I think my 5er battery charging truck when not running

[corn18]

Well, today I finally looked at my Victron app and saw a 4 amp draw on the battery while parked. That was odd since nothing was on in the trailer except the gas detector thingy. So I unplugged the 7 pin connector and voila! the draw went away. I have LFP batteries in the trailer and they read 14.6V. I assume this means that my LFP batteries are trying to charge my truck battery.

And no, the brake disconnect was not activated.

I also saw that I get a whopping 4A going into the LFP battery.

So, maybe I need to look at a DC-DC charger. I wonder where the 7 pin charge line connects into the system? I just redid my entire DC system and I did not see a connection from the 7 pin connector.

If I do go with a DC-DC charger, I'll probably run a dedicated 6ga wire from the battery to an Anderson connector at the back of the truck.

Appreciate your thoughts if I have diagnosed this correctly.

[Hclarkx]

Originally Posted by corn18

Well, today I finally looked at my Victron app and saw a 4 amp draw on the battery while parked. That was odd since nothing was on in the trailer except the gas detector thingy. So I unplugged the 7 pin connector and voila! the draw went away. I have LFP batteries in the trailer and they read 14.6V. I assume this means that my LFP batteries are trying to charge my truck battery.

And no, the brake disconnect was not activated.

I also saw that I get a whopping 4A going into the LFP battery.

So, maybe I need to look at a DC-DC charger. I wonder where the 7 pin charge line connects into the system? I just redid my entire DC system and I did not see a connection from the 7 pin connector.

If I do go with a DC-DC charger, I'll probably run a dedicated 6ga wire from the battery to an Anderson connector at the back of the truck.

Appreciate your thoughts if I have diagnosed this correctly.

I don't think I've ever located the 12V line from the truck in my 5er but I do know it's there. I do have two original terminal blocks that serve the slide motors and the electric brakes via the switch that closes should the truck and 5er separate and a couple of other unidentified circuits. I'd guess mine is on one of those terminal blocks. If you have a clamp-on DC ammeter, you might find it with that.

I'm not sure 4 amps flowing to the AGM battery in your truck is much of a problem. The voltage should eventually equalize at a "float" voltage that both the LFP and truck AGM are happy with; at which point no current will be flowing to the truck battery. Though if you choose to do nothing and want to be able to leave the truck and RV connected, I would watch that current and be sure it goes to zero above 13.3 Volts (where the LFP will rest when fully charged).

If it does not go to zero, the simplest solution is a low-voltage drop diode in the circuit to the truck. This will let the truck charge the LFP in the RV but not vice versa. I.e., I would go to a DC-DC charger only for ample charging of the LFP from the truck, not just to solve this problem.

I had the same truck and did leave it tied to the 5er overnight a few times but didn't notice much current flow.

BTW, does your LFP require 14.6V? What is driving it that high? Consensus is that 14.4V is ample for LFP charging (over 99% SOC even with zero minutes of absorb time).

[Half Ton Heavy]

Lithium float voltage> fla float voltage. So if your truck has no isolation via a relay or diode, the lithium will power it drain out trying to float the fla. A relay would be relatively easy, a low drop out diode will work, but you will not be able to charge as high as you can without the diode. I would put in a victron dc to dc charger, IIRC, they make a 10 amp one.

[Hclarkx]

Originally Posted by Half Ton Heavy

Lithium float voltage> fla float voltage. So if your truck has no isolation via a relay or diode, the lithium will power it drain out trying to float the fla. A relay would be relatively easy, a low drop out diode will work, but you will not be able to charge as high as you can without the diode. I would put in a victron dc to dc charger, IIRC, they make a 10 amp one.

I wonder if the truck has a diode that has "shot through" and is not blocking current flow from the trailer to the truck.

Diodes are available with a forward drop of just tenths of a volt, so would have little impact on the truck's ability to charge the trailer battery ... which in any event is pretty small without the help of a DC-DC converter (i.e., likely just a couple of amps) so not much impact there.

AGM batteries float easily above 13.3V with virtually zero current, so should not drain the LFP in the trailer if the AGM battery is already fully charged. But, yes, there is some risk of doing so if something in the truck is drawing current. My 2015 truck (same as the OP's) would draw considerable current if left in the accessory mode (the infotainment system, internet, and similar alone was over 10 amps). Any current draw in the truck will pull current only from the LFP in the trailer (if not blocked by a diode or similar).

I like the idea of a 10 amp DC-DC charger. That would surely never stress the truck alternator and in a, say, 6 hour drive, would likely let you arrive with enough juice for the night even if the battery was low in the morning. A couple hundred watts of solar combined with that would be nice.

Also, a 10 amp DC-DC charger would likely not stress the harness wire between the engine bay and the 7 pin connector. I think the smallest wire in my 2015 7-pin connector was #14. But, I would suggest anyone going this route check the wire size and fuse rating and check the manual for any advice on the use of the 7-pin 12 V terminal.

[Tabasco_Joe]

Get a large Schottky diode to isolate the truck.
The voltage drop decreases as current decreases so while it may slightly reduce the charge rate it will ultimately get to within 0.2 Volts or so of the truck output voltage.
Mouser has stud types. Should mount on an aluminum heat sink.

[corn18]

Good news and bad news.

Good news: installed the Victron DC to DC converter. Easy install as the battery charge wire was right in the front storage area and easy to get to. Got it all hooked up. 18A @ 14.4 volts going to the battery. Wonder if I need a charge controller for that much current? The wire from the truck is 10ga and it is fused @ 30A.

Bad news: I shut off the truck and measured 18A still going into the trailer battery. I fixed the truck draining the trailer batteries and now my trailer is draining my truck batteries to the tune of about 21A. When the truck is off, the DC to DC converter still spits out 14.4V @ 18A and it does it by drawing 21A @ 12.6V from the truck battery.

So now I still have to unplug the truck when not running. Won't take long to drain the truck if the camper is pulling 21A.

I guess I could add an ignition controlled relay to the trailer charge line. Could have done that in the first place. Or just used the diode. Lesson learned. Crow eaten.

[jimmarako]

I thought the Victrons had a option to connect it to a hot line from the truck to turn it on/off. I also thought they were charge controllers, not just DC-DC convertors.

Jim M.

[corn18]

Originally Posted by jimmarako

I thought the Victrons had a option to connect it to a hot line from the truck to turn it on/off. I also thought they were charge controllers, not just DC-DC convertors.

Jim M.

Yes, they do have an input for hot line, but there aren't any in the 7 pin connector. Wish there was. That would be too easy. I guess I could hook the backup light line to an ignition source line in the truck and use that.

And yes, it is a DC-DC charger. I thought I had purchased just the converter.

[jimmarako]

Set it up so that you have to turn on your trailer parking lights to make the DC-DC turn on. Not sure how the Victrons work... maybe a direct wire into the unit, or a simple relay to "shut the switch".

Actually looking at the manual it looks like the unit can determine when the truck is off and shut itself down.

Good luck

Jim M.

[corn18]

Originally Posted by jimmarako

Actually looking at the manual it looks like the unit can determine when the truck is off and shut itself down.

Good luck

Jim M.

I did not know it could do that. Awesome sauce! Problem solved.