Do I need a DC-DC charger for towing?

[YBen]

I have read through a lot of the DC-DC vehicle threads but I have not found an answer to my question. I think the answer is no, but I just want to make sure.

I have a pop up camper which I am in the process of adding solar to and exchanging a FLA with a LiFePo4 battery. I am not planning on “charging” my pop up battery from the vehicle except through the 7 pin connector while towing. I do not want my alternator to melt, so do I need to a DC-DC converter for this setup?

I appreciate your insight.

 

[chrisski]

I would say no.

I have four golf cart batteries in my fifth wheel that I Charge with solar that are also hooked to the trailer seven pin. The solar on the roof keeps the battery charged enough that it does not need energy from the seven pin.

 

[jwelter99]

I have read through a lot of the DC-DC vehicle threads but I have not found an answer to my question. I think the answer is no, but I just want to make sure.

I have a pop up camper which I am in the process of adding solar to and exchanging a FLA with a LiFePo4 battery. I am not planning on “charging” my pop up battery from the vehicle except through the 7 pin connector while towing. I do not want my alternator to melt, so do I need to a DC-DC converter for this setup?

I appreciate your insight.

Whats the tow vehicle and what do they fuse the 7 pin at?

A DC to DC charger is always the "right" way when you are running a lithium house battery that needs different charge voltages then the vehicle charging system which is setup for lead acid.

 

[YBen]

Whats the tow vehicle and what do they fuse the 7 pin at?

A DC to DC charger is always the "right" way when you are running a lithium house battery that needs different charge voltages then the vehicle charging system which is setup for lead acid.

I tow with a Mazda CX-9, 30amp fused directly from the battery.

I am not really worried about charging the battery as much as not burning out the alternator.

 

[mikefitz]

To keep within the 30A fuse feed a Victron Orion 1212 18 would be a suitable B to B unit and avoid possibe overload of cables and connections.

Mike

 

[jwelter99]

I tow with a Mazda CX-9, 30amp fused directly from the battery.

I am not really worried about charging the battery as much as not burning out the alternator.

The 7 pin cabling and 30A fuse won't allow enough power to be drawn to hurt your alternator so don't let those horror stories worry you. Those are about running a heavy duty cable that can carry a lot of amperage from the engine to the house battery, combined with the high charge rate of lithium batteries. You don't have that situation.

What you do have are voltages on that 7 pin power contact that are not compatible, and perhaps unhealthy, for charging lithium.

So you have a couple choices:

1) Ignore it and just use it as is. The long term impact is the lithium battery might not last as long.

2) Disconnect the 7 pin power contact from the lithium battery in the trailer. Make sure to do so right at the battery so the rest of the trailer safety systems are still powered correctly via the 7 pin connector.

3) Add a B2B between the 7 pin connector and the lithium. Mike points out the right one to use above. Again it will not harm your alternator.

 

[time2roll]

30 amp fuse will protect the wire and the alternator. Most likely there will be enough voltage drop in the wire that the charging will self limit below 25 amps and all will be just fine. Give it a go and have a 30 amp replacement fuse handy. A DC clamp-on ammeter would tell exactly how much current is actually moving. (any excuse to buy a new tool )

Your new LFP battery will be protected by the BMS.

 

[YBen]

30 amp fuse will protect the wire and the alternator. Most likely there will be enough voltage drop in the wire that the charging will self limit below 25 amps and all will be just fine. Give it a go and have a 30 amp replacement fuse handy. A DC clamp-on ammeter would tell exactly how much current is actually moving. (any excuse to buy a new tool )

Your new LFP battery will be protected by the BMS.

I think I should be able to see that through the Smartshunt, right?

 

[MBGPA6]

I tow with a Mazda CX-9, 30amp fused directly from the battery.

I am not really worried about charging the battery as much as not burning out the alternator.

I have been watching this forum for a while because I have the same question. Although, I have been waiting until I pull my trailer out of storage so I can verify some details before I asked the question. Where is this fuse? In line or part of the trailer electrical management system?

 

[YBen]

I have been watching this forum for a while because I have the same question. Although, I have been waiting until I pull my trailer out of storage so I can verify some details before I asked the question. Where is this fuse? In line or part of the trailer electrical management system?

My fuse is about 10 inches from the vehicle battery under the hood. I installed all that myself with a kit from etrailer.

