DC/DC Renogy Trailer Install

[AndyRonLI]

Folks
I have a trailer install. Thus no jumpers from an ignition switch. Just the 7 pin trailer connections.
I have a couple of installation questions about my Renogy 40amp DC DC charger with 2 200AH Lithium Batteries (ampere time)
Question 1: So I have seen were folks put in a switch that sees 12V and turn it to the on position to enable the DC/DC and turn it off when they dont want to pull from the tow vehicle. As I always drive with my running lights on to energize my rear view camera, I was thinking of tapping that voltage. That gives me control from the tow vehicle. Any reason NOT to do that?
Question 2: I was way focused on my alternator capacity and how much current the batteries could draw / need. 40 amps seems fine. I have an F350 diesel with the tow package, I believe that is a 120 amp alternator. Howerver, now that I look at the trailer wiring... 10 ga. Hmm. Thinking I should exchange for a 20 amp. Can I get away with the 40 amp? or was that a mistake?
Question 3: I managed to isolate the individual hot wire from the trailer umbilical to the 12V system. But dammed if I can find an isolated ground. Any reason both ground terminals cant simply go to the chassis ground? I have good continuity from the chassis point to the plug. Meter fluctuates between 0 and 0.1 ohms

Any help is appreciated
Andy

 

[Jasgeer]

Here is my Renogy wiring diagram so I can charge while driving or from shore power. I hope it helps!

 

[AndyRonLI]

Here is my Renogy wiring diagram so I can charge while driving or from shore power. I hope it helps!

Jasgeer, so that is similar to what I was thinking. No specific connection to the trailer umbilical ground. I have a Lithium compatible converter/charger, so that hot source can connect direct to the battery hot side rather than going through the DC/DC. An all grounds are bonded together.

I may add some bus bars. still tracing out the DC loads that do not go through the dc panel.
Thanks
ANdy

 

[AndyRonLI]

This is what I think I have.
I would break the 12V+ trailer line and bring it to the converter.

Any thoughts on 40Amp on 10 ga wire?
Andy

 

[AndyRonLI]

So gotta love amazon sometimes. Returning the 40amp DC to DC and have the 20 amp coming. That really only gives me 10 amps to each 200Ah battery, But an 8 hour drive should top off each with 40 amps or so. That should keep the batteries ahead of the road draw, which is only a residential fridge. Now I need to figure out how best to wire this up.

 

[RF_Burns]

So gotta love amazon sometimes. Returning the 40amp DC to DC and have the 20 amp coming. That really only gives me 10 amps to each 200Ah battery, But an 8 hour drive should top off each with 40 amps or so. That should keep the batteries ahead of the road draw, which is only a residential fridge. Now I need to figure out how best to wire this up.

Andy, Sorry I'm a little late with this, But there is an input on the Renogy DC-DC chargers that limits the current to 50%. Therefore with the 40Amp version you have a choice to select 40A or 20A of charge current.

Also the instructions say the input current will be higher than the output by as much as 30% I believe. So I would run a separate fused positive wire from the battery + and negative wire from the ENGINE BLOCK to the trailer. The size of wire will depend on the length of the wire run and the current draw of the DC-DC charger you select. You will also need a weather proof connector of the appropriate rating at the hitch for disconnecting.

 

[AndyRonLI]

So I am trying to avoid new wiring on my truck, that's a much bigger project than I can get done before my trip. I would also need to upgrade the wiring on the trailer side as well. That last is an even bigger project. So if I use the 20 amp version, I may get to 26 amps. I think the existing wiring should be able to handle that. If the 20 amp version also has the 50% switch, I am down to 13 amps. I dont intend for the tow vehicle to do anything more than try and stay with the towing load. That is lights, camera, whatever parasitic load I have and most importantly the inverter which will be running the residential fridge. That shouldn't be much more than 10 amps on average. I have a generator I can run when I am parked. I will be running two 200AH lithiums which should actually be able to handle a 12 hr drive between charges no problem. But why not take advantage of my 320 amp alternator. My tow vehicle is a F350 Diesel.
Any reason to suspect the 10 gauge existing wiring wont handle 10 amps continuous?
Thanks
ANdy

 

[Short_Shot]

10awg is "rated" for 30a.

I put that in quotes because current handling of wire is a much more complex subject.

That said, it also depends on how much allowance you have for voltage drop over the length of the wire.

Personally I prefer to oversize all my low voltage stuff (12, 24, etc) by one size to allow for the crappy conditions involved in outdoor use.


The whole issue is a sliding scale of demand vs supply and if you don't need significant current due to shore power or other sources when stopped then it sounds like you're set with 20a worth of charger on your 10awg wire.

