Charge wire in 7-way trailer plug and lithium battery

[sunrise]

Hi,
There is a 12v 'charge' connector in a 7-way trailer plug, in my trailer it connects directly to the main battery. My understanding is that while driving this will charge the trailer battery while driving. But this only works well for non-lithium batteries I assume? I am worried due to the voltage of a fully charged lithium battery is higher (14.4v), this will backfeed, not only drain the trailer's battery, but might damage towing vehicle's battery.

Should I just disconnect this connector? Are there any important purpose besides charging trailer battery?

Thanks!

 

[CascadeOffGrid]

The 12V power wire in the 7-way plug should be connected to the tow vehicles battery. It is typically either fused at 30 or 40amps. I wouldn't recommend more than 20amps through this wire though.

This wire is typically used to keep the trailers battery charged while driving. Many trailer manufacturer's install an automatic reset 30amp breaker on the trailer to prevent pulling more than 30amps when you have a dead battery in either your tow vehicle or the trailer. This is not a very impressive solution in my opinion but it is very common.

If you have trailer brakes, you should not just disconnect this wire. The battery in your trailer should be tied to the break-away cable to activate your trailer brakes in the event that the trailer becomes disconnected. You need to have a way to ensure that your trailer battery is staying charged and it may be required by law.

This wire will not charge your lithium battery well. Typically, when you start your vehicle and your alternator is charging the starting battery, you will get some charging to your lithium battery while the alternator is outputting at bulk or absorption voltage. Once the starting battery reaches the float stage, you will get a very minimal amount of charging to your lithium battery. Power can also flow the other way as well. While starting your battery and the few seconds after you start your tow vehicle, the lithium battery can feed back to the starting battery until the alternator kicks in.

Best solution in my opinion is to use a DC-DC charger in the trailer between the trailer connector and your lithium battery. You just have to use a small enough charger to not overload your vehicle wiring.

 

[mapguy525]

If you tow a trailer equipped with electric brakes or electro-hydraulic brakes the 12v power feed at the 7 way rv connector cannot be eliminated. It is there to ensure the breakaway system will work if needed by maintaining the breakaway battery charge.

Large LFP battery is a poor choice to power the trailer brake circuit due to the amps it will deliver in a short circuit event. Emergency trailer brake activation is the equal to a short circuit event in regards to battery discharge.

This is one situation where a hybrid AGM or LA / LFP system works well. If you have an on-board generator the positives of a hybrid system are even greater due to flexibility.

 

[sunrise]

Let's say you start your travel in the morning, your LifePo4 battery is fully charged by shore power the night before. Now you are driving under California sun for 8 hours, At 14.4v your LiFePo4 battery will backfeed and charge your towing vehicle, non-stop, as your solar panel will keep your battery fully charged.
Wouldn't this damage your towing vehicle's lead-acid battery which max charging voltage is around 14.1? Or towing vehicle has some regulator that either prevent backfeeding or higher voltage?

 

[HRTKD]

The electrical system on a modern tow vehicle will not allow the trailer battery to backfeed into the tow vehicle's battery.

As said above, a hybrid system may work better. However, it adds complexity, for sure. I've been thinking - but not too hard - about a hybrid system. The amount of charge that I get from the tow vehicle (2017 Ford F-350, with dual alternators) isn't that great. Not more than say, 20 amps, usually less. So I'm not really looking for the tow vehicle to charge my "house" batteries in my trailer. The new solar system will be responsible for that.

The regular lead acid battery on the tongue would be relegated to braking and running lights as well as being the starter battery for the generator. My existing solar system would be used to keep that battery topped up. The new solar system would be strictly used for the rest of the trailer. The downside would be that I need to keep a lead acid battery instead of getting rid of all of them like I had originally planned.

The challenge would be creating a firewall between the two systems. I can just see it now. "What's this wire for?" "Who's the maniac that wired this system?" "WTF?"

