DC to DC charger? with a genset and battery boost

[Deznut]

I know this is a well explored topic ( and I have tried to find a definitive answer in my forum searches) ,but I could use some help/advice from someone who may have installed one of these "chargers" in a Jayco RV that has a Genset and battery boost option. ( mine is a 2019 Alante 26x), the DC to DC charger is a Renogy unit rated at 40 amps.
1) why install a DC to DC charger?...going to install Solar panels and upgrade to Lithium batteries...the DC to DC charger will protect my alternator from overcharging/overheating.
2) With the help of my trusty multimeter, my coach has 3 wires going to the positive terminal of my coach battery;
Cable "X" = goes to my inverter ( turns 12v DC to 120v AC)...
Cable "Y" = comes from my converter ( battery charger when hooked up to "shore power")...
Cable "Z" which has 12V DC power coming from it when the engine is running, but this cable ( I'm assuming) also sends power to the starter on the Genset.
QUESTION:I'm assuming I connect cable "Z" to the input of the DC to DC power supply ( and output from the DC2DC to my coach's batteries) but will the DC to DC power supply allow DC power back to the Genset starter without destroying the DC to DC power supply?....AND will it allow power to start the engine through the battery boost switch without destroying itself?...I have tried to communicate with Renogy though their site , but it is not very user friendly.
Anyone that may have done this DC to DC install with a Genset and battery boost have an answer?

 

[craigav]

You may want to check with Jayco and see if you can get a wiring diagram. I highly suspect your vehicle already charges the coach battery(s) and that needs to be disconnected and then you could use those same wires. You wouldn't want to have both a DC2DC charger and what already exists in place at the same time. ~CA

 

[Deznut]

Thanks for the quick reply ...I have a schematic...it shows the 3 wires I describe in the first post...the question is... can you use the existing wiring to run the DC2DC power supply without destroying the DC2DC unit when I need the genset to start or the battery boost...the power would be coming FROM the batteries thru the DC2DC pwr sply to the starter in the genset or to the engine starter...is the DC2DC pwr sply isolated from reverse current flow???? ( not to be confused with reverse polarity mix up).

 

[craigav]

Thanks for the quick reply ...I have a schematic...it shows the 3 wires I describe in the first post...the question is... can you use the existing wiring to run the DC2DC power supply without destroying the DC2DC unit when I need the genset to start or the battery boost...the power would be coming FROM the batteries thru the DC2DC pwr sply to the starter in the genset or to the engine starter...is the DC2DC pwr sply isolated from reverse current flow???? ( not to be confused with reverse polarity mix up).

Feel free to share the schematic if you can and I can take a look as well. The DC2DC charger is very similar to a standard battery charger with the difference being that the input is 12vdc instead of 120vac so there are no concerns with backflow damaging the DC2DC charger. If you have a BIM installed (as many Class A's have) then the DC2DC charger can take the place of the BIM (but wouldn't allow the aux start to work without making some changes). I would not think that the generator starter would use those same wires though.

My concern would be that if your RV still has the battery boost setup enabled (I suspect you are using the term "battery boost" to be the same as what I call the "Auxiliary Start" button on the dash) that you still have the BIM (or solenoid) enabled which you may not be aware that the aux start solenoid (relay or BIM) on most motorhomes also charges from the chassis alternator battery back to the coach battery(s) regardless of the aux start switch being pressed in and that setup may need to be disconnected depending on how it is setup from Jayco, otherwise you likely would encounter problems. The wiring diagram should help in determining if this is the case. ~CA

 

[Deznut]

attached is a pdf of schematics ....I own a Alante 26x ....so it is page 1 of the attachment...thanks for your input concerning this project of mine

 

[craigav]

attached is a pdf of schematics ....I own a Alante 26x ....so it is page 1 of the attachment...thanks for your input concerning this project of mine

Thanks for sharing. Hopefully you are not planning to install this today as I will not be able to spend the time needed to look deeper into the schematic you shared until this evening as I will be away from my computer all afternoon, and after I do so I can give you more accurate information on how to best install the DC2DC charger.

