How to install battery isolator for 7 pin positive lead?

[DFrank]

I've installed isolators in my third gens for campers, but I think I cheated by just running a new dedicated 12v wire to the camper batteries and dropping he 12V + from the 7 pin connector.

Truck is a 2019 3500 with tow package, if it matters.

But how do you limit the 7 pin 12V+ to being ignition hot, so the trailer or whatever can't drain the engine batteries?

I'd prefer to avoid janky fire replacement, but the the only way I can think of, would be to replace the fuse that protects that circuit with a jumper wire that puts ignition hot power from the aftermarket isolator to the circuit, and move the fuse into the ignition hot jumper.


is there a cleaner way?

And while we are at it, if I have a good battery in my trailer, do I need to have the little one that is dedicated to the breakaway brake? That battery has been long dead, and it seems silly to replace it, if I have power from a better source.

Aslo, I think my trailer brakes are an afterthought, and I am not sure they are wired right.

I cant find where the break away switch is tied in to them. I have a red and a white line run to the brakes and I am not sure if the white is the ground or the break away, back up power, and they are auto grounded to the chassis in a way that I can't see.

A muliti meter shows a completed circuit when I test those lines, when they are connected to nothing else. Seems if one was the break away switch, it would not complete the circuit, unless the break way tab was removed, right?

And what else can one do to improve the "charging power" of vehicle to trailer?

What would one of those expensive Redarc units do to help this? It seems the alternator is giving 14.4V always, and doesn't slow the power like some modern cars do, when there is less demand.


Thanks,

Dave

 

[AH64ID]

Are you looking to do this for one specific trailer? Or all trailers you tow?

As long as the trailer battery is wired to the emergency brake setup you’re fine. That’s how my trailers are.

The emergency brake switch should be wired online to the brakes from the 7 pin.

A DC-DC charger hard mounted to the trailer is the best way to boost trailer battery charging.

 

[petersonj]

I don't have the answer about the best way to change the battery supplied charging circuit to a switched accessory circuit, but I think I can shed some light on the a couple of other things you mention.

Yes, you can run your breakaway switch from you trailer battery source. I have a small travel trailer with two 6 volt batteries mounted on the tongue. My breakaway switch gets its power from them.

Checking wire continuity with a volt / ohmmeter is not necessarily the best tool to use.

I would use a test light with an incandescent bulb. Incandescent bulbs provide a bigger electrical load in the circuit being tested than a multi-meter does. Consequently, conclusions drawn will be more accurate using the test light. The test light will be bright if the circuit is good and will dim if the circuit is poor. A voltmeter will read the same voltage under either condition and an ohmmeter can read good under either condition. This is because there is very little electrical load imposed by the multi-meter.

And what else can one do to improve the "charging power" of vehicle to trailer?

I am not sure of exactly what you are looking for here. There really is no need to worry about battery voltage at the trailer if everything is operating properly. Voltage at the battery in the truck will vary depending mostly on the ambient temperature since there is a battery temperature sensor under the driver side battery that influences upper limit of the voltage regulator.

The farther the positive and negative wires are away from truck battery, the lower the voltage differential will be between them. Example for a normal operating system:

* Engine idling, 90° day - truck battery at 13.6 volts - trailer battery at 13.0 volts (normal voltage drop through wiring)
* Engine idling, 40° day - truck battery at 14.2 volts - trailer battery at 13.6 volts (normal voltage drop through wiring)

As you can see, both examples show a .6 voltage drop just due to the length of wiring. This natural voltage drop is actually a good thing, as it will not overcharge batteries when driving for long periods of time.

- John

 

[AH64ID]

I am also looking to improve the voltage at the batteries of the trailer. I am not anymore concerned about them overcharging than I am the truck batteries overcharging. I rarely get more than 10A from the truck wiring to the batteries of my 5th wheel. It would take 15 hours of driving to top them off from a 50%SOC in a “perfect” world, and really more like 36 hours in actuality. Most of my camping drives are 3-4 hours max which is insignificant in terms of recharging a 300AH battery bank at up to 10A of charge.

With a separate connection a DC-DC charger will adjust the output voltage to optimal for charging, and even drop to a float on a long enough drive. For a 40A DC-DC charger you just need to supply it with a 60A circuit that’s between 9-32V and it controls it output based on your battery type. With a 2AWG wire you can charge the trailer at 40A and optimal voltage, pretty good deal if your usage warrants this. One has to be campsite hopping frequently without shore power for this to really pay for itself, which we don’t do often enough to justify, yet. We don’t camp with shore power, but we rarely move sites on a single trip.

