2025 PowerBoost Drawing Power from Trailer Battery

[Davexxxx]

I had forgotten about Snakebitten's response on the starter/generator. Thank you for the reminder.

I am still stuck with a known and unwanted draw between my trailer's LiFePO4 and truck, which other forum members seam to deny that it happens with or without saying I'm wrong. So would you conclude that this is a trailer wiring problem more than a truck problem?

I like your "directional diode" idea. If I cannot find a proper fix for this, I may go down that road.

Thinking back about how I tested mine, could be an answer to that question.

I ran test leads from the truck's hot and ground of the 7 pin, to the corresponding points of the trailer 7 pin, to test with an amp clamp, since I was interested in learning potential excess amp draw from the LFPs.

So, that isolated the charging, from all other wiring. A way to check if something is somehow crossed on yours, would be to use the same test leads but add the Victron monitor, so you can confirm not just the amount but direction.

Just a thought.

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[P.S.N.]

Thinking back about how I tested mine, could be an answer to that question.

I ran test leads from the truck's hot and ground of the 7 pin, to the corresponding points of the trailer 7 pin, to test with an amp clamp, since I was interested in learning potential excess amp draw from the LFPs.

So, that isolated the charging, from all other wiring. A way to check if something is somehow crossed on yours, would be to use the same test leads but add the Victron monitor, so you can confirm not just the amount but direction.

Just a thought.

That makes sense. Would not my trailer's shunt recording of a negative number be the exact indication of a draw from my F150? It goes in the positive direction when my WFCO converter/charger or my solar is working, so that is convincing me that my shunt is answering the directional question, or am I looking at it wrong?

 

[Davexxxx]

That makes sense. Would not my trailer's shunt recording of a negative number be the exact indication of a draw from my F150? It goes in the positive direction when my WFCO converter/charger or my solar is working, so that is convincing me that my shunt is answering the directional question, or am I looking at it wrong?

No, you're right that far. I'm just suggesting to definitively affirm whether it is the truck, or something crossed up in the other circuits of the trailer wiring,. Using just 10ga leads from the charging pins and neg pin, spanning the 7 pin connectors would do that.

Make sense?

 

[P.S.N.]

No, you're right that far. I'm just suggesting to definitively affirm whether it is the truck, or something crossed up in the other circuits of the trailer wiring,. Using just a 10ga charge leads from the charging pin and neg pin, spanning the 7 pin connectors would do that.

Make sense?

Okay. Yes, that makes sense.

 

[EricR]

<snip>

If you install a DC/DC charger, which will prevent trailer>tow vehicle current-flow, be sure to do that in way that does not prevent current-flow from the battery to your breakaway-brakes when the trailer is not connected to the truck. Depending on how your trailer is wired and where you install the DC/DC charger, that may require a new jumper from the battery (or battery-side of the DC/DC charger) to the breakaway-brakes switch stud in your trailer junction box.

It was suggested to me to simply things by isolating the truck 12V system from the LiFePO4 system I had installed in our trailer.

This is accomplished by installing a Ken & Barbie breakaway battery.

Breakaway Battery on Amazon

It's pretty simple to open the junction box on the trailer frame to reroute the 7-pin 12V line to this battery to keep it charged.

Thus, you are always breakaway safe, and the truck's 12V system isn't having a less than ideal influence on the trailer's LiFePO4 system in any way.

 

[Davexxxx]

It was suggested to me to simply things by isolating the truck 12V system from the LiFePO4 system I had installed in our trailer.

This is accomplished by installing a Ken & Barbie breakaway battery.

Breakaway Battery on Amazon

It's pretty simple to open the junction box on the trailer frame to reroute the 7-pin 12V line to this battery to keep it charged.

Thus, you are always breakaway safe, and the truck's 12V system isn't having a less than ideal influence on the trailer's LiFePO4 system in any way.

I guess I should be grateful that mine has been plug n play, the whole way.

It just works.

BTW, not all trailers have the junction box.

 

[Buyer2021]

It was suggested to me to simply things by isolating the truck 12V system from the LiFePO4 system I had installed in our trailer.

This is accomplished by installing a Ken & Barbie breakaway battery.

That's certainly a viable approach, though I'm not sure why one would do that if just isolating the truck from the trailer / LiFePO4 system when the LiFePO4 house battery can continue to support the breakaway brakes without any further modification.