 

[sunrise]

A dumb question (one I was hesitating to ask) - Isn't the brake controller in towing vehicle supposed to supply necessary power to engage/activate trailer brake? it's one of the connectors in 7 way plug. I have seen those boat trailers and utility trailers, they look fairly heavy with payload (e.g. a big boat), but the trailer itself is so bare and just a frame, I have not notice they are equipped with onboard battery to drive brakes, they are also partially emerged in water at boat launch. Do they use battery in boat? What about these utility trailers? I am under impression that the towing vehicle is responsible of supplying brake power.
That's not the case?

 

[sunrise]

The electrical system on a modern tow vehicle will not allow the trailer battery to backfeed into the tow vehicle's battery.

As said above, a hybrid system may work better. However, it adds complexity, for sure. I've been thinking - but not too hard - about a hybrid system. The amount of charge that I get from the tow vehicle (2017 Ford F-350, with dual alternators) isn't that great. Not more than say, 20 amps, usually less. So I'm not really looking for the tow vehicle to charge my "house" batteries in my trailer. The new solar system will be responsible for that.

The regular lead acid battery on the tongue would be relegated to braking and running lights as well as being the starter battery for the generator. My existing solar system would be used to keep that battery topped up. The new solar system would be strictly used for the rest of the trailer. The downside would be that I need to keep a lead acid battery instead of getting rid of all of them like I had originally planned.

The challenge would be creating a firewall between the two systems. I can just see it now. "What's this wire for?" "Who's the maniac that wired this system?" "WTF?"

If your F350 only supply 20A, I think I can ignore the amp my mid-size SUV can provide...

 

[HRTKD]

A dumb question (one I was hesitating to ask) - Isn't the brake controller in towing vehicle supposed to supply necessary power to engage/activate trailer brake? it's one of the connectors in 7 way plug. I have seen those boat trailers and utility trailers, they look fairly heavy with payload (e.g. a big boat), but the trailer itself is so bare and just a frame, I have not notice they are equipped with onboard battery to drive brakes, they are also partially emerged in water at boat launch. Do they use battery in boat? What about these utility trailers? I am under impression that the towing vehicle is responsible of supplying brake power.
That's not the case?

Many boat trailers do not use electric brakes. Instead, those are surge brakes that have no connection to the tow vehicle's brake controller. Utility trailers - that have brakes - typically have a small 12v battery that serves as the power source for just the breakaway system.

The battery on an RV trailer serves as the power source should the breakaway cable get pulled. The assumption there is that if the breakaway cable has been pulled, the 7 pin probably has also become disconnected. I had a situation where the breakaway cable got pulled but the 7 pin was still attached. I certainly felt a good tug against the truck, but nothing that would freak you out.

 

[sunrise]

Thanks for the quick response. I searched a bit on this topic and here is what I found out -

1) Travel trailers usually have 2 brake activation systems, one from towing vehicle brake controller through the 7-way plug, and the other one is the 'breakaway' brake system.
2) Brake controller supplies all the operating power to trailer brake. but a battery is needed for 'breakway' brake system on trailer, which makes sense if your trailer is separated from towing vehicle, then you won't have power from towing vehicle.
3) Trailer brake draws less than 20amps, which is well within LiFePo4's output.

https://www.etrailer.com/question-12234.html#:~:text=Expert%20Reply%3A,effectiveness%20at%20full%20load%20capacity

.

"Emergency trailer brake activation is the equal to a short circuit event in regards to battery discharge." could someone please explain this? this seems to contradicting with the link above.

 

[HRTKD]

While I haven't done extensive research on it, I don't think the breakaway system is all that powerful. It isn't going to lock up the brakes on the trailer. Maybe if the brakes are brand new and adjusted to be really tight it would, but that isn't what happened on mine. Note that I'm speaking only about the typical electric drum brakes you find on almost all RV trailers. Electric over hydraulic (disk) brakes are a different matter.

 

[mapguy525]

"Emergency trailer brake activation is the equal to a short circuit event in regards to battery discharge." could someone please explain this? this seems to contradicting with the link above.

