Dual-Battery Trailer wiring diagram

[evldave]

Gonna be at our family cabin for 2 weeks, no electricity and decided last-second to get a fridge. Trying to set up a dual battery system w/cheap locally-sourced parts and stuff from the garage. There's no room in my H3, so I'm gonna have to have the 2nd (and maybe 3rd) battery on the trailer, via a 2nd 'aux' connector in addition to the regular 7-round plug I currently have.

Did I miss anything? I've got a simple constant-duty 80A solenoid laying around, so I obviously won't be reverse jump-starting the truck if the main battery goes dead, I just want to do basic charging of the trailer batteries when the truck is running and disconnect it when I shut the truck off. 6ga should be plenty, right?

[THATSALEXUS?]

The biggest potential proble I see is that your alternator probably puts out more than 75 amps. That means if you completely drained both rear batteries, the alternator would basically put everything it had into charging them, potentially blowing both 75 amp fuses.

You also need to verify that the connector can handle the necessary continuous currrent.

A cursory search on the interweb indicated that H3's have 120 amp alternators.

[ntsqd]

Batteries have an internal resistance. I don't know that an alternator can force it's max output into a battery.

What I see missing is a common ground. Can not rely on the trailer coupler to provide that. It needs a ground wire of the same capacity as the charge wire.

[rusty_tlc]

That is pretty much the same hook-up I've used on every trailer I have owned. As ntsqd (buy a vowel dude ) mentioned don't count on the coupler for ground.
The standard 7pin RV connector has a provision for a ground connection. Just make sure it is connected and has heavy enough wire.

[ntsqd]

I doubt that the ground connection in a 7 pin connector can carry 75 amps, but combined with the coupler, and depending on what the chosen coupler is, it may be all good.

I had to remove 3 letters from my desired very first email 'name' due to length constraints. I've just kept that shortened version ever since. I'll give y'all a hint, two of the missing vowels are the same.

[eugene]

The only real issue with those kind of setups is if your trailer batteries are really low and you turn on the ignition you energize the relay and now put two dead batteries across one good one you could load it down to where the vehicle won't start. Thats why I always preferred a (diode) isolator, but they make delayed or voltage sensitive triggers for relays. If you know electronics a simple RC circuit can be made to delay engagement for a couple seconds allowing you to turn the key to start and then back then the relay engages to prevent loading the starting battery.

I've always preferred the diode isolators, espically since modern systems compensate for the diode voltage drop, but now being faced with replacing mine for a third time due to the studs rusting and wanting to upgrade my alternator and higher amp isolators are about as big as the battery, I'm thinking of just going to a solonoid myseld.

[evldave]

Hey this is all good info...

I'll add a power/ground coupler separate from my 7-pin plug, that should cover the current and grounding issues.

I think until I can put together a delayed relay switch for the solenoid, I'll just plug in the coupler separate after I start the truck and let it run for a bit (it'll be a hyster plug, so the current shouldn't be an issue)...since it's separate from the regular trailer plug, it doesn't affect towing at all.

I've heard that the 0.7v drop for diodes can mess w/modern computer vehicles? I've avoided them for that reason (and the size). I didn't know the electronics could compensate.

H3s have a 135A alternator. I think the basic alternator for most GM vehicles is 120A.

[THATSALEXUS?]

Batteries have an internal resistance. I don't know that an alternator can force it's max output into a battery.

It may not be a problem, but I have seen the current output of a battery charger go to 30-40 amps when connected to a dead battery and set to high.

The only real issue with those kind of setups is if your trailer batteries are really low and you turn on the ignition you energize the relay and now put two dead batteries across one good one you could load it down to where the vehicle won't start.

You can overcome this by triggering the solenoid with accessory, not ignition power, or with a second relay that interrupts the ignition voltage to the solenoid during "start".

[Albin]

It may not be a problem, but I have seen the current output of a battery charger go to 30-40 amps when connected to a dead battery and set to high.




You can overcome this by triggering the solenoid with accessory, not ignition power, or with a second relay that interrupts the ignition voltage to the solenoid during "start".

Definitely do this!

Also, I would recommend a 200 amp constant duty relay vs the 75 amp version. Since a 200 amp only cost about $15 or so, this would probably be a good idea. Western makes a plow relay, I'll see if can dig up the part number.

Good luck.

[AxeAngel]

Definitely do this!

Also, I would recommend a 200 amp constant duty relay vs the 75 amp version. Since a 200 amp only cost about $15 or so, this would probably be a good idea. Western makes a plow relay, I'll see if can dig up the part number.

Good luck.

Or just get a smart isolator or an automatic charging relay like blueseas or a higher end cole hersee