Even if one installs a DC/DC charger to allow the truck to continue to contribute to the LiFePO4 charging, it only takes one simple (usually short) jumper-wire to support the breakaway brakes with the house battery.

Either strikes me as simpler than adding a 'Ken & Barbie' dedicated battery for the brakes, but YMMV, no worries!

 

[JEB]

I had forgotten about Snakebitten's response on the starter/generator. Thank you for the reminder.

I am still stuck with a known and unwanted draw between my trailer's LiFePO4 and truck, which other forum members seam to deny that it happens with or without saying I'm wrong. So would you conclude that this is a trailer wiring problem more than a truck problem?

I like your "directional diode" idea. If I cannot find a proper fix for this, I may go down that road.

As an admitted skeptic that your truck is actually drawing amps from you trailer—or if it is occurring it’s likely insignificant—I’ll try to explain why.

First, I found your testing in your second post illuminating. When you hooked your trailer to your truck and started your truck up, your trailer‘s house batteries were at 13.22V and down over 100ah. Lithiums that depleted are going to draw like crazy from any source it can find. I don’t see any way that they would provide current to a truck battery that is being charged at a minimum of 13.6V (and probably more like 14.4V) by the truck’s charging system. The truck batteries are at a substantially higher voltage than the trailer batteries and if anything would be providing a charge to what is clearly a deeply discharged house battery bank.

Second, people who tow regularly and intend to rely on the tow vehicle’s charging system to keep the house batteries charged won’t ordinarily rely entirely on the stock 12V power supplied through the 7-way. The distance between the truck battery and the house batteries is too great and the gauge of the wire is too small to to provide more than a trickle charge. Thus, the use of a DC/DC converter. The stock pipeline doesn’t get magically larger by reversing the flow of electrons. If the truck can’t provide more than a trickle to the trailer, then the trailer can’t provide more than a trickle to the truck. Same pipeline. So even if there was some backflow occurring—and I’m not convinced there is—it’s not likely to be significant or something to be overly concerned about. I’ve been towing for more than four years with a trailer equipped with two Battle Borns and I’ve neither detected the phenomenon nor experienced a problem.

Third, there may be a built-in system to protect against backflow. The F150 has what Ford calls a “smart“ connector. It will not provide 12V power at the 7-way unless it detects a load. But don’t take my word for it—plug in a 7-way diagnostic tool instead of a trailer and you will see zero volts at the 12V pin. The LED on the tester won’t even light, which has led many to conclude erroneously that the 12V aux power wasn’t working. It was—it’s just dead unless a load is detected. I haven’t confirmed this, but I could see the “smart” system treating no load and a positive “load” from the trailer the same way and simply not open the relay because in both instances there’s no trailer-side draw.

I can’t explain why you are seeing a higher amp draw at the Victron shunt when connected to a running truck. But my suspicion is that there is a trailer side draw you haven’t accounted for. I don’t see how it could be the truck providing the load you are seeing.

Edit: Further thought to my last comment. To try and explain why connecting to a running truck results in a higher amp draw, focus on systems on your trailer that are powered by the house batteries but are only woken up when activity on the 7-way is detected: back up camera, TPMS, running/back up lights etc. may be good candidates to start with.

 

[P.S.N.]

As an admitted skeptic that your truck is actually drawing amps from you trailer—or if it is occurring it’s likely insignificant—I’ll try to explain why.

First, I found your testing in your second post illuminating. When you hooked your trailer to your truck and started your truck up, your trailer‘s house batteries were at 13.22V and down over 100ah. Lithiums that depleted are going to draw like crazy from any source it can find. I don’t see any way that they would provide current to a truck battery that is being charged at a minimum of 13.6V (and probably more like 14.4V) by the truck’s charging system. The truck batteries are at a substantially higher voltage than the trailer batteries and if anything would be providing a charge to what is clearly a deeply discharged house battery bank.

