I'm looking for a thread in which a forum member did research on how many amps the truck was actually delivering to the 7-pin connector. I thought it might have been @SDcrewzer that did it, but I can't find the thread after looking in the three main towing subforums.

 

I’ve never actually measured the current through the +12 circuit from the truck to the trailer. Considering all of the variables, it’s a “hot” topic.

The circuit wiring is typically protected by a 30A fuse. Some of the variables affecting circuit current include:

Tow vehicle engine speed (alternator speed)
Tow vehicle electrical loads
Alternator temperature
Alternator size
Wire gauge and length
Connector pin “cleanliness”
Trailer battery voltage
Trailer loads

I’ve seen reports from some folks reporting as little as 5 A measured through this circuit. I’ve been able to run DC fridges and charge batteries, so that would suggest 20 A or more.

I plan to conduct some experiments on this issue in the near future.

HTH,
Jim / crewzer

 

Well dang, I know I saw a report somewhere. The results were enough to convince a few people that they needed to run a separate line to provide a sufficient charge to the trailer.

 

Jim,

You did read it and it is in the slide-in forum and is a topic that has been covered often on some of the truck camper websites. I made the comment in the slide-in forum here.

To cut to the chase, the limitations of the charge line output have been commented on several times over the years here on FTE and also recently was addressed in one of the posts on the Truck Camper Adventure forum. You can also find it on RV Net.

I obviously not run the numbers on the all the various permutations and combinations, but I have metered it on my own trucks when tethered. When hitched with the engine running and the loads limited to parasitics and the fridge running on propane as though I was driving down the road, I measured 9 amps at the batteries with my Fluke meter. On my last 7.3 dually I saw less than that. In the Truck Camper Forum the poster reported he saw 10 amps.

When higher amp numbers are desired folks normally run much heavier wiring fabricating a high output charge lead and there are both explanations and YouTube videos demonstrating ways in which this has been done. The normal output is usually more than enough to keep things cooking as you go down the road, although operating a 3-way fridge on DC power will exceed the standard charge lead output.

Steve

 

This was another question that came up on the solar forum. A poster there was concerned that the higher output LiFePO4 battery would backfeed into the tow vehicle's battery. I explained that on modern vehicles it simply doesn't work that way.

That transitioned into a discussion on how many amps could the trailer battery bank really see coming in from the two vehicle. I recalled vague numbers and suggested that it wasn't over 20 amps and - at least on my truck - it would never be more than 30 amps because of the relay/fuse in the truck. Folks that think their 170 amp alternator is going to significantly charge up their trailer batteries are in for a rude awakening.

While I'm still in the design phase for my new solar system, I'm considering disconnecting the 7-pin charge from the trailer's LiFePO4 battery bank. This would require a separate (lead acid) battery that is dedicated to trailer breakaway and starting the generator. Keeping that battery kind of defeats one of the benefits of LiFePO4, which is the weight savings.

 

Jim,

Why would you do this? I do not understand you logic. ??

Steve

 

If I decide to change the battery bank to 24 or 48 volts, the alternator can no longer directly charge the batteries. A step-up converter would be required. Keeping the two systems separate is simpler and less expensive.

 

Got it. I didn't know you were considering making that change.

Steve

 

These discussion elements are key to my planned experiment. My hypothesis is that available charging voltage and current from a hot, small alternator spinning at ~3x engine idle speed is relatively low, but that available charging voltage and current from a hot but appropriately sized alternator spinning at ~3x engine road speed should be significantly higher.

I hope to conduct my experiment this Thursday afternoon.

HTH,
Jim / crewzer

 

"Minor" detail I left out.

 

Jim,

Historically, “48 V” PV systems offer performance advantages as well as design challenges. Battery-based PV systems used to be my business specialty at Outback and Schneider (Xantrex); I know a fair amount about the ins and outs.