However I never could find a rating on the amount of power those trailer plugs can handle.

 

[RF_Burns]

The DC-DC converter will compensate for voltage drop on the input side. BUT... the lower the voltage on the input side, the higher the current draw then the voltage drops again till it finds its "sweet spot".

If you are like me, you will be starting out with a fully charged battery, so you only need to keep up to the loads. Even at the current limited output of 10Amps, you will likely stay ahead of the draw because the fridge only runs for about 50% of the time.

 

[Zwy]

So I am trying to avoid new wiring on my truck, that's a much bigger project than I can get done before my trip. I would also need to upgrade the wiring on the trailer side as well. That last is an even bigger project. So if I use the 20 amp version, I may get to 26 amps. I think the existing wiring should be able to handle that. If the 20 amp version also has the 50% switch, I am down to 13 amps. I dont intend for the tow vehicle to do anything more than try and stay with the towing load. That is lights, camera, whatever parasitic load I have and most importantly the inverter which will be running the residential fridge. That shouldn't be much more than 10 amps on average. I have a generator I can run when I am parked. I will be running two 200AH lithiums which should actually be able to handle a 12 hr drive between charges no problem. But why not take advantage of my 320 amp alternator. My tow vehicle is a F350 Diesel.
Any reason to suspect the 10 gauge existing wiring wont handle 10 amps continuous?
Thanks
ANdy

Upsize at least one size after you run the full length thru a calculator that determines voltage drop. https://www.wirebarn.com/Wire-Calculator-_ep_41.html

26 amps, 30 feet and under 2% voltage drop comes to 4 gauge wire. You probably wouldn't need to upsize 4 ga but the option is there in case you decided to increase amps down the road.

 

[AndyRonLI]

Upsize at least one size after you run the full length thru a calculator that determines voltage drop. https://www.wirebarn.com/Wire-Calculator-_ep_41.html

26 amps, 30 feet and under 2% voltage drop comes to 4 gauge wire. You probably wouldn't need to upsize 4 ga but the option is there in case you decided to increase amps down the road.

So at 10 amps using a 10 guage gets me 27.63 ft. That sounds similar the total path length from the truck battery to the plus a little bit. So setting the 20Amp DC/DC to a 10 amp limit I should be ok. If I let it go at 20 amps, I need to upgrade. I will make some checks as to what the inverter really pulls for the refrigerator. If I accept a 5% voltage drop, I am definitely ok even at 20amps. The DC/DC converter should make up for the voltage drop. Though at that amperage I may have some hot wires! So short term 10amps OK as is, long term 20 amps with a wiring upgrade.
Thanks for the link to that tool.
Andy

 

[Zwy]

So at 10 amps using a 10 guage gets me 27.63 ft. That sounds similar the total path length from the truck battery to the plus a little bit.

It will take more length than you think it will because you never get a straight line to and from. And when it comes to smaller gauge wires, always move up to next size, if you input 28 feet and 10 amps, you will find that 10 gauge falls short. So just less than 1/2 foot and you will have more than 2% voltage drop.

8 ga however gives another 16.5 feet, that is what you will need to run 27.63 feet. You don't run things to the limit when it comes to moving electrons, many lose this important rule. If you always run an electrical circuit at full capacity all the time, it leads to shortened lifespan and possible disaster.

So setting the 20Amp DC/DC to a 10 amp limit I should be ok.

No, you would need 8 gauge wire. You will have some resistance in the connections also. You will have more than 2% voltage drop if the length is anything close to 28 feet.

If I let it go at 20 amps, I need to upgrade.

Isn't it cheaper in the long run to do the install where you don't need to "upgrade" later? I spent hours planning and installing my system, I prefer the do it right the first time with no need for upgrades.

I will make some checks as to what the inverter really pulls for the refrigerator. If I accept a 5% voltage drop, I am definitely ok even at 20amps. The DC/DC converter should make up for the voltage drop.

Good luck with that 5% voltage drop. Why not make it 10% or even 15%? Why not attempt to shove 100 amps thru a 10 gauge wire? I mean rules are meant to be broken, right?

Though at that amperage I may have some hot wires!

Any heat is lost energy. Heat in wires also transfers to terminals on each end and heats up the components attached leading to shortened lifespan.

I see it daily in my work where the bean counters want to cut corners because engineers "over design" components and associated wiring. The inadequate wiring leads to a myriad of problems down the road.

But hey, they saved a buck.

So short term 10amps OK as is, long term 20 amps with a wiring upgrade.
Thanks for the link to that tool.
Andy

Any tool is only as good as inputting good data and interpreting the data it calculates properly. Misuse of a tool leads to disaster usually.

