on your 7-pin trailer connector. The relay goes in position #9 and the fuse goes in position #21 in the power distribution box (fuse box) in the engine compartment." This is for a F150 FORD so don't know if it will be the same for the F350 or not. I think the magic words in the manual is to look for TRAILER TOW FUSE and TRAILER TOW RELAY.

This becomes activated when the ignition key is turned ON and will go OFF a few seconds after you turn the ignition key OFF.

One post on the F150 forum said the fuse and relay kit p/n is AL3T-15A416-AA but mine was in an unmarked baggie. I think you can buy a fuse and relay kit from etrailer. I would just go to Ford Parts dept and pick them up.

Like others have said here this should not be use as anything PRIMARY as the small wiring to the 7-way trailer connector is small. Should only be used as a trickle charge PLAN B for the trailer batteries. You DO NOT want to mess with RULE #1 and that is to mess with the truck START battery system as this is the way you get back home.

For camping off the power grid I have installed a smart mode converter/charger on my trailer and have increased the trailer battery power to run all the things we want to run from 120VAC using an inverter and 12VDC things directly connected to the battery bank. When the battery bank gets down to around 12.0VDC (appx 50% charge state) we will run our 2KW generator to re-charge the batteries. The game plan of course is to plan for this to happen around 8AM in the morning when you are allowed to run your generator at most camp grounds for a quick three hour generator run time.

My truck START battery stays fresh, my trailer batteries all get immediately charged back to their 90% charge state for the next day/night camping run, the camping world is running at normal.

Roy Ken

[Chuck_thehammer]

good point Allworth.

if the trailer charger is charging over the TV alternator set point.. even with the engine OFF. the voltage regulator will try to reduce the voltage and overheat itself.. killing the alternator in the TV.

[Allworth]

Given the size of the wire used for the hot line to the trailer, the TV provides a trickle charge at best. It will top off the battery from the power used when hooking up after disconnecting shore power, but will probably not charge a dead battery up to full (Unless: A--the hot line has been rewired. or B--you drive a real, real long way.)

By the way, I don't like to have shore power and truck power hooked up at the same time. I don't think it can hurt anything, but I would rather not mix electrical systems. Particularly complex ones.

[Chuck_thehammer]

most cars/trucks charging systems are 13.8 to 14.2...

and for connecting TV and TT. remember the wire connecting the batteries..
that's about 30 feet total.. so there is LINE LOSS.. and acts like a resistor under high power draw...when TT battery is low.

[afinepoint]

Terryallan wrote:
Yes on Fords, the batteries are only connected when the switch is in the on position. And yes. If you have the towing package relay installed on your Ford. It in fact charges the trailer battery as you drive down the road.

I plan to get new batteries and was thinking about GEL deep cycle. However most manufacturers recommend charging them to at least 13.8 volts, but no more than 14.1 volts. I don't know how this would play out while going down the road. Have never read the alternators output at 2000 rpm on the engine. The dash display is not digital nor calibrated.

Reg

[wa8yxm]

Generally speaking the towed and towing batteries are isolated UNLESS THE ENGINE IS RUNNING and at that time they are conjoined. (So they both charge)

A device on the towing vehicle called an Isolator (Either a diode box or a relay/contactor/solenoid) does the magic.

[afinepoint]

Thanks to all.

[afinepoint]

Thanks to all. I don't know if I have the towing relay. Will look tomorrow.

Nice to know the battery will likely charge on the road.

So I guess the refrigerator pulls to much power to run off the battery when on the road. Per our salesman it's either propane or shore power and on the road it's propane.

[mowermech]

ScottG wrote:
On your FOrd it is indeed separate and they are only connected together when the key is on.
Dodge and GM are always connected together.
FWIW, I've had it both ways and I actually prefer them to be tied together.

Actually, it depends...
Some vehicles are built so that the trailer power lead is only energized with the ignition ON.
Some are not.
Of those that are not, some have been fitted with aftermarket parts so that the trailer power lead is only on when the ignition is ON (like my old '94 Dodge 3500).
You won't know exactly what you have until you test it.
Good luck.

[ryhed]

+1

[Terryallan]

Yes on Fords, the batteries are only connected when the switch is in the on position. And yes. If you have the towing package relay installed on your Ford. It in fact charges the trailer battery as you drive down the road.

As to your electric trailer jack. If your trailer battery is dead, all you have to do to power the jack. Is connect the 7 pin, and turn your truck key to on. Then the truck will send power to the trailer, and power every thing in it, that runs off battery. It's best to have the ruck running when you do that.