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Correct wire sizing for battery to battery (DC-DC) charger installation


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I am doing an installation of a Renogy DC-DC charger aka battery to battery charger on my truck camper rig. 

I know I can call Renogy support and double check all my figuring but I thought it might be helpful to start a thread where experienced people can post information that can help others...

Anyhoo...

The installation instructions call for the charger to be connected to both terminals on the vehicle starting battery, not using the chassis for ground.  (I plan to have a fuse or breaker at the + connection).  

My question:

Wire run from the battery to the device mounting location is approximately 24 feet. Do I use the total length of both cables + and - in calculating wire guage when the device is not grounded "locally" to the chassis? 

 

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22 minutes ago, noteven said:

I am doing an installation of a Renogy DC-DC charger aka battery to battery charger on my truck camper rig. 

I know I can call Renogy support and double check all my figuring but I thought it might be helpful to start a thread where experienced people can post information that can help others...

Anyhoo...

The installation instructions call for the charger to be connected to both terminals on the vehicle starting battery, not using the chassis for ground.  (I plan to have a fuse or breaker at the + connection).  

My question:

Wire run from the battery to the device mounting location is approximately 24 feet. Do I use the total length of both cables + and - in calculating wire guage when the device is not grounded "locally" to the chassis? 

 

https://grealpha.com/resources/dc-load-wiring-calculator/ This one uses one wire lenght for calculation.

Edited by jcussen
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not even. great question,  (Short answer YES I agree with the other fine gents, use the total distance)

 MY FIRST QUESTION is the DC to DC Charger a 20 Amp or a 40 Amp or some other rating ?????????????

           I would size the wires (vehicle Battery to RV Battery):  a) Per the manufacturers minimum (or larger) recommendations.

                                                                                                            b) To have a minimum ampacity of at least 125% of the chargers rating IE if its a 20 Amp max charger, Id recommend wire having a minimum ampacity of 25 Amps, therefore Id use 30 Amp or bigger rated wire  

                                                                                                            c) You compute the line voltage drop and as/if necessary, increase the wire size until the drop is within acceptable limits. 

  YES Line voltage drop (V = I x R)  occurs on BOTH the + and -  lines, so its the TOTAL DISTANCE upon which voltage drop takes place. If the run is 24 feet and there are both + and - wires, voltage drop occurs across the entire 48 feet, NOT on one 24 foot run only !!!!!!!!

 HOWEVER NOTE the online Voltage Drop Calculators, such as this one for example     Voltage Drop Calculator    tells you to use one way or round trip distance and computes the total line voltage drop accordingly ...   

I (like Chad) was always a conservative electrical designer and since bigger wire = less voltage drop and less I Squared R heat energy losses, if I had a 20 amp charger at a distance of 24 feet, Id use AT LEAST 10 Gauge 30 Amp rated wire but more likely go with say 8 Gauge 40 Amp rated wire to be on the safe and have even less line voltage drop.   If its a 40 amp charger wire sizes approximately double in ampacity.........

PS there's a good chance BOTH your engine battery (duh lol) as well as the RV battery already has the - bonded to the frame, but don let that stop you from running both wires per the  manufacturers recommendations..

YES use the proper rated overcurrent protection device (fuse or DC breaker) on the battery Positive posts. 

 Hope this helps and answers your questions HOWEVER don't risk voiding the warranty and go with AT LEAST the manufacturers recommendations  (or bigger) and NOT what I say as I'm a longgggggggg retired and rusty engineer.

Best wishes, John T     Live from the RV in Lakeway Texas                                                                                       

Edited by oldjohnt
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Yes when it comes to DC electrics connected to yuge batteries I design on the side of keeping the smoke inside the components where it belongs.

More info:

The charger is capable of up to 40amps output. The manufacturer states it will draw up to 60amps input. 

The alternator on the truck is ...um...capable of many amps too because. (200 I’m thinking maybe) - like all newer vehicles the engine loafs along at 1500-1700 rpm at cruise... 