 

[AndyRonLI]

Z, Chill out.
Most of the wiring exists on the existing truck.
Straight line distance from the front of the truck to the battery is well under 20 ft.
At a 2% Voltage drop I can go 27 ft on 10 gauge at 10 amps
At a 5% Voltage drop I can go 60 ft on 10 guage at 10 amps
So I may be at a bit more than 2% but I am way less than 5%!
This is the distance to a DC/DC converter whose sole function is to ... RAISE THE VOLTAGE.

If I had a 5% voltage drop at 12 V DC and 10 amps that means I am dumping 6 watts of heat into my wire over its entire length.
I certainly will be checking this system to see what connector temps are doing.

I am not running a computer or a DAC or Linear Actuator or anything else that requires a precise voltage, I am charging a battery

I would be using that device regardless of wire to ensure that my automotive alternator properly charges my trailer battery at the required voltages for lithium and to limit the amperage. In this case the limit is to protect the wires, not the alternator.

I am not saving a "buck" I am saving the one thing money cannot buy. TIME. with a bit of hassle thrown in.
Properly upgrading the wiring from the vehicle battery to the trailer battery is not a trivial task, particularly with the need for a plug for the trailer pigtail. It would take a solid weekend to do it.

I am a licensed PE in NYS, am well aware of codes, while a fluid dynamicist in aerospace by trade, I have dealt with electrical equipment in the past.
So sometimes better is the enemy of good enough. I believe I have an approach that is in fact good enough and will not create a safety issue.

Thanks for your feedback
ANdy

 

[Zwy]

Z, Chill out.

I have no need to chill out, I'm explaining why I wouldn't do what you intend to do.

I remember back when I was in the Corps and the 6 P's.

Prior Proper Planning Prevents Poor Performance

 

[Short_Shot]

Are we really talking about 10 amps here or is that just a generalization?

If it is 10a, then the current involved here is minimal given the cable being used. Andy is right. Install it. Test it. Validate it. Move on.

I intend to do the exact same thing myself.

Voltage drop isn't itself a safety issue. Total current is.

10 amps on 10 gauge is trivial. Even if there's a 10% voltage drop over the length it's still far within current specs. The thing about the NEC and other codes, and even the military, is that the specs are designed with the lowest common denominator and worst case scenario in mind.

This means running at the wire's current capacity and with equipment voltage sensitivity involved. Neither is the case here.

Send it.

 

[Zwy]

Are we really talking about 10 amps here or is that just a generalization?

Actually, it keeps changing. First it was 40a, then 20a, now 10a.

If it is 10a, then the current involved here is minimal given the cable being used. Andy is right. Install it. Test it. Validate it. Move on.

I wouldn't waste the money for the DC to DC for 10a (120 watts). The money would be better spent for an additional 100w solar panel unless the vehicle is driven more than 5 hours per day. And even then I don't think it would still be better than a 100w panel. And money would be left over with the extra panel.

Or one could skip the DC to DC and ensure there is enough resistance in the circuit to accomplish charging thru the 7 pin. Basically that is all that is being done, as long as there is enough resistance present to the house battery, current will be limited. Just install a 10a fuse.

I intend to do the exact same thing myself.

Voltage drop isn't itself a safety issue. Total current is.

10 amps on 10 gauge is trivial. Even if there's a 10% voltage drop over the length it's still far within current specs. The thing about the NEC and other codes, and even the military, is that the specs are designed with the lowest common denominator and worst case scenario in mind.

My guess is the OP intends to run more than 10a thru the DC to DC. He could run 30a thru it, but that always comes at a price.

A 10% voltage drop? Get outta here.

This means running at the wire's current capacity and with equipment voltage sensitivity involved. Neither is the case here.

Send it.

If you only want 10a, don't waste the time with the DC to DC.

 

[Short_Shot]

I mean. Sure.

In your specifically chosen scenario which assumes perfect solar days then yes one additional panel is great.

I go through about 30ah of 12v in a ~12 hour "night". Maybe 40 if I watch a movie. This includes my iceco fridge. However, I've only got ~45 ish ah of usable battery right now.
If I'm traveling then I'm driving more than enough each day to charge that with 10a.

I have solar, but if it's a rainy crappy day then a 100w panel quickly becomes a 5-10 watt panel. Even if OP has 500w on his trailer it isn't enough on the worst days. If you have a lot of battery then you'll get through, but if not increasing it costs a lot more than a dc to dc unit and another panel's not the answer either.

Then this suggestion falls apart and the small dc to dc unit becomes not only viable, but the only real option.

Suddenly the 10a of tow vehicle charging is the only thing getting me through that next night.


