2fpower's going to build a camping trailer

80t0ylc

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The largest wire that most 7 pin connectors can accept, on only some of the pins, is 10 ga. That's not big enough to do much even over a full day of driving. It's better than nothing, but not by a lot.
Good point. OP @2fpower must not have brakes on the trailer if he doesn't have the 7 pin connector. Might be a better idea, unless brakes are in future plans, to just separately add larger AWG wire through an Anderson connector to charge while on the road. I did that on mine in additition to a 7 pin connector. Also have a DVSR that will prevent draining the towing rig's batt if left connected without engine running for long period of time. Yeah, that means dealing with 2 connections to the trailer, but so what? The benefits are worth it.

Here's some pics for reference:
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GLTHFJ60

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Wires should be sized to the amperage load. Voltage drop of less than 10% is more than likely acceptable, so pick your size based on how much current you're going to send through it. Don't run huge pipes all the way back there just to minimize voltage drop if you're only ever sending 10a through them.

I'd rather invest in a DC-DC charger if you can tolerate zero voltage drop, rather than waste money on huge cables.

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ntsqd

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10% Voltage Drop (VD) is OK for most things, but when you're trying to charge a battery at a distance it isn't good enough because the regulator's charge voltage doesn't leave you much room to wiggle. With some alternator's a 10% VD just removed all ability to charge over a long wire/cable. Say the alternator outputs 14.4 VDC and you have a 10% VD at the battery hopefully being charged. A 10% VD drops the voltage at that battery to 12.96 VDC. Most things will run just fine at that voltage, but it's not going to do much charging at that voltage.

If the battery is really down it's going to pull some current in the Bulk charging stage. The way that I look at these decisions is to decide what is the realistic max output of the alternator and find the cable size that results in a less than 3% VD at that amperage over the circuit's total length. Memory is telling me that as the SoC increases so does the internal resistance of the battery, but I didn't find anything to back that up directly. Given that, the added VD during bulk charging due to a current that may exceed my estimation of max output is of less concern than if it were during the later charging stages.

The problem that I've seen with DC-DC converters is that once you get above about 20A-30A the cost goes way up, and even 30A is not really "bulk charging". Well past the cost of larger cables. With a so-called "Smart Alternator" you probably don't have any choice because adding a second battery w/o a DC-DC converter will likely trigger a trouble code.

A very, very hinky way around this that I've seen some folks do on the Tear-Drops & Tiny Trailers forum is to hard wire an inverter next to the battery and run a dedicated extension cord to the TR's rear bumper. Then put a dedicated charger in the trailer and plug it into the extension cord. They wire the inverter so that it only runs when the engine is on. It works, but it just seems so incredibly wasteful to do it that way.
 

GLTHFJ60

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Imo if the use case is to charge over the road, meaning a charging window over hours or days of driving, and the battery array capacity is less than say 400ah, no one will never need more than 20a of charging power.

Besides, you're assuming that with the large cable, there will be zero voltage drop, which isn't true either.

Nothing you're saying is wrong, but it is expensive and unnecessary. Good large gauge cable is very expensive, fuse holders and whatnot are expensive, the hydro crimper needed to work with it aren't super bad but the terminals are expensive, and everything large gauge is in general hard to work with.

Sizing for a 20a charge rate is much much better for this use case.
 

ntsqd

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I'm not assuming that the voltage drop will be zero. I am assuming that it will be in the noise with an appropriately sized cable. I'm not recommending 4/0 for this. For an 80A charge current to our camper batteries the charts called for 4 ga. I went with 6 ga. because I figured that I'd rarely see 80A and the battery monitor has proven that to be correct. I have seen more than 20A semi-frequently. At least I did before I got the solar installed. Now I could probably leave the camper disconnected from the alternator and never know the difference.
 

GLTHFJ60

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I see over 20a with my LiFePO4 setups also, but it charges so fast that I have changed my stance on wire sizing and charging rates. If it takes me a week to discharge my batteries, then an hour to charge them (while also still using them) that's great, but not necessary. If it was an ev, absolutely! I spent a ton on 1/0 wires all over my cruiser, and don't really regret it, but see now that there's a better way.

With a long span from charge source to batteries, DC-DC charging just makes more sense in my eyes. While the batteries could pull way more, mine can pull over 200a while charging, it's just not needed. DC-DC chargers can current limit down to your wire size and everything is easier, while still fulfilling all use cases.


One last point to add (for everyone), alternators can be overdrawn and fried pretty easily. If an alt is rated for 60 or 90 or 120a, that's how much it's *rated* to produce. Put a 200a load on it and the alternator will try its best to keep up, but will kill itself in the process. DC-DC chargers help prevent this issue, but so can the bms of most modern LiFePO4 batteries.


Hth.
 

2fpower

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Thanks for all the info. My plan is to do the 7 pin so when I am driving, I can get some charge on the battery, as well as add the solar panel in front of the RTT, with a solid mount triangle frame that is bolted to the panel, with 2 quick connect pins on both sides. design idea from @kc_chevota .... There will be two aluminum angle irons on the existing flat part of my trailer that it will attach to. When I pull the pins, the angle frame will go with the panel, and should be a decent angle to put on the ground to collect solar energy. Maybe even figure out a place to put camping chairs behind it, seems like that is always taking up lots of space in the trailer.

I will post up pictures to explain better....
 

ntsqd

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Yeah, the new chemistry batteries have caused a reset in what the ideal charging system design is, for them.
When I want/need the alternator to pick up the camper batteries I'm looking for bulk charging, so as much amps as the situation will allow w/o hurting anything. I use the solar for long duration charging and use a charge controller. Unfortunately the new to us camper's solar charge controller is a PWM, but that will be changing to an MPPT when I'm able to spend the time on it. For now, it works.
Not killing an Alt. is why I don't size the cables for it's rated max output. I've no RoT for how much to down-rate them, I just do.
 

2fpower

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I got the install completed…. For ball detent quick pens attach it…. Don’t really like the wires flapping in the wind so I might figure out a way to make a Velcro attachment point

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