hovr

Hello all,
I have just installed a DC-DC charger (12v, 18amp max) on my travel trailer, and I connected it to the 7pin charging/ground wire from my ML 350. Since this wire has a 20amp fuse in under the rear seat fuse box, I was hoping to get 10-15amp charging current from ML350. Today after I connect the trailer 7pin plug, and put my ML in idle next to the trailer, I found I only get 5-6 amp from it.
Since I am still working on other wiring stuff in the trailer I can't take the trailer to the highway and test it at cruising speed (60+ mph). But I heard the alternator should put out over 100a at idle. Is the charging wire in 7pin plug heavily regulated down (to 5-6amp)?

I know people run dedicated large gauge wires (2-4awg) directly from the alternator when they have a large DC-DC charger(like 50A+ ones), I have a relatively small trailer and 10-15amp is all I needed, so I don't want to complicate things and will stay with 7pin plug/wire.

Thanks!

-Hovr

chassis

Be careful you don't send the vehicle and trailer up in smoke.

100A at idle? Where did you see this documented?

Which pin are you measuring 6A? How are you measuring 6A? Please post a photo of the current probe actively measuring with the display visible and showing the connector with the pin in question.

mikapen

What is the reason for a DC-DC converter? Are you changing voltage? Why?
6A is probably sufficient to run your trailer, even a fridge and the breakaway battery. Maybe even 2A.
I'd sure try to avoid a direct, separate connection from the alternator to the trailer - a can of worms.

I do not know how (if) the Rear SAM regulates that wire. It may need a load to unleash its full power. Or maybe not.
What are you trying to accomplish?

hovr

I think ML350 uses a 180A alternator, 100A at idle is just a ballpark estimate, the point is that the alternator at idle puts out way more than 6a.

I measured the amp from the Lithium battery BMS in my Airstream. the BMS reports charging current. I am confident the wiring is correct as the 12v charging wire(black) and ground wire(white) are 12 awg and the rest are 14awg. I have also tested other wires (green, red, brown, white, blue, etc) for brake, turn signal they all works.

hovr

Thanks @mikapen , I need the DC-DC charger for 2 reasons - 1. I use lithium batteries in my Airstream which have higher voltage than ML's starter battery - without this it would charge VERY slowly or not charging at all. 2. I need isolation between 2 batteries so it won't back feed starter battery (again the voltage difference)

You are right that 6amp is sufficient for my small fridge and keep the trailer battery enough juice for breakaway brake. But it would be nice to have 15a so after a 3 hour highway travel it gives 40ah additional charge.

I also put the car in neutral and stepped on gas to ramp up RPM to 3000 briefly, no difference in current on the battery side. so I think the current is probably regulated.

chassis

Can you post a photo of what you are dealing with? Please post a photo showing the DC-DC charger indicating 6A. What brand and model of DC-DC charger do you have?

hovr

Thank you @chassis , the dc-dc charger is a Victron Orion-Tr 12/12 - 18 Smart Charger with bluetooth

https://shop.pkys.com/Victron-Energy...er_p_8042.html

The reading of 6a was from an iPhone app that connects to my lithium batteries' BMS. It's pretty accurate, I have tested it with lab grade power supply, the amps between BMS reports and power supply display were within 0.05amp.

chassis

Do you have an owner's manual? Alot of trailer-related information is there.

Also, please don't take offense - do you know how electrical charging systems work? It may be the system you are working with only requires, or has decided that, it only needs 6A at the moment. The Airstream system will not ask for more than it needs, or that it can use. The Mercedes system will not deliver more than has been asked for by the Airstream.

What is the state of charge of the Airstream battery(ies)? What are the loads turned on in the Airstream (lights, fans, furnace, etc.)?

mikapen

I understand the need to charge LiPO4 batteries at a higher current than Lead Acid. However I don't think you need to do it while driving - your major charging time will probably be when parked and plugged into a pedestal, or a generator.
And I believe 14.6v (normal driving voltage) is a good recommended maintenance rate for LIPO4's, so you are at least maintaining them while driving.

If you are trying to recharge from a deep-discharge state, I doubt that you can count on any 7 pin to do the job. It's essentially trickle charging.
I'd spend my time / $$$ on as solar setup, which will feed your converter and charge the lithiums.
And just accept the lower driving rate.