BTW and just FYI, I also have the Renogy 40a DC2DC charger but I haven't taken the time to install it yet and I see from a quick look at your schematic that your setup is very similar as on my Class C's (in regards to the aux start switch and relay used for charging and boosting). I have a similar plan on mine that should work well on yours also and can give you more details later this evening, possibly tomorrow morning if I get in later this evening than I plan. ~CA

 

[Deznut]

No problem...gotta take out the batteries and drill some holes to mount the DC2DC along with a fuse block or 2....have a good day...talk to to you later - Ron

 

[craigav]

I highlighted in Yellow the wiring that currently is setup to charge the coach battery(s) which is listed as a 4GA (awg) Black wire. If you notice that there is a 100 AMP Start Solenoid inline which is turned on by either pressing the Battery Boost Switch and also when the KEY is on which is the 14GA (PUR for Key on and ORG when pressing the Battery Boost). This is the Solenoid I mentioned earlier.

You could use the 4GA Black at the Coach Battery as the input to the DC2DC charger and then output the DC2DC charger to the Coach Battery. That would be the simplest approach but keep in mind when you do that and because you can't backflow the power through the DC2DC charger that your Boost Switch will no longer allow the Chassis battery to be boosted.

Without some additional wiring and changes made around the Aux Start Solenoid your options are limited.

Where did you mount your DC2DC charger? ~CA

 

[Deznut]

Yes...great minds think alike...
1) that solenoid is controlled by the ignition switch...I believe the violet wire energizes the solenoid
2) the orange wire is the controller of the "Battery Boost switch"....I was going to just remove it ...but as you can see in the picture ....I have to cut it...this way no accidents with hitting the switch at the wrong time
3) I have installed a small battery tender ( switched on/off from the driver's seat) for the chassis battery that is AC .powered ...so the AC inverter will maintain the chassis battery when the engine is not running
I'll try and add some more .pictures once I get it "finished"....you can see a corner of the DC2DC in one of the images...it is mounted vertically with the wire input and output front to back...gotta have access to the front to connect the power "on" wire and the dip switches for battery config.

 

[Deznut]

Progress so far...
1) cost of 2ga wire is over $3 a foot...need to determine lengths and runs...and take out a bank loan!
2) Joined a forum sponsored by Renogy ("Discord") posted my original question and a forum member suggested I run a parallel system with the chassis battery charging a AGM battery and the Solar array charging a Li battery (On shore power the LI would get charged...and the battery boost function would be available)
3) got me thinking about the size of the battery tray...but note in the pictures the large rivets protruding into the battey box area ...made some spacers out of wood to insure my current setup ( 2 six volt AGM) doesn't get damaged
4) the mounting holes in the Renogy DC2DC charger (IMO) are too small
....back to work

 

[craigav]

Looks good, Just FYI, for the DC2DC 40 amp charger you really don't need anything larger than 6 AWG on the input side and 8AWG would be fine on the output side. I would likely use 6ga though on both sides. ~CA

 

[Deznut]

Thanks ....I'll compare pricing 2ga ve 6ga....I know the Solar is all 10 ga - Ron

 

[Deznut]