Second, people who tow regularly and intend to rely on the tow vehicle’s charging system to keep the house batteries charged won’t ordinarily rely entirely on the stock 12V power supplied through the 7-way. The distance between the truck battery and the house batteries is too great and the gauge of the wire is too small to to provide more than a trickle charge. Thus, the use of a DC/DC converter. The stock pipeline doesn’t get magically larger by reversing the flow of electrons. If the truck can’t provide more than a trickle to the trailer, then the trailer can’t provide more than a trickle to the truck. Same pipeline. So even if there was some backflow occurring—and I’m not convinced there is—it’s not likely to be significant or something to be overly concerned about. I’ve been towing for more than four years with a trailer equipped with two Battle Borns and I’ve neither detected the phenomenon nor experienced a problem.

Third, there may be a built-in system to protect against backflow. The F150 has what Ford calls a “smart“ connector. It will not provide 12V power at the 7-way unless it detects a load. But don’t take my word for it—plug in a 7-way diagnostic tool instead of a trailer and you will see zero volts at the 12V pin. The LED on the tester won’t even light, which has led many to conclude erroneously that the 12V aux power wasn’t working. It was—it’s just dead unless a load is detected. I haven’t confirmed this, but I could see the “smart” system treating no load and a positive “load” from the trailer the same way and simply not open the relay because in both instances there’s no trailer-side draw.

I can’t explain why you are seeing a higher amp draw at the Victron shunt when connected to a running truck. But my suspicion is that there is a trailer side draw you haven’t accounted for. I don’t see how it could be the truck providing the load you are seeing.

Edit: Further thought to my last comment. To try and explain why connecting to a running truck results in a higher amp draw, focus on systems on your trailer that are powered by the house batteries but are only woken up when activity on the 7-way is detected: back up camera, TPMS, running/back up lights etc. may be good candidates to start with.

Thank you for the response and advise.

Being that I have a 2025 PB, I don't know how accurate your first paragraph is about the ICE side 12V system, given it lacks the starter/generator you have with your PB.

To your last "Edit:' paragraph, backup camera only gets powered when the marker lights are on, and they were off. There is no TPMS on my trailer. The running and backup lights were off and have to be forced on by rotating the headlight switch for they do not come on by default unless it is dark and it was the middle of the day.

So I feel like I am still not understanding what is happening.

 

[JEB]

Thank you for the response and advise.

Being that I have a 2025 PB, I don't know how accurate your first paragraph is about the ICE side 12V system, given it lacks the starter/generator you have with your PB.

To your last "Edit:' paragraph, backup camera only gets powered when the marker lights are on, and they were off. There is no TPMS on my trailer. The running and backup lights were off and have to be forced on by rotating the headlight switch for they do not come on by default unless it is dark and it was the middle of the day.

So I feel like I am still not understanding what is happening.

DIfference in equipment between your truck and mine is a dead end. Both trucks have charging systems to recharge the 12V starter battery. Given the data you have provided so far, highly unlikely the trailer is charging the truck. Your trailer batteries started out way too depleted to be at the level of the truck’s starter battery, particularly since the truck was running in the third test. You said it yourself. Battery systems like to balance one another. The balance doesn’t tip toward the truck. 13.22V and 107Ah down is a deep discharge on the trailer side. No getting around that unless you measured the truck’s starter battery voltage and determined that it was below 13.22V even with the truck running and charging the battery. Possible? I suppose. But not likely.

I’m at a loss as to what’s causing the draw on he trailer side. You have ruled out serveral candidates. But my list was not exhaustive. Nevertheless, your initial testing data does not confirm the truck as the culprit.

I would continue trying to rule out trailer systems turning on due to activity on the 7-way by eliminating as much activity on the 7-way as possible. Turn the headlamp switch to full off, not auto. Keep your foot off the brake. Turn off the trailer sway control. Anything else you can think of. If none of this works to reduce the draw, check 12V system voltage on the truck while it’s running and compare it in real time to battery voltage on the trailer. Which one is lower?

EDIT: One final thought, and I admit it is the remotest of possibilities. Was your 12V refrigerator on when you did your testing? It would be a coincidence but it’s probably worth considering that the compressor kicked on in the brief time between the second test when the truck was connected but off and the third test when the truck was connected and on. The amp draw is about right for a 12V fridge with a Danfoss compressor.

 

[Gros Ventre]

There is no If's, and's or buts about it, the two battery systems will seek to equalize terminal voltage. Unless you need power running into the truck from the trailer, get a disconnect plug put in. Don't plug it in until the trailer drops to truck battery system (eg PB has two AGM batteries). The systems will then share load proportionate to battery capacity.

 

[P.S.N.]