@DFrank I bought one of these years ago to install to isolate my TT from the truck and never got around to it. We camp so rarely where the truck and trailer stay connected that I never prioritized the install.

https://www.bluesea.com/products/7601/m-Series__Automatic_Charging_Relay_-_12_24V_DC_65A

If I were to install it now I would probably put it under the hood. I’d find the battery + wire coming off the fuse box, cut it, and put that ACR inline. This would work for any trailer and with the 5th wheel or bumper pull 7-pin.

 

[DFrank]

Thanks AH, and PetersonJ. I still don’t follow what one of these dc to dc chargers do that won’t happen if the 12v+ wire goes from the 7 pin wires to the trailer’s battery?

they are very proud of the Redarc version of this device, so it must do something ?

Peterson, thanks for the tutorial about circuit testing. I feel
Like I know just enough about that to be dangerous, or at least inefficient with my circuit testing.
I need to buy one of those incandescent test lights that has its own built in power source.

Another project that is stymieing me on this trailer is how to wire reverse lights that I can make come on with reverse, but also use as scene lighting at camp, without back feeding the truck, or even each other for that matter. I’ve got some flood lights on the back and each side of this snowmobile trailer, and it’s easy to wire them to all come on with reverse, but how can I also make them individually switched (from inside the trailer) for use at camp, without back feeding the truck, or even back feeding each other, as is happening now? I have the rear and each side individually fused, and the plan was to put a diode fuse in each, but I’ve only found these as 1 amp Air Conditioner fuses, which would blow even on the low draw of these Led flood lights.

Thanks

 

[Turbo Terry]

I think rv places could either do what you need done or explain your problem areas. They do this stuff all the time. They put a isolator in my new truck to charge the slide in camper battery. It charges going down the road or truck running but never drains the two truck battery’s when I use the camper . It’s mounted under the hood

 

[AH64ID]

With the standard 7 pin 12V wiring you get a voltage drop, like @petersonj mentioned, which provides inefficient charging. Voltage drop varies based on amp draw, so the more depleted a battery is the more voltage drop there is. When the trailer batteries are full, or near full, that voltage drop is less. I’ve seen as little as .1-.2 voltage drop, but the camper batteries were above 95% SOC and the amp draw was minimal.

What a DC-DC charger does is take lower voltage DC power and increase the voltage to a better charging voltage. This does come at a cost thou, meaning that it may take 60A of power from the tow vehicle to put 40A of charge into the trailer batteries. They aren’t quite that inefficient, just showing an extreme for easier understanding. Let’s take that voltage drop into account and say that the DC-DC Charger is drawing 50A and is getting 11.5V from the truck. It will take the 11.5V @ 50A and boost the power to 14.4V @ 40A, which is what the trailer battery sees and gets charged by.
You would install a DC-DC charger as close to the taller batteries as you could manage, this way there is minimal voltage drop on the charging side of the charger. With a DC-DC charger you do want to disconnect the 12V+ on the trailer side of the 7-pin to keep from trying to charge the truck and wasting amps.

You can put a relay in for the lights and have both the R circuit and a switch power the relay. A diode isn’t needed, which makes it easier. This is similar to how @John Jensen wired the aux backup lights on his rear bumper.

I’ve done lots of electrical testing over the year and personally I’ve never found a good reason to own a test light. Just get a digital voltmeter, it tells you so much more.

 

[DFrank]

Thanks again AH for the deeper description.


TT, if I wanted to drop my truck off to have something installed, I would not be here asking how's to do it, but thanks for the suggestion.

Maybe the two stroke oil from the snowmobiles will help fix it.

 

[lpennock]

@DFrank I bought one of these years ago to install to isolate my TT from the truck and never got around to it. We camp so rarely where the truck and trailer stay connected that I never prioritized the install.

https://www.bluesea.com/products/7601/m-Series__Automatic_Charging_Relay_-_12_24V_DC_65A

If I were to install it now I would probably put it under the hood. I’d find the battery + wire coming off the fuse box, cut it, and put that ACR inline. This would work for any trailer and with the 5th wheel or bumper pull 7-pin.

With factory wiring putting a VSR in the camper doesn't work very well. The voltage drop of the stock wiring causes the VSR to constantly cycle. If the wiring on the truck side is upgraded to 10ga or larger it somewhat works with Lead Acid batteries but will still cycle with LiFePO4 and a DC/DC. Best to put the VSR as close to the truck batteries/alternator as possible so it doesn't see any voltage drop. I've done a VSR on three of my trucks now and love the the way it works. Truck running it charges the camper, Truck not running it isolates the camper. Can't get much more idiot proof.

