12 vdc to 12 vdc Charger

[Wrknrvr]

     I just stumbled into this type of charger a few days ago.   Search for them on the net. I apparently just never heard of them before.

 

    So from what I understand is it will take say 12.9 volts on a charge line and raise the voltage up to properly charge batteries that are a distance from the regular charging source.

 

   I have 3-0 welding wire from the truck batteries to the rv batteries in the Fifthwheel. The fifthwheel batteries are just in-font of the entrance door.      I did have trouble keeping the batteries charged on our trip from South Carolina, to Montana a few weeks ago. Never had that problem before.

    We have a full size residential fringed. Travel with the ice maker off. So with the temperature outside and the rv being hot inside during the day. 85 plus degrees, the fridge apparently had to work more than normal.      So when it was really hot outside and running the a/c in the truck, the inverter would shut off at times apparently because of low voltage.

 

   They look interesting.    I remember years ago I think Mark Bruss had stated that you could loose a full volt from the alternator to the rv batteries. Has anyone installed one of these.

 

    I need to do some more reading on this subject 

 

 

[DG N. AL]

I have a Victron 12/12 30 amp charger on my fifth wheel.  I have 4 lithium batteries.  Lithium needs a higher charging voltage than lead acid.  The Victron steps up the voltage so the lithium batteries reach full charge.  I have  4 gauge wires running from the truck batteries to the hitch on the truck.  They run thru a relay that is switched in the cab.  I also have a residential fridge and my batteries stay 98% or better on long travel days.  The Victron has adjustable charging profiles that can be set for the battery type.

[craigjanelle]

My concern with the DC-DC is the stress on my stock alternator. I was reading up on the DC-DC chargers for our 12 volt mini-split in our Volvo. Seemed like many were talking upgrading to a higher power alternator so as no to fry the original. The DC-DC sounded like my answer until that came up. I had an extra inv/charger that I ended up using with a small genny & 225ah of AGM's. Interested & following. Safe Travels.

Edited August 5, 2023 by craigjanelle
left out info

[Jaydrvr]

6 hours ago, craigjanelle said:

My concern with the DC-DC is the stress on my stock alternator. I was reading up on the DC-DC chargers for our 12 volt mini-split in our Volvo. Seemed like many were talking upgrading to a higher power alternator so as no to fry the original. The DC-DC sounded like my answer until that came up. I had an extra inv/charger that I ended up using with a small genny & 225ah of AGM's. Interested & following. Safe Travels.

This is why it's important to size the DC to DC to stay well within the parameters of your alternator. It depends on your mission. If you just need to maintain and maybe add a bit, I think the 20 amp would do fine and not tax your alternator. If you need to step it up significantly, you might want to consider a large alternator. You don't want a lithium bank to overload your alternator, hence the DC to DC charger. Jay

[Wrknrvr]

   Since I just noticed these chargers, I need to do some research about them...

 

    years ago I was working on a Monaco coach and it needed a alternator on the engine replaced.

    So I called Wilson alternator rebuilders.   I was talking to the tech at there facility.

   He said what is this on.   It has a higher voltage rating than HDT do. I said it is on a motorhome. He stated that since it is going through a battery isolator, the voltage needs to be higher than normal to charge the batteries to where they need to be.  As the isolator restricts the voltage some.

  If I remember correctly the voltage output on that alternator was at15.1 vdc at it’s highest rating.

   I also think the alternator had a sense wire on the batteries to control the proper output voltage that was needed.

 

   Now that has been probably 8 years ago, since I did that job.

[Jaydrvr]

27 minutes ago, Wrknrvr said:

   Since I just noticed these chargers, I need to do some research about them...

 

    years ago I was working on a Monaco coach and it needed a alternator on the engine replaced.

    So I called Wilson alternator rebuilders.   I was talking to the tech at there facility.

   He said what is this on.   It has a higher voltage rating than HDT do. I said it is on a motorhome. He stated that since it is going through a battery isolator, the voltage needs to be higher than normal to charge the batteries to where they need to be.  As the isolator restricts the voltage some.

