DC-DC charger (B2B) options and general discussion

[Dzl]

ok, not trying to confuse the two or make a reference to isolated vs non, but in a b2b alternator charging system, ensuring that the chassis batteries are isolated when no power is supplied by the alternator seems like an important feature to note (unless all b2b chargers have this feature, which i would not take for granted).

I'm not 100% positive, but I'm 95% sure all of the B2B chargers designed and marketed for charging the house/auxiliary battery from the vehicles starting batttery/charging system have a method of isolating the two batteries when the vehicle is not running.

I believe most tap into the ignition wiring, some may have a voltage trigger or something more complicated.

However the above may not apply to the DPS, since as you noted it looks to be just a power supply, not a charger.

What I'm hoping is that as this thread gets more attention, we get more people that actually own each of these devices that can chime in and fill in details for the devices they own and are familiar with.

 

[DerpsyDoodler]

I'm not 100% positive, but I'm 95% sure all of the B2B chargers designed and marketed for charging the house/auxiliary battery from the vehicles starting batttery/charging system have a method of isolating the two batteries when the vehicle is not running.

I believe most tap into the ignition wiring, some may have a voltage trigger or something more complicated.

However the above may not apply to the DPS, since as you noted it looks to be just a power supply, not a charger.

What I'm hoping is that as this thread gets more attention, we get more people that actually own each of these devices that can chime in and fill in details for the devices they own and are familiar with.

The two ways I’ve seen are ignition (like you noted) or automatic voltage sensing (detects voltage high enough to be considered charging voltage, when found, allows charging).

 

[dozynsleepy]

Hi,
I also experienced 2 issues.
slow charging via alternator. Not sure if Renogy is draining the starter battery with IGN connected, assumed a smart control. But unsure it this is working .

and promised tickle charging of starter battery not working. at least I did not see any current and LED Blinking when battery LED is green and solar is charging.


Anyone having same topic?

Or did I miss anything and it is a user error

Thanks T

I have seen the full max 50 amps from the alternator, although my solar wasn't connected at the time.
My understanding is that once solar is connected the max is split 25 amps solar, 25 amps alternator.

25mm2 (4 awg) from the vehicle battery (alternator) to the alternator terminal on the charge controller (5 metre length).
25mm2 (4 awg) from the charge controller to the house battery.
Battery at 51 % state of charge (SOC)

1. Traditional alternator so nothing connected to the BVS terminals on the charge controller.
From the manual
BVS - Battery Voltage Sensor (RVSCC) port for measuring the battery voltage accurately with longer line runs. Due to the connection and cable resistance, there can be discrepancies in voltages at the terminal vs the BVS.

2. LiFePO4 batteries so nothing connected to the BTS terminals on the charge controller.
From the manual
BTS - Battery Temperature Sensor port utilizing data for accurate temperature compensation and charge voltage adjustment

3. Battery Type Indicator - Colour Blue - Status ON - Description - Lithium
2 x 12V 100Ah smart lithium iron phosphate battery in parallel so charger controller set to Lithium

It is worth purchasing the Renogy BT-2 Bluetooth Module , allows you to use the Renogy DC home app when plugged into the Renogy DCC50S 12V 50A DC-DC on-board battery charger with mppt.
Screenshot from the Renogy DC Home app showing the current being limited to 48.15 amps from the alternator

 

[Reed Cole]

Ahh, well that seems to make sense, and thanks for that explanation. I guess part of what had me concerned is some of the first dc dc charger isolators I looked at several days ago, did specify a maximum input amperage. I will have to retrace my steps and make sure I understood what I was looking at. Thank you.

If it were an issue nobody would use these devices because most alternators can supply more current than these devices draw.

Sterling rates their chargers according to their input, and may have been worded as the maximum input, but that just means the maximum amount of current the charger can draw.

 

[DerpsyDoodler]

If it were an issue nobody would use these devices because most alternators can supply more current than these devices draw.

Sterling rates their chargers according to their input, and may have been worded as the maximum input, but that just means the maximum amount of current the charger can draw.

It seems like there are very few options for 24v dc-dc and the ones that do exist are absurdly expensive. Does anyone have insight on this or recommendations on ways around it? I have both 12 and 24v source I can tap into but it seems absurd to have to step up to 24v to be able to charge when my chassis batteries already function as both a 12v and 24v bank. I suppose I could put an isolator in place then use either a pwm or other charge controller to regulate charging to the house lifepo4s. Are there any potential pitfalls to doing something like this? Of course, I have not priced that approach. Could be that it isn’t any cheaper than purchasing one of the more expensive 24v units.

 

[Dzl]

It seems like there are very few options for 24v dc-dc and the ones that do exist are absurdly expensive. Does anyone have insight on this or recommendations on ways around it? I have both 12 and 24v source I can tap into but it seems absurd to have to step up to 24v to be able to charge when my chassis batteries already function as both a 12v and 24v bank. I suppose I could put an isolator in place then use either a pwm or other charge controller to regulate charging to the house lifepo4s. Are there any potential pitfalls to doing something like this? Of course, I have not priced that approach. Could be that it isn’t any cheaper than purchasing one of the more expensive 24v units.

I skipped coffee today so that may be a contributing factor but I'm not quite understand the situation your are describing or your goal. Could you explain in a little more detail or maybe draw a simple schematic?

And specifically, what do you mean by:

I have both 12 and 24v source I can tap into but it seems absurd to have to step up to 24v to be able to charge when my chassis batteries already function as both a 12v and 24v bank

What do you mean your chassis batteries already function as a 12V AND 24V bank? And what are the 12 and 24 volt sources?