There are lots of opinions on this these days, since this is relatively new technology. I just bought a new RV converter, Just In Case I buy Lithiums, but I'm not convinced it was a necessary purchase, even if I do that migration.

Etienne Lau

You DC to DC charger maybe rated for 18A. However that only means it during use will draw no more than 80% of that 18 A, meaning nominal current draw will be 14A. The question is why should it draw 14A? What in the trailer will force the DC to DC charger to draw 14A, what devices do you have that will 14A, if nothing is turned on or if the devices that are turned on in the trailer only draw 6A that you have measured. Then nothing wrong here.

hovr

@Etienne Lau Thanks for your comment. To my disappointment, Victron tech support actually clarified that 18a was output current and the charger could draw over 20A from the TV. As to your question why it should draw 14a - I have a 40% charged 200ah LiFePo4 battery that would devour anything the charger could throw at it due to its low internal resistance. Hope that makes sense.

Etienne Lau

What's controlling the charging rate of the LiFePo4 battery? Is it the Victron?

I don't believe the issue is the TV, the issue is the Victron itself. If the Victron is truly able to draw the 20A or more than 20A, you would be blowing the fuse in the TV. The fuse in the TV is to prevent over current from burning the wire. The alternator is not the restriction, it puts current to "recharge" the TV main battery. As an example, if your vehicle consumes/draws more than the 100A that the alternator is putting out, it only means you will be discharging the TV battery. And then you would need to either drive the TV to recharge the battery or plug in a charger to recharge the TV battery.

Diabolis

@Etienne Lau is correct - the current is a result of what the trailer draws, not what the car's alternator can output. AFAIK the aux output pin is fused with a 20A fuse, and the car will give you anything up to 20A (or blow the fuse if the trailer resistance is low enough to need more) depending on what the trailer requires.

chassis

@hovr Can you please provide the LiFePO4 battery manufacturer and model number?

hovr

I want to give an update on this - it turned out the way I wired the 'remote on/off switch wire to the Victron DC/DC charger, requires disabling 'engine on/off detection' from the VictronConnect app settings. Once I disabled that, the charge starts to draw a much higher current - I had to lower absorption voltage in settings from 14.4 to 13.8 to avoid blowing the fuse - it turns out the rated 18A charger can draw close to 25A for ~10s when it starts charging according to some discussion on Victron community forum.

Etienne Lau

The "Engine on/off detection" is a safety feature to prevent over drawing the TV electrical system. So I would advise to to enable that feature. In other words, you want to allow higher current draw only when the engine is running and not when the engine is off. With the engine running you, the TV will be able to provide higher voltage and higher current (UP to 20A fuse), but with the engine off, you want to err on the cautious side and not draw that much current. It would become a nuisance to keep replacing the 20A fuse on the TV if you always drew the 20A....

hovr

Thanks Etienne, the Victron charger has multiple switch options, The way I wired it is also listed in their manual, and preferred by many professional Airstream technicians - it gives you control over when to turn that on or off. if your engineer is off, it's unlikely you will keep your running lights on.
Another thing I discovered that is much better than using engine on/off detection - the charger is powered only by the 12v charging wire, having a separated on/off wire allows me to change settings and have charger disabled, I learnt this the hard way - when I change settings, somehow the charger doing some random stuff, it blew the fuse several time before I figure this off, now I always disable charging first, change settings, and enable charging again,

johnand

The issue is the wiring gauge of the 12VDC aux to the 7way is too small ~20gauge. It is fused at 20amp, but with a heavy 12VDC refrigerator draw, I could never get more than 6-8amps out of it as well. I installed a circuit breaker and 6gauge wiring directly to battery to an Anderson SB50 connector next to the 7pin. Then added the mating SB50, breaker, and 6gauge wire to my trailer. I am running the 30A Victron DC-DC charger and can get the full 30A out of the ML350 to run refrigerator and change the lithium batteries. I use still use the factory 12VDC aux to turn on the Victron dc-dc charger.

hovr

Could you share a couple of pictures on how you route the 6awg wire? Thanks!

johnand

Sorry, other than the battery area, I don't have pictures of routing, but I did route the wires from battery to door sill channel, then over wheel well on passenger side, and out exiting grommet where treiler wiring comes out from cargo area. Here are a few pictures of the battery, circuit breaker, and hook ups:

​​​​​​https://www.icloud.com/sharedalbum/#B0s5yeZFhDB9ei

hovr

Thank you @johnand , this is helpful!