DC-DC charger (B2B) options and general discussion

[MetalChef]

Added a column for parallelable (only for the single purpose DC-DC chargers), So far I only filled in Victron and Sterling. I'd like more confirmation on the Renogy, Amazon Answers have a poor track record when it comes to technical questions. And I've no idea about the last two options.

Not sure if this helps but someone here in another diysolarforum.com thread also mentioned someone from Renogy tech support said 2 of their DC to DC chargers could not be used together. Here is a screen shot.

 

[Dogsareloud]

I decided against a B2B for now - The distance from my F250 alternator to the battery bank in my towed 26ft RV will end up being almost 40 feet. Using the Victron Cable Calculator, for a 30 amp charge, I would need to run 1/0 cable....I cannot visualize this being a good option yet. It makes sense for a lot of applications but not for me.

 

[Yuphorya]

Hi @Dzl , great thread to collect all the information. I was at this stage months ago and remember seeing many questions on this subject. I read through this thread and see that Bogart Engineering SC-2030 isn't included. It's intended for a Solar > Battery charger but can also function as a DC-DC charger limiting current to 30A with an AGM charge profile. (it can be combined with a control unit at extra cost to make a custom LFP profile if needed). I will be testing this with my battery bank in the coming weeks. @$125 it's a budget option.

High Efficiency Solar Charger Controller

Compact, powerful and smart solar charger controller for RV, off-grid. 30 Amps 12V and 24V charger for Gel, AGM, lead acid, lithium LFP. WiFi remote capable

www.bogartengineering.com

 

[DerpsyDoodler]

Agreed mostly, but amazon answers are also answerable by anyone who has purchased the product. If you look at a lot of electrical component Q/A's on amazon, maybe as much as half have answers mistaking volts and amps, amps and watts, etc. Sometimes its the seller, more often its a well meaning but misinformed customer. And concerning Renogy, their ground level tech and sales reps don't have the greatest reputation/track record.

Good things to know.

 

[RGIvy]

Most of the Votronic chargers have this in their specs:

• Integrated Vehicle Power Supply Filter: Unproblematic parallel operation of charging sources (EBL, chargers, motor- driven and petrol-driven generators, solar systems) on one battery.

e.g. https://www.votronic.de/images/Manuals/04. B2B Chargers Booster/3324_e.pdf
or https://www.votronic.de/index.php/en/products2/series-vcc/standard-version/vcc-1212-50-new

 

[MetalChef]

Not sure if this helps but someone here in another diysolarforum.com thread also mentioned someone from Renogy tech support said 2 of their DC to DC chargers could not be used together. Here is a screen shot.

A very common set of questions in the beginner and the vehicle mounted systems subforums concerns charging a LFP house bank from a vehicles starting system (alternator). The most commonly used and recommended devices for this purpose are DC-DC chargers (also often called Battery to Battery (or B2B) Chargers). This thread is intended to be an index of the various options available as well as a catchall thread for questions and answers, discussion, etc.

Disclaimer (and request for help/collaboration): I do not own and have not used any of these. I have compiled this list because its a very common area of interest and confusion and also for my own interest. Some of these products I have researched in moderate depth, others I have barely read the product description and/or datasheet. There are bound to be errors, omissions, inaccuracies, etc. And there are some aspects that are just hard to represent neatly in a table. So take the accuracy with a grain of salt (especially anywhere you see a red ? or a *) and please help me improve the info if you see something that needs correcting or can think of another specification that should be added.


Charger Only
(DC-DC / B2B)

Brand / Model	Max Current 12/24	Input Voltages	Output Voltages	LFP compatible	Parallelable
Victron Orion-TR Smart (Isolated)	30A / 17A	12, 24	12, 24	Yes	Yes
Victron Orion-TR Smart (Non-Isolated)	30A / 17A	12, 24	12, 24	Yes	Yes
Sterling Power Pro Batt Ultra**	70A / 35A ***	12, 24	12, 24, 36*, 48*	Yes	Yes
Renogy DCC1212**	60A / --	12	12	Yes	?
Redarc DPS	40A / 25A	12, 24	12, 24	Yes	?
Votronic VCC	TBD	12, 24	12	Some Models	?

*Not available in all input voltages
**Not all models shown/listed in link or table
***Input current (output current not specified)

Dual Function Charger
(DC-DC + MPPT)

Brand / Model	Max Output	Input Voltage	Output Voltage	Max Solar Input Voltage	LFP Compatible
Renogy DCC	50A / --	12	12	25 (?)	Yes
Kisae DMT	50A / 30A	12, 24	12, 24	50	Yes
CTEK D250SE	20A / --	12	12	23 (?)	Yes
Redarc BCDC	50A / 20A	12, 24	12, 24	32 / 28 (?)	Yes

Triple Function Charger
(DC-DC + MPPT + AC Charger)

Brand / Model	Max Output	Input Voltage (DC)	Output Voltage	Max Solar Input Voltage	LFP Compatible
Redarc Manager 30	30A	12, 24	12, 24 (?)	30 (?)	Yes
Votronic VBCS	TBD	12	12	TBD	Yes

Today I called Renogy tech support directly to ask if 2 Renogy 60a DC to DC chargers can be combined to increase the amp to 120. Renogy tech support technician said "we do not recommend combining more than 1 Renogy DC to DC chargers in parallel as one device might detect the charge from the other charger and cancel it out or turn itself off or it could damage the battery."