  If I remember correctly the voltage output on that alternator was at15.1 vdc at it’s highest rating.

   I also think the alternator had a sense wire on the batteries to control the proper output voltage that was needed.

 

   Now that has been probably 8 years ago, since I did that job.

The higher voltage would only be needed if it's going through an old style diode isolator. If it's a relay style, that passes the full voltage through.

Every alternator I've replaced in the last few years has the sense wire. If you have a shop do the alternator, you med to check behind them to make sure it's properly connected. I have yet to see a shop mechanic who understood the concept. Jay

[NeverEasy]

Just some info that might help you design your system.  

Assumptions:

1.       You are running 3/0 AWG on the positive lead and the negative lead.

2.       The one-way distance is about 50 feet making the two-way run 100 feet.

3.       You are using some good, high amperage wire connectors between the truck and trailer like Orion Motor Tech 175-amp connectors.

4.       You have great connections at the cable crimps and battery connection points.  However, nothing is perfect.  Say you have .1 ohms resistance in all those connectors and connections.  Does not seem like much, does it.

Given that the resistance of 3/0 is 0.630 ohms per thousand feet then the resistance of 100 feet is 1/10 of that or .0630 ohms.  Adding that to the .1 ohms loss at cable/battery connections is .1630 ohms. 

Not knowing your load this is a table of voltage drop with resistance x amps (V=IxR)
10 amps.  Drop of 1.63 volts.

20 amps   Drop of   3.26  volts

30 amps drop of 4.8 volts

40 amps drop of 6.532 volts

You get the idea.  The more current you draw, the bigger the loss.

Thoughts:

Each time you plug truck to trailer, spray the contacts with DeoxIT spray cleaner.  Then add a touch of DexoIT D1000L-2DB to the contacts.  Both available on Amazon.

Get a clamp-on DC ammeter to see what the real DC amp load is.  Then load the system with trailer 12 VDC components to get something you can read.  Then read the voltage at the trailer battery.  If it is dropping, you are loosing the battle.  When the trailer battery gets down to the voltage being provided by the truck, then you can do the math to see what the resistance loss is.  

Example:  Current on 3/0 cable is 5 amps.  Voltage at truck battery 13.8 volts.  Voltage at trailer battery 12.6 volts.  Difference of 1.2 volts.  Resistance equals (R= V/I) or R=1.2/5  or .24 ohms.  This means the system would need to be checked for loose or bad connections as it is greater than the .1630 ohms (above) on a very good system.

[Wrknrvr]

Thanks for that technical explanation of a system like I have.

   It has been on the truck since 2007.

   I will explain more tomorrow when I can get to measure the cable length.

[NeverEasy]

Here is a thought out of the box!  The problem with 12VDC is the number 12.  It does not take much to eat it up.  120 Volt AC is much more forgiving.  A 10% drop is no big deal.

Why not use an inverter on the truck that will sustain a 30 amp output (overkill).

(Power = voltage times amps or 120 x 20 = or 2400 watts  (Error here, I used the DC voltage and AC amps yesterday).  Therefore, the price of a 2400 watt inverter is a bit steep.  Kind of throws a monkey-wrench in the whole idea of saving some $.  😞  I will run a test with my current rig using a 1000 Watt inverter which will pump out around 9 amps.  A 1500 watt inverter would be better with an AC output of about 12 amps.

Another edit:  I did the math on my trailer 60 amp converter.  The maximum AC input to provide that 60 amps of 12 VDC is only 6.5 amps.  Thus, a 1000 watt inverter at the truck should be sufficient.

Anyway, disconnect the trailer's converter from the trailers AC source and feed the truck inverter output to the existing converter. 

The converters will take an AC input as low as 105 volts. Run the whole trailer charging system while driving down the road.    

Safety would be a concern of course.  Use 12 AWG heavily insulated extension cord.  Three wire is a must.  Wrap with extra insulation at all metal contacts.

Fuse at the truck inverter at 15 amps.  

Use twist-lock connectors to connect truck to trailer.