Sure you can charge a bit from the tow vehicle during the drive without one but it's often negligible, and mine definitely does not keep up because of the voltage drop of the factory installed wire causing it to be below the battery voltage, which a dc to dc charger doesn't care about as long as you don't burn up the wire.

And if you're using a lithium you'd be rather silly not to use one.

 

[Short_Shot]

At the end of the day I'm not sure why you're arguing so passionately against it.

The cost difference between the charger and panel only really holds true on perfect days.

Surely you understand that the consistent availability (if you are on the road daily) adds value beyond the roughly 20 to 30 dollar price difference?

And if he can use it at 20a, which he most likely can, then it's a no brainer.

 

[Zwy]

I mean. Sure.

In your specifically chosen scenario which assumes perfect solar days then yes one additional panel is great.

I go through about 30ah of 12v in a ~12 hour "night". Maybe 40 if I watch a movie. This includes my iceco fridge. However, I've only got ~45 ish ah of usable battery right now.
If I'm traveling then I'm driving more than enough each day to charge that with 10a.

OP had 400 Ah's of battery, how long would it take driving to recharge at 10a from 80% depleted?

In your case, you have to drive a minimum of over 3 hours to recharge everyday if you have no solar. But he doesn't have a 45Ah battery, he has 400Ah. That's almost 10 times the size. 10a is similar to peeing to fill a swimming pool. It will take a long time if he can keep ahead of evaporation. It will certainly require plenty of beer to fill that swimming pool.

I have solar, but if it's a rainy crappy day then a 100w panel quickly becomes a 5-10 watt panel. Even if OP has 500w on his trailer it isn't enough on the worst days. If you have a lot of battery then you'll get through, but if not increasing it costs a lot more than a dc to dc unit and another panel's not the answer either.

500w at 80% yield is 400w times 4 to 5 hours peak sun. 2000w

100w at 80% yield is 80w times 4 to 5 hours. 400w

If you don't have enough battery, there is always a cure for that. In his case, he has two 200Ah batteries. He has batteries, just not enough generating capacity running off the engine alternator at 10a.

Then this suggestion falls apart and the small dc to dc unit becomes not only viable, but the only real option.

At 10a, the DC to DC is not needed.

Suddenly the 10a of tow vehicle charging is the only thing getting me through that next night.

And to think you aren't getting the full 10a due to voltage drop as you mentioned below.

Sure you can charge a bit from the tow vehicle during the drive without one but it's often negligible, and mine definitely does not keep up because of the voltage drop of the factory installed wire causing it to be below the battery voltage,

Right there, you admit it. The alternator can easily charge a LFP bank provided the voltage drop isn't high. That has been the whole point of this discussion. And where the DC to DC is located will make a huge difference. Install it on the vehicle and you still have the VD. Install it at the trailer with undersized cable, I can assure you it will not charge 10a either. Watts are watts and with reduced voltage, the DC to DC will not perform at capacity.

The money and time spent for DC to DC install for 10a would be better spent just upgrading to a heavier wire from the tow vehicle to trailer.

which a dc to dc charger doesn't care about as long as you don't burn up the wire.

Watts are watts, they don't come from the air and we don't have perpetual motion machines. You reduce volts input, watt output will decline.

And if you're using a lithium you'd be rather silly not to use one.

For 12v nominal LFP, there really isn't any difference in peak charge volts compared to FLA or AGM.

I'm all for using a DC to DC charger but not for 10a. The 10a can be accomplished without the DC to DC easily.

If you want to run small wires, then you need to up the voltage on the circuit. A 5% loss on 24V for example is one half the wattage loss of 5% on 12V. One could even run an inverter on tow vehicle and run 120V AC power back to the trailer for more watts. There is also a member here that uses a boost converter to increase amperage over small wires. https://diysolarforum.com/threads/r...harge-solar-in-the-trailer.20730/#post-242871

 

[Short_Shot]

Lmao since when is a dark rainy day 80% yield on solar?

And suddenly it's become clear you don't understand the issue at hand.

Reducing voltage does not "decrease watts" if a device such as a dc to dc charger is capable of increasing voltage at its output. That's the whole point of a boost converter.

If you have 10 volts at input and output is 14v @ 10a then your input current will be 14a. Including the efficiency of the dc to dc charger will bring up the input current a bit more still.

This holds true because the alternator is capable of much higher current. You aren't pulling more from "thin air". A 100 amp @ 13.6v alternator for example has 1360 watts available to use. A 5% voltage drop brings this down to 1292 watts. You might recognize this as "significantly more than necessary for a 10 amp charger". Thus, the necessary ~14.x amps needed is available.


Furthermore, an alternator connected straight to lithium without any means of regulation is a really bad idea.