Question about wiring/charging trailer battery

[adventurebuddies]

Hi. I just got a trailer and have a couple of questions regarding charging the trailer batteries.

Tow vehicle has a starting battery and an AGM battery linked via a selectable 200A solenoid. The second battery runs the winch and all accesories so the starting battery is used only for fully stock applications.
I have #4 cables already routed from the second battery to the back of the vehicle to power my fridge, compressor, and rack mounted aux lights.

The trailer has two Optima blue tops. How should I charge them? I believe the "standard" method is to route #10 wires from the main battery to the 7pin RV plug for trickle charging the trailer batteries. The trailer is already setup up for this configuration, but the tow vehicle is not.

The other method (I think) would be to have two plugs - one for the standard RV 7pin and another for the #4 cables. That way the two trailer batteries would essentially be connected to the tow vehicle's second battery and thetrailer batteries could be brought in and out of the charge loop by my 200A solenoid.

I read somewhere that AGM batteries allow current to move very quickly and gell (optima) batteris require a very slow charge. If the AGM and optimas were linked, would I risk damaging the optimas if they were low on charge and the AGM rapidly flooded them with current?

So - which method is preferable? can I wire both methods? Any opinions or experiences? Thanks!

 

[Airborne Lee]

As a yellow top owner I thought the blue tops were marine batteries (I could be wrong). I know you can use a yellow top. Marine batteries do need a slow charge. I am not sure if the trailer power supply qualifies for this or not. You might ask a trailer shop a general question like that. You could also ask optima.

 

[Mr. Leary]

I would invest in some sort of voltage regulator. A trickle charge is always best for the batteries in my experience. What kind of axle do you have in the trailer? If you have the rear end out of a truck in it, you could run a belt on it and charge your batteries off of an ordinary alternator. I haven't tested this yet, but have been kicking it around in my trailer design plans! (another one of my hairbrained ideas!)

 

[brentbba]

Standard method would be to have the cruiser wired for a 7 pin adapter with power coming from either your main or secondary battery on the truck. Mine, when I had only one battery was, obviously wired there. When I installed a second battery, I rerouted the power to the 7-pin from the primary to the secondary battery.

The only way those trailer batteries are going to charge is when you're hooked up.

When I get around to putting a battery on my trailer, I'll look into a shore power adapter readily available at marine stores as it's a very standard item for boats in dock. No research on them or $$ yet. AT sells them as an accessory for their trailers.

For what you've already got wired to the back of your truck, I'd just go from there to the 7-pin adapter. Run the wire down and out the grommet behind the rear quarterpanel to under the truck and you're done.

 

[R_Lefebvre]

I'd hook 10ga up to the solenoid you've got to charge the second truck battery. That way if the trailer is hooked up, but truck is not running, the trailer won't be drawing from the main truck battery.

I also think 10ga all the way to the front will help slow down the charge rate to a manageable level.

 

[adventurebuddies]

I'd hook 10ga up to the solenoid you've got to charge the second truck battery. That way if the trailer is hooked up, but truck is not running, the trailer won't be drawing from the main truck battery.
I also think 10ga all the way to the front will help slow down the charge rate to a manageable level.

Had not considered that but it sounds like a great idea. I agree that the length of #10 wire will mitigate any risk of fast charging. I'm going to run the charge wire in the 7pin either way so I have a universal setup.

 

[elmo_4_vt]

Unfortunately, the wire size won't change the rate of charging... Only the difference in potential between the two batteries and their internal resistance.

That said, the resistance in the batteries should keep the charging current down to what they can handle. I'll have to look into that part a little further and I'll reply back, but that's how your alternator works, so I think it still makes sense.

My plan is to just run a #10 tap to my 7-pin connector from my larger wiring that I will have already going to my rear winch. Those will be run from a second, isolated battery just like your setup. And I will fuse the #10 wire in order to protect the wire from burning up and potentially catching fire. Running a completely separate #10 wire from you isolator is unnecessarily and will result in more of a voltage drop to the trailer batteries, and they will therefore not charge as fully.

Don

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[R_Lefebvre]

I'm sorry, but you're wrong on most counts.

The wire size will change the rate of charging. A long, smaller wire has a higher resistance to current flow, which will naturally reduce the charge rate of the trailer battery. This is basic Ohm's law stuff.

However, as the battery comes up to full charge, and current drops off, so does the voltage drop through that 10ga wire. Eventually, current will approach zero, so the voltage drop across that wire will also approach zero, and the trailer battery will achieve the exact same voltage as the charging system. You will achieve a full charge.

The problem with your suggested setup, is that any time that trailer is connected, but the vehicle isn't running, you will have a destrcutive see-saw set up between the trailer battery and your secondary battery. They will both drain eachother.

That made me realize my suggested setup through his charge solenoid will have the same effect as you'll have a current transfer between the trailer battery and secondary through that solenoid terminal.

You should, therefore, use a second solenoid switched off accessory power, to link the trailer battery in individually. You could simply use a standard 30Amp automotive relay, fused for 30 Amps (obviously).

 

[adventurebuddies]

you will have a destrcutive see-saw set up between the trailer battery and your secondary battery. They will both drain eachother.

I was thinking the same thing. Technically, connecting the trailer #10 charge wire to the #4 cable at the back of the vehicle would "link" the vehicle second battery and the two trailer batteris in series so it's like having 3 trailer batteries. The consequences of this really aren't that severe though they will drain each other. At least the starting battery remains unaffected. I could also make a habit of unplugging the trailer connector in camp. Or I can wire the relay as suggested.

 

[OlympiaFJ60]

To keep the trailer isolated from the truck, use a 40amp relay triggered by the acc/run ignition maybe?

 

[R_Lefebvre]

Yeah, that's basically what I'm saying. If you can get a 40 Amp, would be even safer than a 30.

 

[OlympiaFJ60]

Ah, yes, so it is :ylsmoke: I guess I was skimming too fast, or it was too late

Yeah, that's basically what I'm saying. If you can get a 40 Amp, would be even safer than a 30.

 

[adventurebuddies]

This has been very helpful. Thanks. I think the best solution is:

Wire the #10 trailer charge battery wire from the #4 cable from the vehicle second battery already at the back of the vehicle. Setup up a 40A relay triggered from an acc/ign wire to the #10 charge wire. So, the trailer batteries will only link into the vehicle charge loop when the second vehicle battery is connected via the 200A solenoid AND the key in on in the ignition.

Wouldn't have thought of it on my own but it's a great solution that requires minimal user interaction.

 

[R_Lefebvre]

Should probably work. I would just be careful with the currents you get out of that system, since you'll have less 10ga. You could fuse it with a 20Amp fuse and just go trial and error. But whatever you do, don't fuse it for more than the max that a 10ga can take, obviously.

 

[adventurebuddies]

Just thought of something else to address. There will be an Engel 60 fridge in the trailer. Is the #10 charge wire sufficient to powere the fridge while driving without it slowly draining the trailer batteries?