For example, 48 V inverters are more efficient then their 12- and 24 V cousins. And, the DC wiring is smaller AWG. However, the NEC design voltage for a “48 V” system’s PV array is typically in the 150 V range, and we typically run into the DC voltage spec issues for some circuit breakers and fuses.

Should you decide to pursue a 48 V architecture, I hope you’ll allow me to offer some pointers here and there, all pro bono, of course.

Regards,
Jim / crewzer

 

if you look at the current through the charge wire from an electrical stand point, you will see that the current through the wire is a variable based on the voltage difference between the plug (truck side) and the coach battery.

As the delta between the two decreases, so does the current. So in reality, the coach battery will never see a full charge when hooked to the truck. In theory, it will see the same state of charge as the start battery on the truck, but in reality there is some resistance and voltage drop at the rear plug so the state of charge on the coach is Limited by the voltage available at the plug - which will always be less than the alternator voltage.

add to this the newer “smart alternator” strategy deployed to save fuel and the coach just never sees a full charge from the TV.

the best solution for a conventional system is a 12v-12v DC-DC charger. These work in the same way as an AC-DC charger in that the charger senses what the battery needs and pulls the required current from the source to supply the proper charge to the battery. If the voltage at the plug is less than the voltage required to charge the battery, it simply pulls more current to make that happen - up to the limit of that circuit.

there are 12v-48v chargers available too. They are not cheap but they would be a great solution to charge your high performance battery bank you are talking about.

food for thought.

 

I'm trying to stick to a 12v system in my trailer. Once the 12v standard is deviated from, the level of complexity and the cost go up. One of the attractions of a 48v system is "efficiency". Yes, I acknowledge that certain components within the system function more efficiently at 48v. But the rest of the trailer is still 12v, which requires step down converters. More batteries are also required to get to 48v.

I was provided a link to quality DC-DC chargers yesterday. Good grief those things are expensive. $400 for a decent one. Wow!

Figuring out how to handle the charge circuit from the tow vehicle for a 12v system is fairly easy. I may not like the charge profile the trailer batteries get from the tow vehicle, but the BMS on my battery bank will prevent any overcharging, should that actually happen. Through the many posts I've read here, I've come around to the thinking that the charge from the tow vehicle is enough to maintain a battery, but don't expect to bring it up to 100% from a low state of charge. It is what it is, I'm not complaining about it. No big deal.

My current design will use a 2p4s battery bank made up of 3.2v 280 Ah cells. Eight of them, with a net 12v and 560 Ah. I could rearrange them for 24v and 280 Ah. If I were to abandon 12v, I might as well go 48v, but finding the room for 16 cells is the challenge.

 

I wasn’t advocating for a 48v system, I just thought that was the direction you were heading

http://www.walmart.com/ip/Renogy-12V...rger/258749281

i can’t speak to its quality but the price is right and it proclaimed to be good for several different types of batteries.

im not sure Of all the ramifications of trying to charge some of the high performance batteries from a 14.4v vehicle system, but as I understand it the best bet is to do it with a dedicated charger.


As you mention above, the 12v charge wire is best viewed as a voltage stiffener, rather than a charge wire. It will never fully charge a battery but it will keep it from going flat when in tow and subject to a system draw (e.g. refrigerator).

 

Renogy is a popular brand on DIYSolarForum. I can't speak to their quality either. The DC-DC chargers I saw yesterday are from Sterling Power. $425 for a 12v to 12v, 60a charger. Mucho dinero.

If I felt the need for additional charging from the tow vehicle, then a DC-DC charger would be appropriate. However, if I have enough solar panels I think I can avoid it. Even increasing the number of panels by one and perhaps upgrading to a stouter Solar Charge Controller, I would spend less money than for a DC-DC charger like I mentioned above.

The purpose of starting this thread was - mostly - to baseline how little power a trailer gets from the tow vehicle with the factory setup. Given how little power the tow vehicle produces, I'm not worried about it negatively impacting my trailer's system.

I'm still learning about all this. 95% of what I know about PV and LiFePO4 in an RV was learned within the past six months.