Add a simple on/off/on switch and land the original trailer converter AC wires to one side and the truck AC to the other thus insuring no other trailer AC systems can grab the trucks AC.  

 

Edited August 6, 2023 by NeverEasy
error in calculating wattage

[Deezl Smoke]

11 hours ago, NeverEasy said:

Here is a thought out of the box!  The problem with 12VDC is the number 12.  It does not take much to eat it up.  120 Volt AC is much more forgiving.  A 10% drop is no big deal.

Why not use an inverter on the truck that will sustain a 30 amp output (overkill).

(Power = voltage times amps or 13.6 x 30 = or 378 watts.  So jump up to 500 watts.  Feed that to the trailer converter.  The converters will take an AC input as low as 105 volts. Run the whole trailer charging system while driving down the road.    

Safety would be a concern of course.  Use 12 AWG heavily insulated extension cord.  Three wire is a must.  Wrap with extra insulation at all metal contacts.

Fuse at the truck inverter at 15 amps.  

Use twist-lock connectors to connect truck to trailer.

Add a simple on/off/on switch and land the original trailer converter AC wires to one side and the truck AC to the other thus insuring no other trailer AC systems can grab the trucks AC.  

 

👍 I think they make a shielded HD 3 wire abrasion and weather resistant, for mobile use. IIRC....(ya right, I'm old) I bought some at Platt many years ago. Or do you think shielded would even be a good idea?

[NeverEasy]

11 minutes ago, Deezl Smoke said:

👍 I think they make a shielded HD 3 wire abrasion and weather resistant, for mobile use. IIRC....(ya right, I'm old) I bought some at Platt many years ago. Or do you think shielded would even be a good idea?

Thanks, I will look into that.  Shielded should not matter at 60 HZ.  It only matters at very high frequencies like that in Cat 6 Ethernet.  In the meantime, I think I will run a functional test without running a cable through the frame.  

Also, please note that I had to edit yesterday's post on the calculation of the inverter wattage.  

 

[Wrknrvr]

    When i first installed the 12vdc welding wire, there was some discussion about the use of 110 vac power as you stated. 

    But I was cautious about ac power. I had the wire, so I went with the 12vdc system. It has worked as intended, until we traveled in the hot weather going from South Carolina to Montana this summer.

 

    Now I need to tell that I did something really stupid last fall????

 

   But it worked. Some how.

 

   I cleaned the battery terminals on the truck last fall before we left Montana for South Carolina.    After we got to our campground, and unloaded the Jeep, I noticed the Jeep needed a starter.

   Drove the truck to town and picked up a new starter.   

 

   Now get this!    The wife said there is something hanging down under the truck.

 

  Bad, bad Vern?     I was sorta doing my thing getting ready to travel cross country.     Bad, really Baaaddd Vern ?    That thing hanging down was the negative wire for the battery charge line. The cable end was ground off from dragging on the road.

 

   But the fridge and everything worked as expected.

 

  Batteries charged as expected.

 

    But that was badly stupid. Or stupidly bad.

 

   When I do something really stupid, I tend to leave my mind think about it before I admit it.

 

 

Edited August 6, 2023 by Wrknrvr

[Wrknrvr]

    The other thing that I had trouble with on the trip back to Montana. Was the generator would run in the morning, when the weather was cool.

   But it would only run for 15 minutes at most in the hot part of the day. Apparently the gasoline was vaporing from the heat.

   The welder is a restored Miller Legend model from back in the 1970’s. It is in a protective covering. So with the hot air flowing over the carburetor. I believe it it was not leaving liquid fuel flow through the carb.

 

    Yes there is better ways to do the charging. But since I never noticed the subject of 12vdc to 12vdc Charger. It has given more thoughts on how to do what is needed.

  I had let a inverter back in Montana that would have worked for my situation at the time.

 

   Yep,     Ssssstupiiddd,    Vern

    

[NeverEasy]

1 hour ago, Wrknrvr said:

 That thing hanging down was the negative wire for the battery charge line. The cable end was ground off from dragging on the road.