BTW: It does have a behavior that if you hook up to shore power before the voltage drops to the trip point it will allow your shore power to keep the truck batteries topped of also. This is because the dropout voltage is about 12.7V so if you get shore power hooked fast enough the truck side won't get down to 12.7.

Another project that is stymieing me on this trailer is how to wire reverse lights that I can make come on with reverse, but also use as scene lighting at camp,

Don't over think this. It is actually a very easy solution. Cut the wire for the reverse lights and install a single pole double throw switch (I prefer a on-off-on but a on-on works). The common lead goes to the the reverse lights. One of switched leads goes to house batteries and one goes to the trailer connector reverse lead. You just throw the switch to house side when you want the lights and back to truck side when you don't. Note: this assumes your ground is common for the lights and house power (most are). If it isn't just use a double pole double throw switch and switch the ground side to the lights also.

 

[DFrank]

Thanks Ipennock.

What does VSR stand for. Do you have one you'd suggest?

Seems like relays in the trailer may be easiest at this point, and it also keeps the feature working with other trucks.

I have switches in the trailer that select for on and off at camp, which will trigger a relay, which can all be triggered by the back up circuit from 7 pin.

Ay reason not to stick with this plan?

Sounds like I should get a DC to DC charger, with isolator wired under the hood, run fat wires to a new power port in the truck bumper, and add a dedicated 12v power line from trailer tongue to the coach trailer battery.

Does the ground need a thick upgrade, or does the chassis ground pretty much cover this?

Do you have a pic of where you placed the isolator?


Thanks again.

 

[lpennock]

Thanks Ipennock.

What does VSR stand for. Do you have one you'd suggest?

Seems like relays in the trailer may be easiest at this point, and it also keeps the feature working with other trucks.

I have switches in the trailer that select for on and off at camp, which will trigger a relay, which can all be triggered by the back up circuit from 7 pin.

Ay reason not to stick with this plan?

Sounds like I should get a DC to DC charger, with isolator wired under the hood, run fat wires to a new power port in the truck bumper, and add a dedicated 12v power line from trailer tongue to the coach trailer battery.

Does the ground need a thick upgrade, or does the chassis ground pretty much cover this?

Do you have a pic of where you placed the isolator?


Thanks again.

Voltage Sensitive Relay. I've used the approximately 140A models. You can use the Blue Sea that was linked earlier but I've just used the ones on Ebay or Amazon without an issue.

I don't see any issue with your plan to use a relay other than the additional current draw which is pretty small.

My personal feeling about a DC/DC is it isn't necessary unless you are running LiFePo4 batteries in the camper. The Current to Voltage curve of Lead acid pretty well aligns with the wiring voltage drop. With LiFePo4 the current doesn't roll off until the StateOfCharge is 100% so the DC/DC is required to get to 100%SOC. Lead Acid Batteries charging current rolls off as the Battery SOC increases so the voltage drop of the wiring also decreases and the voltage comes up.

Ground should be at least as heavy as the power lead if not heavier. I wouldn't depend on the ball connection to provide a good ground and the 7 way ground is just as bad. You don't need to run a ground lead all the way to the battery and can ground to the chassis of the truck but the connection to the camper should provide a real ground and not depend on the 7 way or ball connection if you are upgrading the hot side.

I'll try and shoot a picture later today but I have my VSR tucked pretty out of the way.

 

[AH64ID]

Yes DC-DC is more of a requirement on LiFePo4, but I would still see a large improvement in trailer battery charging while driving with a DC-DC on my Lifeline AGM's since the stock wiring just can't move enough amps and even if you put larger wiring in from the truck to the trailer you wouldn't get the benefit of smaller wiring with a DC-DC charger.

 

[lpennock]

I'll try and shoot a picture later today but I have my VSR tucked pretty out of the way.

Not much to see but here is where I mounted my VSR.

I ran parallel 10ga back to the in-bed 7pin to reduce the voltage drop on the truck side. I also replaced the in bed 7 way with a lug style 7way rather than the connector style factory 7 way so I could connect the heaver wire to the + lug and heavier ground to the ground lug.

My 7 wire cable uses 10Ga for plus and ground. This is something to check on your camper as there are 7way cables that use as small as 16 gauge wire for everything.