 

[Annie C]

Am I correct in thinking that the alternator is protected if a DC-DC charger is used? My alternator is a Bosh 90amp in my Westy. The battery, I think, will be an SOK 206 Lithium. There will be the appropriate isolation and wire sizing. I am leaning toward the Renogy DCDC MPPT 30. I will add Solar on for next summer and keep the AC charging separate as I want to have the battery in the house during winter for the occasional power outage.

Edit: I did see in the video it seems to limit to 25 amps if solar and alt is running at same time. I will search more for confirmation.


Comments...

Let me know if this isn’t the appropriate place to ask.

 

[Dzl]

My understanding of the purpose of the DC-DC is to protect the alternator. However the method of this is by limiting the current between the alternator and the lithium batteries. So the role of the DC-DC is to (1) to limit this current to a level that won't overburden the alternator (2) apply the proper charging parameters optimal for your battery type. In order to do #1 it has to be sized properly (small enough) to limit current to a level your alternator can handle, or possibly have a selectable output (not sure if any have this ability).

 

[Annie C]

That is my understanding as well. So is it safe to say then the DC-DC MPPT doesn’t increase the amps to the charging side for the battery? IE, you can not have a 50amp charger for a 50amp alternator.

If that is the case, where is the safe balance? A rule of thumb?

Thank you for answering so quickly.

 

[Annie C]

This is what I just found. Estimate max should be 50% or less.

DC-DC Charger vs Battery Isolator for Camper Alternator Charging

DC-DC Chargers and Battery Isolators are the two ways to charge a camper house battery bank with the alternator. This blog post will teach your the pro's and con's of each method.

www.explorist.life

 

[Dzl]

I've heard about a 1/3 is a safe estimate, but I can't say how true or not that rule of thumb is. What I've heard is to set aside 1/3 for overhead/safety margin, 1/3 for the vehicles systems, and the last 1/3 is free to use to charge the house banks. I think at best this is a very rough estimate, it seems like there are a lot of variables.

In your situation a 30A charger would be under the 1/3 rule of thumb, as well as the 50% estimate you found so its probably a good size but my understanding is somewhat shaky here. There isn't a ton of reliable in depth info on this that I've been able to find (so far at least).

 

[sshibly]

I decided against a B2B for now - The distance from my F250 alternator to the battery bank in my towed 26ft RV will end up being almost 40 feet. Using the Victron Cable Calculator, for a 30 amp charge, I would need to run 1/0 cable....I cannot visualize this being a good option yet. It makes sense for a lot of applications but not for me.

I have a duramax and I have a 40A trailer power wire option, the factory wire is pretty filmsy, guys are running with it fine, yes the volts drop a bit. My truck is 24.5 ft long add to that the trailer tongue and then the boat.
I plan on doubling a 10awg wire to hitch plug and then to the boat.

I am using a simpler solution

13v chevy alternator 40A max > 12v to 24v sealed waterproof converter > 14.4v sealed converter [pwm].

 

[MetalChef]

Hey folks so I'm finally getting closer to install day for the electrical upgrade to my 2004 Lance 1161 Truck Camper. I've spent hours researching DC to DC chargers and Battery to Battery chargers and have decided all of the ones currently on the market are meant to protect low or medium output alternators and would defeat the purpose of the premium I spent to purchase my 370a high output alternator I currently have on my 2000 Ford F350 SD. So as an alternative to using DC to DC chargers, I have decided instead to isolate my 2 - 400ah LiFeON4 batteries into 2 separate battery banks using a 4-way manual switch so that my alternator will never to trying to charge 800ah of LiFeON4 batteries at once. On my previous truck camper my 370a alternator had no problems charging 3 - 150ah LiFeON4 batteries at once so am hoping this solution will work. Would welcome some feedback or suggestions. The fuses, breakers and busbar placements are random and my installer will decide where those should go. Cheers

 

[DerpsyDoodler]

I have a duramax and I have a 40A trailer power wire option, the factory wire is pretty filmsy, guys are running with it fine, yes the volts drop a bit. My truck is 24.5 ft long add to that the trailer tongue and then the boat.
I plan on doubling a 10awg wire to hitch plug and then to the boat.

I am using a simpler solution

13v chevy alternator 40A max > 12v to 24v sealed waterproof converter > 14.4v sealed converter [pwm].

Is that a stock alternator? I wouldn’t Continuously push it to its max. Based on what I’ve read, it is ideal to keep the alternator’s continuous output at 1/2 to 70% its max rated output, otherwise you will burn it up quickly. YMMV but thought i’d give you the chance to look into that yourself, before you get caught replacing an alternator on the side of the road.

edit: and the numbers I quoted is for total continuous alternator output (running systems, charging chassis battery, AND your added load of charging house batteries).