Thanks for the replies.

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2 hours ago, noteven said:

The charger is capable of up to 40amps output.

Thanks for the update. As I mentioned if its a 40 amp charger ampacity doubles. Id use A) Manufacturer recommendations AT LEAST or bigger wires,,,,,,,,,,,,,,,,,B) Wires having a minimum ampacity of 125% x 40 or 50 Amps (or bigger),,,,,,,,,,,,,,,,C) If necessary upgrade if voltage drop is excessive D) Provide overcurrent protection at batteries (fuse or DC rated breaker) of 50 Amps if 50 Amp wire is used or 60 Amp if 60 Amp wire is used E) Bigger wire = less voltage drop. 

PS The ampacity of single conductors in free air is typically (other factors count) much greater then conductors enclosed in conduit or sheathing subject to size of enclosure and how many conductors of what size enclosed therein etc etc etc. If it can deliver 40 amps Id use no less then 50 Amp wire even better 60 Amp  subject to length of run and voltage drop 

John T

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I am back with more questions / ideas. I post these stories so maybe the ideas and replies will be of help to other forum readers too - 

The majority of "you tube installers" with trailers or slide in campers are installing these chargers on board the trailer or camper.  I have found a location in the camper but it be a bit of a rewiring project, uses up a handy storage spot, but mostly the job will be a pain in the ass.  

I am wondering why not install it on the truck and connect to the camper using Phillips 15-336 dual pole connector set? I see some advantages with the unit being available for "other trailers" etc. 

The DC-DC charger is turned on by a circuit connected to an ignition on source on the vehicle. The truck has factory upfitter switches so I can connect one of those to the charger for manual control of the charger without wiring all the way back to the camper. Just the 2 number 4 cables through the 15-336 connector are needed to the camper.  

The connection from the camper's battery switch to make up a 15-336 camper cable is very easy to access via the under belly service panels. 

And yes, my "high end" camper (Cirrus 820) has a WFCO converter load center. (Booooo....) but Zamp solar (yaaaay) so I am thinking of disconnecting the WFCO non lithium battery charger altogether and using DC-DC and solar for 2 lithium batteries. I have a heated place in the camper that will accommodate 2 batteries nicely.  I ran some tests and from a cold start at -10C inside the heating system brings the battery area up to +1C (i.e. above freezing) in 20 minutes. 

Ray at "Love Your RV" channel did a test of lithium charging methods on their rig. He found the OEM WFCO charger would maintain the lithium batteries at around 56% SOC charging at 13.6 volts, so it does not "hurt" the batteries.  If I left it connected the solar and/or DC-DC lithium charger will top them off properly. 

Lithiums apparently don't need or necessarily "like" constant max top off charging anyways? 

 I do not sit for days and days relying on shore power for this rig. If I found truck and solar aren't sufficient I can add a lithium configured charger from Progressive Dynamics to the WFCO, or buy a $70 lithium charger from the battery supplier and use that. The battery supplier also has a recommendation for a controller to turn off the solar charging if the battery temperature is below 0C. The batteries also have low temp cut off in the BMS, but the supplier says it is meant as a fail safe and it is better to use a stand alone adjustable controller for day to day. 

I will fuse at the + terminal right at the battery set. 

 

 

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noteven CONGRATULATIONS you're really getting into this and doing your homework. FWIW here are my thoughts and opinions.

1) I have watched several You Tube how to videos and while many provide what I (strictly in my opinion as a 49 year RV owner and engineer, but no warranty as I'm long retired n rusty) consider correct and valuable information regarding camping and the RV lifestyle in general, when it comes to fire, life and electrical safety, some (certainly not all) are simply NOT trained experienced professional electricians or engineers (and it shows lol) and I have helped and corrected a few of them if I (in my opinion) considered their advice severely lacking or dangerous even. 

2) I would choose to install the DC to DC Charger on the tow vehicle instead of say back at a trailer even if either location will work. Your choice not ours..........