DIfference in equipment between your truck and mine is a dead end. Both trucks have charging systems to recharge the 12V starter battery. Given the data you have provided so far, highly unlikely the trailer is charging the truck. Your trailer batteries started out way too depleted to be at the level of the truck’s starter battery, particularly since the truck was running in the third test. You said it yourself. Battery systems like to balance one another. The balance doesn’t tip toward the truck. 13.22V and 107Ah down is a deep discharge on the trailer side. No getting around that unless you measured the truck’s starter battery voltage and determined that it was below 13.22V even with the truck running and charging the battery. Possible? I suppose. But not likely.

I’m at a loss as to what’s causing the draw on he trailer side. You have ruled out serveral candidates. But my list was not exhaustive. Nevertheless, your initial testing data does not suggest the truck as the culprit.

Thank you all for taking the time to try and help me.

You continue to state that the PB's 12V system, with it's standard batteries, have a higher voltage than the LiFePO4 in my trailer. The only time I can think this would be true is during the truck's charge cycle when the hybrid system fills the depleted batteries. I don't know the charge cycle of my 2025 PB but I assume it's not continuous.

Note that I did this test five times, including when my LiFePO4 was at 78% SOC and got the exact same numbers. In other words, the LiFePO4's change in SOC was not having an impact on the numbers.

I understand your list of power consuming sources was not inclusive, but a very good suggestion. I know my trailer's system well and the only things pulling power were what I listed in my original post (2W worth of draw). Nothing in my trailer "wakes up" due to power flowing through the seven-pin, unless I turn the parking lights or headlights on in the truck, which were off. If there were things to "wake up," it would be due to my trailer drawing power from the seven-pin (my truck), not the trailer battery, otherwise it would already be "awake."

TO ALL: If I get my hands on a multimeter, would measuring the starter battery's positive and negative terminals directly during and after engine start, with my trailer connected and disconnected, give me the correct measurement as to what the 12V side of my PB is doing or do I need to be probing/monitoring something else on my 2025 PB to get meaningful values?

 

[JEB]

Thank you all for taking the time to try and help me.

You continue to state that the PB's 12V system, with it's standard batteries, have a higher voltage than the LiFePO4 in my trailer. The only time I can think this would be true is during the truck's charge cycle when the hybrid system fills the depleted batteries. I don't know the charge cycle of my 2025 PB but I assume it's not continuous.

Note that I did this test five times, including when my LiFePO4 was at 78% SOC and got the exact same numbers. In other words, the LiFePO4's change in SOC was not having an impact on the numbers.

I understand your list of power consuming sources was not inclusive, but a very good suggestion. I know my trailer's system well and the only things pulling power were what I listed in my original post (2W worth of draw). Nothing in my trailer "wakes up" due to power flowing through the seven-pin, unless I turn the parking lights or headlights on in the truck, which were off. If there were things to "wake up," it would be due to my trailer drawing power from the seven-pin (my truck), not the trailer battery, otherwise it would already be "awake."

TO ALL: If I get my hands on a multimeter, would measuring the starter battery's positive and negative terminals directly during and after engine start, with my trailer connected and disconnected, give me the correct measurement as to what the 12V side of my PB is doing or do I need to be probing/monitoring something else on my 2025 PB to get meaningful values?

It would be easier to get a scanner and plug directly into the OBDII port and read the 12V system voltage from there. FORScan, FDRS and most apps that work with wireless adapters work well.

Was your refrigerator on when you did your testing?

 

[P.S.N.]

It would be easier to get a scanner and plug directly into the OBDII port and read the 12V system voltage from there. FORScan, FDRS and most apps that work with wireless adapters work well.

Was your refrigerator on when you did your testing?

Another good idea, thanks.

No, my trailer's 12V frig was off.

 

[Davexxxx]

TO ALL: If I get my hands on a multimeter, would measuring the starter battery's positive and negative terminals directly during and after engine start, with my trailer connected and disconnected, give me the correct measurement as to what the 12V side of my PB is doing or do I need to be probing/monitoring something else on my 2025 PB to get meaningful values?

You need a multi meter anyway. RV ownership practically demands it but yes, this would show, unless something is wrong with your PB, that the voltage at the starting battery is as high or higher than the LFPs.

And yes, if the engine is on, it should be charging.

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