The path for ground then must have come from the trailer/truck hitch connection.   The white wire ground on the trailer to truck plug in would have too much resistance so an easier path would likely be the frame to frame through the hitch.  That would not be a safe connection point.  The only other patch would be via your trailer/truck plug.  That white wire might get hot.

This might be a better ground for your system.  At the truck, near the high amp truck/trailer connector, cut the 3/0 ground wire.  Attach it directly to the frame with a star washer on both sides of the frame bolt.  In these cases, I use brass or stainless hardware.

1 hour ago, Wrknrvr said:

 The welder is a restored Miller Legend model from back in the 1970’s. It is in a protective covering. So with the hot air flowing over the carburetor. I believe it it was not leaving liquid fuel flow through the carb.

 

Carb could be "vapor locking" but the problem is likely vapor lock prior to the carb.  Here is an interesting article relative to that: https://www.carparts.com/blog/vapor-lock-symptoms-causes-and-solutions/

 

[Wrknrvr]

    I am thinking I may try the inverter on the truck idea as a test. It will probably be next week or later till I find time to try it.    

   I have a 750 watt inverter, for the truck. And a 60 amp charger in the fifthwheel. It can be plugged into a separate power cord for the test.

 

   Other things to be doing this week. But will get the inverter out of the van today.

   Will look at the specs on the alternator on the truck also.

[Jaydrvr]

As a point of reference, my '99 Volvo has always had a 135 amp alternator, which I also use to charge my 525 a/h agm battery bank, in addition to my starting batteries. I think in the last 19 years, I've replaced it maybe (?) 3 times.. Maybe 4??? I'm completely satisfied with that kind of life and I never felt the need to get a larger alternator. Jay

[NeverEasy]

I performed the test of using my truck inverter to power the frig and converter.

1.       Test one: feed directly into the receptacle for the trailer inverter that feeds the frig (bypassed the trailer inverter).

2.       Test two: Feeding the trailer shore power input with all breakers off except the frig. Trailer inverter on. This test did nothing as the trailer inverter went to bypass mode. Pretty much the same as above.

3.       Test three: Turn only the frig breaker and converter breaker + the 50-amp main.  Still feeding the shore power input.

Summary:  All the setups powered the frig.  The frig got down to 34 degrees and the freezer to -2 degrees in 1.5 hours from a dead start with ambient temps at 87 outside (hotter inside).  The best setup would to be to feed truck inverter AC to the trailer converter and inverter.  The battery will stay charged and the inverter will be in bypass mode unless the AC input from the truck failed and then the inverter would revert to converting DC to AC (thus discharging the batteries).

The truck inverter maximum AC current/voltage (1.87 amps/122 volts) results in 228.14 Watts.  My truck inverter was showing one bar load.  That means a 500-watt inverter full-sine wave inverter should be sufficient.  Now, finding a reliable one. 

Safety is still a concern.  The installer must insure that either the converter or the inverter cannot be powered by shore power and the truck inverter at the same time. 

On my trailer, the inverter is on the basement wall and plugged into a receptacle that is fed from the trailer load center. I have not traced out that circuit to see how power gets through the inverter to the frig.  Might be as simple as putting a receptacle next to the wall receptacle so the cord can be moved to the other receptacle.

The converter is easy.  It is behind the basement wall so I will mount an on/off/on switch to select either power source.

Can anyone recommend a 500-750 watt inverter? I would not want to add this load to my already installed truck inverter as it has enough to do.  

[GlennWest]

I use the 1100 watt Victron Phoenix unit. Likely best small unit on the market. 

[NeverEasy]

9 hours ago, GlennWest said:

I use the 1100 watt Victron Phoenix unit. Likely best small unit on the market. 

Thanks! They have an 800 watt unit that is shorter.  Going to do a test and see what opening a slide would do to the load on the inverter.  Would be nice to be able to run the slides or feet off the inverter when dry camping.  From past testing, 1/2 of the power comes from the trailer batteries and the other half from the converter.