3) Cable size and connectors. I would size the cables with an ampacity of no less then 125% of the rating IE for a 40 amp Charger I'd use at least 50 amp rated cables, compute the voltage drop, and if necessary increase the size. Bigger cable = less line voltage drop and wasted I Squared Energy heat losses. Of course use quality connectors having adequate rating. 

4) I would choose BOTH the DC to DC charger as well as the 120 VAC Converter/Charger that have specific settings for proper charging of Lithium batteries. If I were to spend $1,000 bucks for a battery hoping it will last 10 or so years  I would NOT use a charger designed for ONLY lead acid or AGM  but one that's approved for Lithium even if it will "work" to some degree and even if it can provide at least "some" (although NOT perfect) charging. Compared to the cost of Lithium batteries a few more bucks for a proper and correct charger is worth it in my opinion.  

5) YES install overcurrent protection on BOTH batteries. Those batteries and their huge cables could provide dangerous levels of current heat and sparks should there be a short !!!!!!!!!!!  

 There ya go, again do NOT do as I say but continue to research and do your homework as these are ONLY one mans opinion and others may have different opinions. Its YOUR money YOUR RV and YOUR choice none of ours.

Best wishes, thanks for the update

John T  Live boondocked in the RV from Matagorda Bay Nature Park in Texas, headed to Galveston Beach area later.

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15 minutes ago, Darryl&Rita said:

Same as solar charge controllers, mount as close to the batteries as practical to minimize line losses. That's the idea behind the dc/dc conversion.

Thanks John T and Darryl.

Darryl - the DC-DC charger practical location in the truck camper needs about a 10 to 11ft run of cables to the battery practical location.  If I mount it in the truck it needs about a 14ft run of cable... ya I know... it took a minute of gazing for me to figure that out. 

Connection to trailers is a future thing - maybe not the best in that application. 

I am all ready developing a couple more questions... 

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21 minutes ago, noteven said:

Thanks John T and Darryl.

Darryl - the DC-DC charger practical location in the truck camper needs about a 10 to 11ft run of cables to the battery practical location.  

Repurpose the existing cables to charge only, add cables to get back to remote loads, and mount the converter close to the batteries. The further from the batteries, the lower the benefit of the dc/dc conversion.

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noteven, the DC to DC charger can still "work" regardless of location, its just that line voltage drop may need to be taken into consideration and bigger cables and/or shorter runs reduces it. If at a trailer far from the source it still takes raw input energy (even if lesser voltage due to line drop to it) and uses it to still supply the CORRECT voltage to charge your batteries. HOWEVER if installed in your tow vehicle there can be more line voltage drop back to the trailer (which big enough cables can account for) but its handy if you want to tow different trailers IE you don't need a charger for each and every or if you change trailers. I can envision pros and cons each way..........

 USE WHATEVER WORKS FOR YOUR SITUATION and with big enough cables and quality connectors voltage drop is less of an issue HOWEVER if the cables are too small and/or the run too long, that's where installation back at the trailer may be preferred as there's little drop from the charger to the batteries, yet the charger can likely (subject to design) still perform correctly AT ITS OUTPUT END even at reduced voltage AT ITS INPUT END due to line drop ......... Its function is to take raw input energy (even if not exact voltage) and convert it to the correct output to properly charge the batteries. What I'm  saying is Id worry less about voltage drop on the chargers INPUT then over on its OUTPUT side. Regardless, the closer to batteries the better

Do the math and weigh the alternatives, then do what's best for YOUR situation even if larger cables are necessary to reduce voltage drop it can still work in either location....

John T

 

Edited by oldjohnt
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afternoon update :)

So having some heavy gauge volts and amps coming from the truck while running down the road should make my stupid "3 way" fridge work better too, right? 