No contradiction. Electric trailer brakes are really a main circuit of normal operation driven (powered) by tow vehicle 7 way brake lead wire. Then a sub circuit that overrides normal operation under Emergency breakaway activation The sub system is driven(powered) by battery on trailer. If activated -it does not automatically reset. Magnets at the end of the circuit are a resistivity load and will continue 100% engagement of brakes until battery dies or wiring smokes. Think about the discharge potential of 200amp 12v LFP battery in this situation.......

While I haven't done extensive research on it, I don't think the breakaway system is all that powerful. It isn't going to lock up the brakes on the trailer. Maybe if the brakes are brand new and adjusted to be really tight it would, but that isn't what happened on mine. Note that I'm speaking only about the typical electric drum brakes you find on almost all RV trailers. Electric over hydraulic (disk) brakes are a different matter.

If your plain electric trailer brakes won't lock up something is not right in the system. I can lock up the electric trailer brakes on my 14K 2 axle toyhauler at 60 mph by stomping on the pedal. It will easily skid tires using manual over ride lever.

Yes, I have fixed all the wiring problems that were OEM installed on the assembly line. There were a bunch of problems -too small gauge wiring, bad splices, nicked wiring in axle housings, did i say too small wiring gauge, and poor wiring scheme.

 

[HRTKD]

Here's a graph that hopefully won't add to your confusion. It doesn't address the amps delivered to the trailer. It's showing that as the alternator heats up, it produces fewer amps. Something to keep in mind while driving, to temper your expectation of how much juice the tow vehicle can deliver to the trailer.

On my F-350, the trailer charge circuit goes through a 30 amp relay. I'm throwing that out to say that on my truck I would NEVER see more than 30 amps. Due to the wire gauge and length, even if it started at 30 amps after the relay, what I see at the trailer wouldn't be 30 amps.

 

[sunrise]

No contradiction. Electric trailer brakes are really a main circuit of normal operation driven (powered) by tow vehicle 7 way brake lead wire. Then a sub circuit that overrides normal operation under Emergency breakaway activation The sub system is driven(powered) by battery on trailer. If activated -it does not automatically reset. Magnets at the end of the circuit are a resistivity load and will continue 100% engagement of brakes until battery dies or wiring smokes. Think about the discharge potential of 200amp 12v LFP battery in this situation.......

If your plain electric trailer brakes won't lock up something is not right in the system. I can lock up the electric trailer brakes on my 14K 2 axle toyhauler at 60 mph by stomping on the pedal. It will easily skid tires using manual over ride lever.

Yes, I have fixed all the wiring problems that were OEM installed on the assembly line. There were a bunch of problems -too small gauge wiring, bad splices, nicked wiring in axle housings, did i say too small wiring gauge, and poor wiring scheme.

Thanks @mapguy525 , so for breakaway braking, since there isn't a reset system, the engaged brake will draw way beyond normal max draw ~20A because it's constantly engaged, why? This doesn't seem to make sense, say if both TV and trailer are stationary, and if I step hard on TV brake, and won't let go, this would simulate a breakaway event, at least to the trailer brake, and this might fry the battery on trailer or trip breaker/fuse?

 

[HRTKD]

If your plain electric trailer brakes won't lock up something is not right in the system. I can lock up the electric trailer brakes on my 14K 2 axle toyhauler at 60 mph by stomping on the pedal. It will easily skid tires using manual over ride lever.

Brake pedal activation and breakaway activation may not be the same force. I have my factory brake controller set so that my trailer brakes will NOT lockup with full force. This was set with the manual lever at full engagement. Locking up the trailer brakes could be counterproductive in an emergency situation. Having the trailer slide out of control due to the tires locking up could lead to a loss of control.

I had an emergency braking experience a few years back with a different truck and trailer. I was hauling logs. The truck's brakes did lock up, activating the ABS. The trailer brakes did not lockup and the trailer stayed right behind the truck where it should be.