 

[Annie C]

I have a duramax and I have a 40A trailer power wire option, the factory wire is pretty filmsy, guys are running with it fine, yes the volts drop a bit. My truck is 24.5 ft long add to that the trailer tongue and then the boat.
I plan on doubling a 10awg wire to hitch plug and then to the boat.

I am using a simpler solution

13v chevy alternator 40A max > 12v to 24v sealed waterproof converter > 14.4v sealed converter [pwm].

I am lucky in that my run would be no more than 6 feet. Westy has engine in back and battery will be in the middle of the vehicle.

 

[Annie C]

I've heard about a 1/3 is a safe estimate, but I can't say how true or not that rule of thumb is. What I've heard is to set aside 1/3 for overhead/safety margin, 1/3 for the vehicles systems, and the last 1/3 is free to use to charge the house banks. I think at best this is a very rough estimate, it seems like there are a lot of variables.

In your situation a 30A charger would be under the 1/3 rule of thumb, as well as the 50% estimate you found so its probably a good size but my understanding is somewhat shaky here. There isn't a ton of reliable in depth info on this that I've been able to find (so far at least).

I guess the lower the better.

 

[Annie C]

View attachment 36560

Hey folks so I'm finally getting closer to install day for the electrical upgrade to my 2004 Lance 1161 Truck Camper. I've spent hours researching DC to DC chargers and Battery to Battery chargers and have decided all of the ones currently on the market are meant to protect low or medium output alternators and would defeat the purpose of the premium I spent to purchase my 370a high output alternator I currently have on my 2000 Ford F350 SD. So as an alternative to using DC to DC chargers, I have decided instead to isolate my 2 - 400ah LiFeON4 batteries into 2 separate battery banks using a 4-way manual switch so that my alternator will never to trying to charge 800ah of LiFeON4 batteries at once. On my previous truck camper my 370a alternator had no problems charging 3 - 150ah LiFeON4 batteries at once so am hoping this solution will work. Would welcome some feedback or suggestions. The fuses, breakers and busbar placements are random and my installer will decide where those should go. Cheers

That is a huge system!

 

[sshibly]

Is that a stock alternator? I wouldn’t Continuously push it to its max. Based on what I’ve read, it is ideal to keep the alternator’s continuous output at 1/2 to 70% its max rated output, otherwise you will burn it up quickly. YMMV but thought i’d give you the chance to look into that yourself, before you get caught replacing an alternator on the side of the road.

edit: and the numbers I quoted is for total continuous alternator output (running systems, charging chassis battery, AND your added load of charging house batteries).

My head hurts from crunching number,
I had to look up the UPLIFTERS guide to figure out what you mentioned:

1. Alternator RPM vs Amp output
2. Find my high idle option for my ECU
3. Drink a few adult beverage and say f-it and pulled the bussman fuse for the trailer power and put my little genny in my truck bed ;-)

The duramax factory high output alternator has enough to cough out 40A at the trailer power.

At hwy speed I am at 1800 rpm give or take a few.

Unfortunately, this spring I have to put the fuse back in and finish this project.

 

[sshibly]

View attachment 36560

Hey folks so I'm finally getting closer to install day for the electrical upgrade to my 2004 Lance 1161 Truck Camper. I've spent hours researching DC to DC chargers and Battery to Battery chargers and have decided all of the ones currently on the market are meant to protect low or medium output alternators and would defeat the purpose of the premium I spent to purchase my 370a high output alternator I currently have on my 2000 Ford F350 SD. So as an alternative to using DC to DC chargers, I have decided instead to isolate my 2 - 400ah LiFeON4 batteries into 2 separate battery banks using a 4-way manual switch so that my alternator will never to trying to charge 800ah of LiFeON4 batteries at once. On my previous truck camper my 370a alternator had no problems charging 3 - 150ah LiFeON4 batteries at once so am hoping this solution will work. Would welcome some feedback or suggestions. The fuses, breakers and busbar placements are random and my installer will decide where those should go. Cheers

It does not matter how big your system is,
Think current limiting device.
You need to work backwards and figure out your max Amps.

My transit van as 60amp CCP behind the driver seat, and I plan on putting the batts few feet away. Even though I have 60a (not sure what rpm I need though). I am putting in a 13A CC CV charger so my draw will NEVER to more than 200W with efficiency loss.

You can connect 2 13A 13.8v chargers to each of your banks and turn on your high idle switch and smoke your fav cigar while you wait for the batts to charge.

FYI: I love Lance TCs

 

[MetalChef]

That is a huge system!

Yeah, my plan has alway been to have a large enough system to run an air conditioner in my truck camper during the summer. I was even originally planning to swap out the Coleman 13,500 BTU roof air conditioner to swap in a mini split but since its an old used truck camper, I'm think to spend a little money at a time just to make sure it's not a lemon.