Back to the charger project update:

I looks at the DC-DC charger and ideally it would like to have a battery temperature sensor connected. It uses a "telephone wire" connection. Plus the ign on wire to turn it on.  So nearby the batteries will be better... I can either install a 3rd wire from the truck to turn on the charger or install a constant duty solenoid under the hood in the #4 cable and power the charger from itself with a jumper from the input... I'm thinking wire and not solenoid - simpler...

Ok - so my nice heated place under the dinette seat which would keep 2 lithiums super cozy - will not hold the DC-DC charger too. It is 1/4" too tall to fit along with batteries.  So option 2 spot is over there across the camper in another storage compartment that requires careful drilling, routing #4/2  cable from the truck connector and then back down and across the camper in the basement then back up to the seat compartment then.... the aforementioned pita spot to work...

But wait... the existing battery compartment is 3ft from the nice place for new lithium location.  I could refurbish it to hold the charger and a junction bar for the nest of connections that are already terminated in there and run 2 #4's and the temp sensor up to the new battery location... 

this compartment is not heated and is vented of course for flooded batteries. I've camped a couple short nights at -30C which means the flooded batteries are at -30C by morning. Can you say voltage sag? 

Tomorrow my outdoor workshop is going to be sunny and +2C so I'll get the basement opened up and look over the situation...

And Darryl! This just in! My Fort Garry guy has 4/2 black insulated commercial truck grade cable in stock! So I can build a perfessional like cable to match the RV 7 way one hanging out the camper now... except this one will bend in the cold... 

 

 

Edited by noteven
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About your cables...... yes, you can use low oxygen stranded copper welding cable as many of us do.  But...... you will eventually experience the "blue hue" where any copper is exposed (like at the Anderson plug and lugs) to the road salts and environmental crap encountered along the way that a welder would not see. This oxidation will seep into places you would never expect, even with the addition of an anti-ox coating.  Yea, you can use marine heat shrink for protection but that is no guarantee.   Thus, even if it is more expensive, it is best to use "tinned" Marine Grade cable which will also have the jacket or outer covering needed for UV, salt, and diesel exhaust soot (very corrosive) protection.  I have seen multiple copper cables literally eaten alive where the jacket has broken or cracked creating a high resistance point that generates heat and a voltage drop that grows like an avalanche.  I don't mind spending someone else's money on marine grade cables - they aren't just for boats anymore.

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Hi RandyA - 

Around here I might have to send an order out over the pass by dog sled to get tinned cable 🤣

I am planning to use a Phillips 15-335 and 15-336 2 pole connector set - used for commercial trailer lift gate charge wire connections - I have used Anderson in the box of a previous truck but I wouldn't use one down by the trailer hitch in the salt and chit .... 

I am planning a marine breaker up under the hood which I will open to shut the power off to the Phillips connection when not in use... 

Open to advice I am ...

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noteven, Thanks for the update.  Having owned boats for years I can attest to the quality of Marine Grade wiring and components BUT ITS EXPENSIVE GRRRRRRRRR. Fortunately an RV isn't typically as badly exposed to harsh marine environments such as salt water (unless one beach camps often which I do)  but if one uses Marine grade cable and switches etc. that's great.

FYI you asked about correct wire sizing and above I furnished what would be my MINIMUM ampacity (but bigger if voltage drop was excessive) so FYI here's a voltage drop calculator tool I find useful.

     Voltage Drop Calculator

Of course use appropriate DC rated circuit breakers for overcurrent protection, and hey any Marine Grade wiring or components would be great. 

Hope this helps, keep us posted as to your choices and progress or more questions.

John T  Live in the RV from Galveston Texas

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Update - 

It was a pleasant afternoon in the outdoor shop, bright and sunny and just above freezing so I removed a basement service panel to take a gander at the situation. It has been determined that the wiring and rewiring and proposed placement of Li batteries will require a disassembly of the rear lower body work of the camper. It comes apart mechanically, but its a fair amount of work and would require resealing all the sealed seams.

So I will apply Red Green's Principle of Leave 'er.   I am not doing this project in the winter outdoors. 

Thanks everyone for the information. 

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