...back at it - the 6ga wire is good to work with and has a high enough Amp rating to handle the charging load from the DC2DC charger.
1) 6 ga wire from Amazon- brand "Spartan" welding and auto wire...added benefit my old craftsman stripper crimper had a place for stripping 6 ga
2) Terminal Lugs - you have to plan this out, Fuse blocks and battery posts have 5/16" dia studs , but the disconnect switches have 3/8" studs, plus I'm working with existing 4ga wire, the new 6ga wire and the solar panels are 10ga...again from Amazon a supplier "Wirefy" makes Heavy duty tin coated copper lugs in all the sizes I needed.
3) Crimping the lugs - I purchased a hydraulic crimper from Amazon - Amacnc brand came with cable cutters and a set of dies...the instructions state if the crimp is not strong enough ( I pull tested all my crimps - initial efforts failed) to mis match the die set ( ie use a 6ga in the top and smaller 8ga in the bottom) that solution worked but the crimp produces some "wings" on the lug - which you can file off or sand off with a small belt sander.
4) used a 90 degree drill attachment from Harbor Freight to ge into the tight areas to drill the holes to mount the Solar MPPT and the DC to DC charger...a "center drill" ( if you ever ran a lathe- you know what it is) to start the holes, and then cut off some drill bits ( shortened the bit with a Cut off wheel) to make the correct size holes in the steel panels.
5) built a "sub panel" ( 1/4" plexiglas) to mount on the front of the battery "drawer" - purchased MPPT controller/history logger, battery status with a shunt, other battery status gauges I had before and toggle switches to control the DC to DC charger. also battery disconnect switches , but the existing cables are not very flexible ...so the disconnects will only disconnect the Dc2 DC charger from the batteries ( left side) and the Solar panels right side.
....waiting for fuses from Amazon- hope to test it Saturday

 

[Deznut]

So Hold the phone....you mean I DON"T have to cut the orange wire?
I watch some You tube info on devices to see the pros and cons...one of the videos had a person comment that a DC to DC converter made by Li Time can handle the Battery Boost function....so I emailed LiTime with a description of my coach and the Battery Boost function as I understand it.... here is their reply:
Dear Ron,
Thanks for reaching out LiTime. This is Mavis and I'll be here with your assistance.

This won't hurt the circuit, it's a design of the circuit itself.
In reverse charging, the source of the charge is the living battery,
and the one being charged is the starter battery.

Hope this info will be helpful.

Kind Regards,
Mavis
so If I understand this reply correctly...it can handle the Battery Boost option ( Reverse charging) ...I added this post because at this time the 40 amp unit is on sale for around $100! ( and the mounting holes appear to be heavy duty)
https://www.litime.com/products/lit...4-lead-acid-sla-gel-agm-and-calcium-batteries
Sale only lasts a couple days...if you buy and install one any feedback would be appreciated - Ron

 

[craigav]

I don't think they (Li Time) understood your question. A DC2DC charger is a one way device with an input and output (not two-way). You cannot change this without turning the charger around (or the connections). I think that is what they were trying to tell you when they stated "it's the design of the circuit itself" meaning not the charger's design.

Also, if you were to run a parallel setup (keeping the existing boost and charge circuit in place) then you would be bypassing the DC2DC charger and in addition the output of the DC2DC charger would be electrically connected back to the input which the charger may or may not like (depending on the protection circuit in the charger) and at minimum would defeat the purpose of the DC2DC charger.

The above being said if I understand correctly what you have stated above. It all looks good though in the pictures. ~CA

Feel free to share a link to the Youtube and perhaps I could get a better understanding of what the person stated in the comments, without seeing it I would suggest the person who stated that the converter can handle the "Battery Boost" is incorrect as you can't get more out of the charger than it is rated for and it is not a bidirectional device.

 

[Deznut]

Thanks for the clarification...dealing with the language barrier is tough for both sides....onward with the CUT Orange wire and further wiring "clean up" in the battery bay.

 

[Kevin Cooper]

I highlighted in Yellow the wiring that currently is setup to charge the coach battery(s) which is listed as a 4GA (awg) Black wire. If you notice that there is a 100 AMP Start Solenoid inline which is turned on by either pressing the Battery Boost Switch and also when the KEY is on which is the 14GA (PUR for Key on and ORG when pressing the Battery Boost). This is the Solenoid I mentioned earlier.

You could use the 4GA Black at the Coach Battery as the input to the DC2DC charger and then output the DC2DC charger to the Coach Battery. That would be the simplest approach but keep in mind when you do that and because you can't backflow the power through the DC2DC charger that your Boost Switch will no longer allow the Chassis battery to be boosted.

