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DC/DC charger help


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#1 joebob25

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Posted 23 August 2022 - 07:04 PM

Hello all!

 

I've been lurking here for months trying to absorb everything I can regarding electrical/solar information. I'm so grateful for all the threads and have elevated my knowledge from a 0 to maybe a 2... and hope to continue learning.

 

A bit of history on my current setup. I have a 2018 FWC Hawk on my 2005 F150. I have a dual 6V 190 AH AGMs with 200w Renogy flexible solar panels on the roof. I have a Renogy solar controller and battery shunt for monitoring my battery levels. I bought the camper used and I think the guy maybe used it twice, and put it in storage for the last 4 years. The actual camper is immaculate, but I don't think he maintained the 6V AGMs well. After a night of fridge and light heater use I am usually in the range of 12.3-12.4 volts on my batteries and showing around 30-40% of my usable AH left on my battery monitor. It's not a huge deal currently as I'm in Colorado and we get lots of sun in the summer, but I would like to move my system toward a more robust setup that I can use for a few days in the winter/colder months when sunlight is harder to come by.

 

I just bought a new truck to haul my camper (2019 F150 with the heavy duty payload package). I'm eventually planning on going lithium (I'm looking at the SOK 204 AH LIFEPO4 mainly for the form factor and where I need to fit the battery into my camper.) I know that I'll need a DC/DC charger for my truck to properly charge the lithium batteries, the rest of my system is already lithium capable as long as I get the lithium dongle for my Iota shore power charger. I'm probably going to go with Renogy again for the DC/DC as their products are economical and have worked well in my system thus far.

 

Here is where I need the help. I'm not the best when it comes to electrical knowledge, but I want to learn more and do the install myself. I'm fairly handy (I managed to install the FWC on my current truck and even wire it up using the recommended FWC method). I know from research that the usual 10G wire won't cut it for the DC/DC charger to work efficiently. I want to install the DC/DC charger in the camper with my battery system. Is this a problem? I can run 2 or 4 gauge wire to the bed of the truck but everything after that is the stock 10G wire FWC uses to wire their campers. Will I need to replace some of the camper wiring with larger gauge wiring to make this work? I was hoping to just remove the blue sea ACR that FWC uses and drop the DC/DC charger in it's place. This would make my future lithium upgrade a simple drop in installation (i think). I'm willing to go lithium now if needed but was hoping to eek a little more life out of the AGMs and go lithium this winter. 

 

Sorry for the long post, thanks for any insight for the electrical novice!


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#2 Vic Harder

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Posted 23 August 2022 - 10:17 PM

Sounds like you are on the right path. The DCDC should be as close to your camper batteries as is practical.

The Renogy DCDC is 30A, right? You can pull 30A over the existing 10g wire, but the voltage drop will be higher than ideal. That’s ok, because the DCDC will compensate.

Ideally, 4G (6G is good enough) should be run all the way from the truck battery back to the camper’s DCDC inputs.

You have to add breakers at both ends to protect that new wire too, and I use Anderson Power Pole connectors in the truck bed to join the two systems.
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#3 joebob25

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Posted 24 August 2022 - 01:50 AM

Sounds like you are on the right path. The DCDC should be as close to your camper batteries as is practical.

The Renogy DCDC is 30A, right? You can pull 30A over the existing 10g wire, but the voltage drop will be higher than ideal. That’s ok, because the DCDC will compensate.

Ideally, 4G (6G is good enough) should be run all the way from the truck battery back to the camper’s DCDC inputs.

You have to add breakers at both ends to protect that new wire too, and I use Anderson Power Pole connectors in the truck bed to join the two systems.

 

Thanks for chiming in Vic! I was hoping you would, I always seek out your threads and comments, they have been so helpful on my journey to understand electricity!

 

Renogy has a 30A, 40A and 60A model. I was thinking about getting the 40A for slightly faster charging. The SOK battery I'm looking at takes up to 50A input. Do you see the 40A model being an issue over the camper's 10G wire? I'm not opposed to running 6G wire all the way to the DCDC if it will be significantly better, I was just hoping to keep things as simple as possible.

 

I'm not familiar with Anderson pole connectors. Would they be significantly better than the Atwood trolling motor plug that I'm using currently?


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#4 Vic Harder

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Posted 24 August 2022 - 03:01 AM



Thanks for chiming in Vic! I was hoping you would, I always seek out your threads and comments, they have been so helpful on my journey to understand electricity!

 

Renogy has a 30A, 40A and 60A model. I was thinking about getting the 40A for slightly faster charging. The SOK battery I'm looking at takes up to 50A input. Do you see the 40A model being an issue over the camper's 10G wire? I'm not opposed to running 6G wire all the way to the DCDC if it will be significantly better, I was just hoping to keep things as simple as possible.

 

I'm not familiar with Anderson pole connectors. Would they be significantly better than the Atwood trolling motor plug that I'm using currently?

There are a number of voltage drop calculators online, I like this one:Voltage Drop Calculator

 

Using 14v coming from your alternator over 25' (that's what it was on my crew cab truck) you get a very hefty voltage drop of almost 17%.  You want that to be 3% or less.  I get 2AWG when I do that at 40A.  30A and you can get away with 4AWG.  Now that is for optimal efficiency, which you don't need because of the DCDC picking up the slack.  6AWG at 40A should be ok.  You lose 7% at that point.  Which brings up an interesting point, as the DCDC will pull enough current to put out its top amperage, meaning it will pull at least (some inefficiency inside the box too) 7% more than 40A, so figure at least 44A draw.  