 

[DerpsyDoodler]

Chassis batteries are two 12v fla batteries wired in series, as per the mfg specs. it’s a 99 MCI D3 series 40’ motorcoach. The wired disconnect has a 12v side and 24v side. For instance, if my batteries were dead, I could get a 12v jump or a 24v jump. They’re charged in series, however, by the alternator and have an active equalizer. There is a converter/charger on board. as well as stepdown for 12v inside. I won’t be using that stuff for anything other than it’s mfg intended purpose and as a tapping point for an isolator/dcdc alternator charging of house batteries.

edit: so what I am saying is I have plenty of options for achieving my goal.

edit2: sorry if my original post was confusing.

edit3: and am wondering if anyone has advice on the matter and/or agrees that the 24v versions of these devices seem absurdly priced.

 

[MetalChef]

Some areas for further research/documenting:

1. Which of these B2B's are isolated (or offer an isolated model)? I am aware Victron does, Kisae doesn't. Not sure about others. Anyone have more info to share?
2. Power sharing, for the dual and triple input devices, how does the device manage inputs? The sub questions here are is it Solar and B2B or Solar or B2B, or a middle ground, and which source takes priority.

Perhaps another column of this chart might answer if multiple devices can be combined in parallel to increase the amps per hour of charging. Sterling in their YouTube videos says "yes" you can combine their B2B chargers and the amps don't even need to match to increase the amp per hour charged. Renogy on Amazon questions says "no" you can not use 2 Renogy DC to DC chargers in parallel because for instance if 1 Renogy 60a DC to DC charger detects any other power source including another Renogy DC to DC charger trying to increase the amps above 60a, it will regulate the amps back down to 60amps.

 

[Dzl]

Perhaps another column of this chart might answer if multiple devices can be combined in parallel to increase the amps per hour of charging. Sterling in their YouTube videos says "yes" you can combine their B2B chargers and the amps don't even need to match to increase the amp per hour charged. Renogy on Amazon questions says "no" you can not use 2 Renogy DC to DC chargers in parallel because for instance if 1 Renogy 60a DC to DC charger detects any other power source including another Renogy DC to DC charger trying to increase the amps above 60a, it will regulate the amps back down to 60amps.

This is a good idea,
Its not info I have currently, but it would be useful

I am pretty sure Victron has stated there devices can be parallele

 

[MetalChef]

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
Victron Orion-TR Smart (Isolated)	30A / 17A	12, 24	12, 24	Yes
Victron Orion-TR Smart (Non-Isolated)	30A / 17A	12, 24	12, 24	Yes
Sterling Power Pro Batt Ultra**	70A / 35A ***	12, 24	12, 24, 36*, 48*	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

Victron in one of their YouTube videos says that their Victron Buck Boost DC-DC Converter 25A / 50A / 100A can also be used to regulate the amps from your alternator charging your batteries. At 100amps at 12v I believe that would be the most amps possible from any of the DC to DC device.

 

[Dzl]

Victron in one of their YouTube videos says that their Victron Buck Boost DC-DC Converter 25A / 50A / 100A can also be used to regulate the amps from your alternator charging your batteries. At 100amps at 12v I believe that would be the most amps possible from any of the DC to DC device.

Interesting, I would like to watch that video, I bet others would find it useful to, do you have a link or ballpark title to search for?

 

[MetalChef]

Interesting, I would like to watch that video, I bet others would find it useful to, do you have a link or ballpark title to search for?

Here is the link to the Victron webinar where they was discuss using an Alternator to charge Lithium.

 

[MetalChef]

Just started playing with my Sterling BB242435 (24v to 24v rated at 35a). So far through the BMS I've seen a max of 27.15a basically at idle. Somewhere I read it could go as high as 40a but I can't find that documentation currently. Still working on the rest of the system...

I think this Sterling YouTube video shows how adjust to get a few more amps than what the B2B is rated for.

 

[Perrybuilds]

Scotty AI 3kW Bi Directional DC to DC 12V to 24,36,48V

SCOTTY 3kW 12-48V CANbus Bi-directional DC to DC

www.safiery.com

This is the one I referenced earlier for reference, it's there take on a beefed up version of the victron buck boost. They also have a tiny one the size of a iphone coming out that's apparently 100/150 amps that is military grade.

These Canbus based systems looks incredible but so expensive... And at those levels however you need commercial high output alternator or dual alternator kit in addition.

 

[Mad_Texan]

I think this Sterling YouTube video shows how adjust to get a few more amps than what the B2B is rated for.

Thanks for the video... I heard him say it was adjustable but not how. Currently I'm happy with the output as I plan on adding solar as well. I was watching several of their videos yesterday that give me some more insight into the charger on the settings as I've found the directions a little confusing. Most likely my personal ADD... ? squirrel....

 

[MetalChef]

Thanks for the video... I heard him say it was adjustable but not how. Currently I'm happy with the output as I plan on adding solar as well. I was watching several of their videos yesterday that give me some more insight into the charger on the settings as I've found the directions a little confusing. Most likely my personal ADD... ? squirrel....

I thought he pointed to one of the buttons on the device.

 

[Dzl]

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.

 

[DerpsyDoodler]

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.

Really depends on whether the answer comes from the mfg or a seller, usually. Mfg should be considered authoritative. While seller response might carry some weight....notice the word might. ?

 

[DerpsyDoodler]

Really depends on whether the answer comes from the mfg or a seller, usually. Mfg should be considered authoritative. While seller response might carry some weight....notice the word might. ?

And anyone else...well....ymmv

 

[Dzl]

Really depends on whether the answer comes from the mfg or a seller, usually. Mfg should be considered authoritative. While seller response might carry some weight....notice the word might. ?

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.