Without some additional wiring and changes made around the Aux Start Solenoid your options are limited.

Where did you mount your DC2DC charger? ~CA

Craig, I looked at the wiring diagram and I do not see a DC to DC converter in the system. On our 2018 Alante 29S the unit labeled MP between the chassis battery and the house batteries has 80 amp circuit breaker on it. So, I’m not understanding where the 40 amp Dc to Dc is or what it does.
If it were me, I would be very concerned that adding another path for alternator current to house batteries would allow too many amps to the output conductor on the alternator and cause insulation damage on it.
Let me know where I’m missing it in the diagram.

 

[craigav]

Craig, I looked at the wiring diagram and I do not see a DC to DC converter in the system. On our 2018 Alante 29S the unit labeled MP between the chassis battery and the house batteries has 80 amp circuit breaker on it. So, I’m not understanding where the 40 amp Dc to Dc is or what it does.
If it were me, I would be very concerned that adding another path for alternator current to house batteries would allow too many amps to the output conductor on the alternator and cause insulation damage on it.
Let me know where I’m missing it in the diagram.

You didn't see one in the wiring diagram because there isn't one in the wiring diagram. The OP is installing a new DC2DC charger and what I highlighted is the existing charge circuit (and Aux Start Boost) that in part can be used for the DC2DC charge line instead (not to duplicate) of using completely new wiring and as you mentioned not to be used in parallel with a new charge circuit.

A DC2DC charger is very similar to any 120v battery charger and most (if not all) have a charge profile for optimally charging LifePo4 batteries where most all engine alternators and their regulators are not regulated for optimally charging LifePo4 batteries. The main difference is that a DC2DC charger uses ~12v (plus or minus a few volts) as the input instead of a 120v input and the output side charge profile can be set for Lead Acid, AGM, LifePo4 style batteries. In this case the input amperage would likely max out around 50 amps and the output amperage max out right at 40 amp which is less than the existing charge circuit wiring is capable of.

The main benefit of a DC2DC charger (as in the OP's plan) is to be able to charge LifePo4 coach battery(s) optimally (higher voltage and a 40A constant current until the LifePo4 reach ~14.4v even if\when the input voltage from the chassis engine's alternator is outputting a lower voltage) which will also charge the LifePo4 battery(s) to 100% quicker than without using such a charger. The use of a DC2DC charger also limits the current draw from the engine's alternator to the coach battery(s) although the original charge circuit on most motorhomes I have seen have 80a self resetting breakers as well for protecting the circuit. ~CA

Just a note, you can find DC2DC chargers with different amperage ratings, 40a is fairly common though.

 

[Kevin Cooper]

Thanks Craig. Somehow I thought he said he was replacing the DC2DC charger. So it is should now not close the 100 amp solenoid when started, the only time it is used is with the power to jump start the engine or the generator from chassis power. Correct? This is all necessary because of lifePO batteries charging profile. The DC2DC could easily be connected to the post of the 100 amp solenoid on the 4ga red post for power, but would need a fuse or breaker.
Ok, I get it! Thanks Craig!

 

[Deznut]

Success - got it all wired up....had wifey jump in the driver's seat and first just turn on the ign switch - relays clicked power to the DC2DC was evident, nothing was smokin, all the fuses were still conducting electricity. Second had her start it and I measured the current flow from the DC2DC power supply...almost 9 amps (the batteries have been disconnected from the coach and shore power for about a week) ...with the engine running, I had her hit the starter for the genset...measured about 20 amps from the DC2DC to the battery bank, again nothing smokin and all fuses intact.
I did go back and revisit connecting all power leads to the disconnect switch ( one from the converter , one to the inverter and one from the DC2DC power supply),now the power from the coach's batteries are switched as recommended by many schematics when adding Li batteries.
Hope this post helps some of you, Thanks to Craig for his valuable advice!