 

I'd use 60A Blue Sea 285 or 185 surface mount breakers at each end and Anderson Power Pole connectors for the size/gauge wire you choose to use - Amazon.com : anderson power pole connectors

 

vdrop

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#5 Dadocut

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Posted 24 August 2022 - 06:21 AM

Hi,

 

Add a fuse at your truck battery before the heavy wire running from the truck battery to the camper's DC-DC converter.  Something like a terminal mount MRBF fuse works great.  The heavy wire allows for a larger fuse, but something like 80A is plenty large enough for your use.  The battery protects the system in the case of the heavy wire shorting out.

 

I have the small Renogy DC-DC converter.  I ran a "key happy" 12v signal from the truck (i.e. a 12v line that is on only when the truck is running) to the Renogy's enable input.  That way the DC-DC converter only turns on after the truck is running.  I also added a switch in series with the enable to allow me to fully disable the DC-DC converter.  I don't need it on ever time the truck is running.  My solar typically keeps everything charged.

 

You mentioned getting a commercial LiFePO4 battery.  Get one with an integrated BMS (Battery Management System).  The BMS provides over & under voltage protection and monitors temperature to prevent charging when freezing.  You can get a BMS with bluetooth so you can see all the details of your electrical system on your phone. Very handy. 


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#6 Marvap

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Posted 25 March 2023 - 04:33 PM

New here, looking for expert/experienced thoughts about DC-DC chargers and supply wire gauge. It seems to me that the concern is voltage drop, but not functionally for charging (which it is for a simple battery isolator), Rather, it is heat in the wires, which in extreme cases can cause spectacular failures when insulation melts. My idea is that the resistance, current and voltage drop in the run all relate to each other and to the power (heat) dissipated in the run: for a given wire, as current increases, voltage drop increases, which causes a DC-DC charger to draw more current to compensate, which in turn increases the voltage drop. Power is current times voltage, in this case the voltage drop in the run, so as the system ramps up the heat generated goes up.
Do I have that right, or is Herr Ohm chuckling politely at my ignorance?


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#7 Jon R

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Posted 25 March 2023 - 08:55 PM

If you have a 20 plus foot 12 volt wire run from your truck engine compartment to the camper battery, voltage drop will determine the required wire size rather than wire temperature rise from resistive heating.

If you are running a dc to dc charger located at the camper battery, it can raise the voltage at the charging current to properly charge the battery, but you still need to limit voltage drop so the voltage-based engine running detection of the charger works properly. Alternatively, many dc to dc chargers will accept a separate ignition on circuit trigger signal, but that’s more hassle to connect and run all the way to the charger.

There are a bunch of threads where various people have talked about the wire size they use or how to calculate the minimum gage with an acceptable voltage drop. I used 6 awg for a roughly 30 foot run to my 30 amp (output) Victron Orion. You need to size the wire based on the voltage drop for about 36 maximum input amps for that charger. From a resistive heating standpoint that size wire is good for 80 amps dc.
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#8 Vic Harder

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Posted 25 March 2023 - 09:10 PM

New here, looking for expert/experienced thoughts about DC-DC chargers and supply wire gauge. It seems to me that the concern is voltage drop, but not functionally for charging (which it is for a simple battery isolator), Rather, it is heat in the wires, which in extreme cases can cause spectacular failures when insulation melts. My idea is that the resistance, current and voltage drop in the run all relate to each other and to the power (heat) dissipated in the run: for a given wire, as current increases, voltage drop increases, which causes a DC-DC charger to draw more current to compensate, which in turn increases the voltage drop. Power is current times voltage, in this case the voltage drop in the run, so as the system ramps up the heat generated goes up.
Do I have that right, or is Herr Ohm chuckling politely at my ignorance?

yup... power loss over the run = heat in the wires.  Plus, with sufficient drop, the DCDC decides the truck isn't running and cycles on/off too much.  Unless you use the override switch and force the DCDC to stay on and pull even more current.  


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#9 TacomaAustin

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Posted 25 March 2023 - 09:30 PM

... but you still need to limit voltage drop so the voltage-based engine running detection of the charger works properly.

 

That was my rational for selecting #6 gauge wiring for my 30 amp DC to DC controller - which has a cut-off sensing voltage of 12.7v.

 

My seat of the pants estimate was to limit the voltage drop when the controller was drawing full current to less than 3%.  My truck has an old-school 130 amp dumb alternator.  I only have a wire run that is, if I include the camper is less than 15 feet to truck's starter battery.

 

If I had a newer truck with a Mr. Smarty Pants alternator - then I would add the optional sensing circuit between a live ignition wire on the truck and the provided ignition sensing port that is on the DC to DC controller.


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Andrew in Austin, TX


#10 Marvap

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Posted 27 March 2023 - 02:56 PM

If you have a 20 plus foot 12 volt wire run from your truck engine compartment to the camper battery, voltage drop will determine the required wire size rather than wire temperature rise from resistive heating.

If you are running a dc to dc charger located at the camper battery, it can raise the voltage at the charging current to properly charge the battery, but you still need to limit voltage drop so the voltage-based engine running detection of the charger works properly. Alternatively, many dc to dc chargers will accept a separate ignition on circuit trigger signal, but that’s more hassle to connect and run all the way to the charger.

There are a bunch of threads where various people have talked about the wire size they use or how to calculate the minimum gage with an acceptable voltage drop. I used 6 awg for a roughly 30 foot run to my 30 amp (output) Victron Orion. You need to size the wire based on the voltage drop for about 36 maximum input amps for that charger. From a resistive heating standpoint that size wire is good for 80 amps dc.

Thanks!


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