One of the suggested adjustment procedures for a trailer brake controller is to get the trailer brakes to barely lock up, then back off of that setting so that the trailer brakes do NOT lock up.

I pull an 11k bumper pull toy hauler. I've had that trailer for about three years and have over 10,000 miles on the road with it.

 

[HRTKD]

Thanks @mapguy525 , so for breakaway braking, since there isn't a reset system, the engaged brake will draw way beyond normal max draw ~20A because it's constantly engaged, why? This doesn't seem to make sense, say if both TV and trailer are stationary, and if I step hard on TV brake, and won't let go, this would simulate a breakaway event, at least to the trailer brake, and this might fry the battery on trailer or trip breaker/fuse?

No, it doesn't work that way. The trailer brakes get power from the tow vehicle. The breakaway system gets power from the trailer battery.

 

[sunrise]

No, it doesn't work that way. The trailer brakes get power from the tow vehicle. The breakaway system gets power from the trailer battery.

I meant from trailer brake (the magnetic parts) power consumption perspective, that has nothing to do with where the amps come from

 

[HRTKD]

I didn't state my post quite right. Normal braking that is initiated from the brake pedal of the tow vehicle uses power from the tow vehicle. The trailer's battery does not come into play in that situation.

All power for the breakaway system (subsystem of the trailer's brake system) comes from the trailer's battery. I'm not convinced that the breakaway system goes to 100% power when activated.

 

[mapguy525]

I meant from trailer brake (the magnetic parts) power consumption perspective, that has nothing to do with where the amps come from

It's all about inrush and lack of disconnect in relationship to battery chemistry characteristics. Review discharge characteristics of lead acid and AGM batteries compared to LFP - especially the "C" rate. IE -how fast the battery will(can) discharge.

Trailer brakes are just one of the complications of adding LFP batteries to towable RV's.

 

[HRTKD]

It's all about inrush and lack of disconnect in relationship to battery chemistry characteristics. Review discharge characteristics of lead acid and AGM batteries compared to LFP - especially the "C" rate. IE -how fast the battery will(can) discharge.

Trailer brakes are just one of the complications of adding LFP batteries to towable RV's.

Hmm, that's interesting. For a EOH (electric over hydraulic) system I would expect that there would be enough smarts in the on-board components that it would draw only as many amps as are needed for the requested braking force. In that case, I wouldn't expect the battery chemistry to matter.

For the breakaway system, I don't think there is a direct connection - without a governor - between the battery and the brakes. But I could be wrong, so I'm going to post this question to some guys that know more about this than I do. One of the guys is an RVDA/RVIA Master Certified RV Technician, so he should be able to help.

EDIT: I found this blurb on the RV Doctor blog:

When the pin is out or not fully inserted, the brake magnets will continue to draw power and eventually drain the battery bank. The brakes can draw a lot of current – upwards of 3 amps or so for each magnet, depending on the size of the brakes. So the bottom line is never remove the pin except once a year for cleaning and testing.

 

[mapguy525]

Brake pedal activation and breakaway activation may not be the same force. I have my factory brake controller set so that my trailer brakes will NOT lockup with full force. This was set with the manual lever at full engagement. Locking up the trailer brakes could be counterproductive in an emergency situation. Having the trailer slide out of control due to the tires locking up could lead to a loss of control.

I had an emergency braking experience a few years back with a different truck and trailer. I was hauling logs. The truck's brakes did lock up, activating the ABS. The trailer brakes did not lockup and the trailer stayed right behind the truck where it should be.

One of the suggested adjustment procedures for a trailer brake controller is to get the trailer brakes to barely lock up, then back off of that setting so that the trailer brakes do NOT lock up.

I pull an 11k bumper pull toy hauler. I've had that trailer for about three years and have over 10,000 miles on the road with it.

@HRTKD -I do understand the function and adjustment procedure to adjust an electric brake controller properly. I have a MaxBrake controller and it works flawlessly once all the adjustments are made for your driving style. You are correct that skidding is not wanted typically.