 

[MBGPA6]

My fuse is about 10 inches from the vehicle battery under the hood. I installed all that myself with a kit from etrailer.

Thanks. So this is between the alternator and the trailer wiring? I’m really new at this.

 

[YBen]

Thanks. So this is between the alternator and the trailer wiring? I’m really new at this.

My trailer wiring is connected directly to the battery, so it does not connect to the alternator “as such”. I am sure the alternator is connected directly to the battery as well, so I guess it is connected directly to the alternator.

 

[jwelter99]

30 amp fuse will protect the wire and the alternator. Most likely there will be enough voltage drop in the wire that the charging will self limit below 25 amps and all will be just fine. Give it a go and have a 30 amp replacement fuse handy. A DC clamp-on ammeter would tell exactly how much current is actually moving. (any excuse to buy a new tool )

Your new LFP battery will be protected by the BMS.

This is not totally true. The BMS, depending on emergency shutoff programming, may disconnect but the tolerances are pretty wide.

 

[YBen]

Thank you all for your help!

 

[Zwy]

I tow with a Mazda CX-9, 30amp fused directly from the battery.

I am not really worried about charging the battery as much as not burning out the alternator.

Depending on the year, that's about a 100amp alternator.

I've fried 130amp already by continuous charging thru the 7 pin when traveling for a few weeks, no solar to recharge house batteries. I went to 165amp now and B to B charger.

I'd verify what the amp rating is on that alternator. Second, charging only thru the 7 pin takes time as the resistance thru the wire can create a VD of over 4% for 20 to 25 feet. This won't allow full charging of the house battery.

You said you used a "kit". What size of wire?

 

[JMc]

My trailer wiring is connected directly to the battery, so it does not connect to the alternator “as such”. I am sure the alternator is connected directly to the battery as well, so I guess it is connected directly to the alternator.

The vehicle starter battery becomes irrelevant once it’s fully charged by the alternator. At that point whatever is connected downstream is at the mercy of the alternator’s voltage regulator, ideally it outputs ~ 14V.

 

[YBen]

Depending on the year, that's about a 100amp alternator.

I've fried 130amp already by continuous charging thru the 7 pin when traveling for a few weeks, no solar to recharge house batteries. I went to 165amp now and B to B charger.

I'd verify what the amp rating is on that alternator. Second, charging only thru the 7 pin takes time as the resistance thru the wire can create a VD of over 4% for 20 to 25 feet. This won't allow full charging of the house battery.

You said you used a "kit". What size of wire?

Apparently the alternator is 150 amp.
The kit came with 10 gauge wire, if the kit from etrailer is still the same.

 

[Zwy]

Apparently the alternator is 150 amp.
The kit came with 10 gauge wire, if the kit from etrailer is still the same.

Let's say length is 25 feet, 10 gauge wire and 10 amps, alternator output is 14.0V. That comes to 13.41V at load. 30 amps you get 12.2V.

You will never get to full charge just running a wire back. If full charge isn't important then you would be fine. You should probably install an isolator if the kit didn't include one. Or you install a solenoid that switches on when engine is running, manually or automatic. The isolator will reduce V at the load, the solenoid (continuous duty only) will not.

 

[willo]

You *might* want one if....

• You want to automatically have the charger turn off when the engine is off
• You plan to run a non-12v system (I use one for 12v to 24v)
• You want to regulate how much power is exiting your vehicle (fixed to the value of the charger)
• You want to check status/performance with your phone
• You want a different charging voltage than your vehicle puts out.
• You want to control (enable/disable)the charger with your BMS or other device or switch.

I have a victron isolated dc-dc for tow charging. It provides bluetooth config options. Charge current is fixed, but it provides boost conversion for my 24v pack from my 12v truck. It does provide cutoff based on detected engine status and I am able to integrated it as a controlled charging source from my BMS.

 

[YBen]

Let's say length is 25 feet, 10 gauge wire and 10 amps, alternator output is 14.0V. That comes to 13.41V at load. 30 amps you get 12.2V.

You will never get to full charge just running a wire back. If full charge isn't important then you would be fine. You should probably install an isolator if the kit didn't include one. Or you install a solenoid that switches on when engine is running, manually or automatic. The isolator will reduce V at the load, the solenoid (continuous duty only) will not.

Thanks for this info, do you have any links to more information about isolators and solenoids?