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DC-DC charger (B2B) options and general discussion

I just posted this on another thread, but this might be a better place to ask.

I currently have a Victron shore power charger on my group 31agm truck battery linked to my Victron smart solar and a 100w panel. I'm going to add a 12v lithium 100ah diy with overkill bms soon. Can I add in the Victron Orion dc-dc charger in the mix? Could I leave the shore power set to AGM charging the starting battery and have the dc-dc charger/mppt charge the lithium when the truck is off? Or do I need to disconnect the shore charger from starting battery and set it to lithium?

I'd like to have both batteries charged, if possible.
 
I've definetely considered/pondered it, but my depth of knowledge in electrical is not sufficient to feel confident going beyond pondering. I do not know if it advisable or doable or not. Not with those cheap converters at least. I am pretty sure (almost positive) Victron's converters (some of them at least) can be used in that manner, but at that point the price difference between a Victron DC-DC converter and Victron DC-DC charger (meant for alternator charging) is not that substantial.

If these cheap converters (from the link you posted) would work and don't introduce additional unacceptable risk, it would be a very cheap solution, and I actually think 13.8V (3.45Vpc) sounds like a good voltage for dumb fixed voltage (non 3 stage) charging.
My scheme, oriented towards travel trailers, uses a cheap boost/regulator converter under the TV hood and works great. (It has been implemented by several other persons over the years. My own has been working since 2016). It can send about 460 watts through the Bargman umbilical cable, at only about 13.3 Amps. I currently use and always recommend boosting to 36.0 Volts, using this ultra-cheap device.

But I definitely DO NOT go straight into the Trailer Battery Bank. Within the Travel Trailer, a "Detector Relay" with a 36-VDC coil, is used to switch the "PV +" input power connector of an MPPT Solar Controller away from the genuine panel Array "+" wire, connecting the Bargman cable "TBC" into "PV +" instead. The SCC sees that the virtual panel has changed, and rapidly tunes the PV input Voltage to match the 36.0V input. In my own case, the "real" panels present about 80V under sunlight. My relatively cheap "Tracer BN" Series Controller, form EpEver, adjusts to the new max power point very quickly. It can also rapidly adjust from 0 power to the 36 Volt MMPT at night, with no ill effects.

In the Tow Vehicle dashboard, we have all added basic switch to "enable" the Boost output wire path, path on-the-fly. This switch controls a high current Relay into the Boost device "input +" terminal, connected to the battery on an 8-AWG wire with a 50A fuse (this is chassis wiring, and very short.) The Boost switch only gets power while the ignition is "on". Leaving the Boost Switch "off" allows you to tow other normal trailers, which can't handle 36V on TBC, and also allows you to turn it off when your engine is idling. (At idle speeds, relatively few SUV and Pickup Truck Alternators are "happy" to have the Trailer sucking in almost 40A of power).

The two disadvantages of my scheme are: #1, "real" solar gets disconnected from the MPPT when the 36V boost voltage is present on Bargman TBC. You cannot run "real" Solar and the Boost scheme at the same time (although you could bypass this limitation, by using more than one MPPT controller). And #2, the Boost Converter must NEVER be overloaded. Choose MPPT maximum output power by starting with a slightly conservative limit of utilization within the Boost device (perhaps using it at only 90% of its rated maximum). Make sure that the current through the Bargman cable won't exceed 15A. 36V at the under-the-hood Booster, travelling about 25 feet each way through the Bargman, will have considerably less Voltage at the MPPT input terminal (typically around 34.8V when running at full load). Available power at the MPPT terminals determines your maximum input power for the MPPT SCC, and the output limit (battery charging voltage * maximum amps allowed) must not exceed about 94% of the input power, due to power loss within the MPPT SCC.

EXAMPLE: In my own configuration, the MPPT charges at up to 14.5V. (A "12 volt" LFP battery bank). From under the hood, the device is rated for 540 watts, but I want to limit its use to only 85 percent of that figure - 460 watts. This would require about 12.8A, until we consider Voltage Drop along the way. That typically ends up around at about 34.8V, if the TV wiring of TBC from the Booster to the 7-pin socket is at least AWG-12, and the Trailer Wiring from the Bargman end is AWG-10. So instead of 460 watts we're down to about 440 watts. (For calculating the Voltage Drop through that output current path (Boost Device to 7-pin connector within the TV, Bargman wire, and some length from the Bargman termination to reach the SCC) you can note that the Bargman TBC is typically only 14-AWG, while the return ground wire is typically a bit larger (I've usually seen 12-AWG when repairing those cables). On the Return Path, only the Bargman length needs to be used (because the TV frame and Trailer Frame are capable of higher current).

We're now delivering only 440 watts as input to the SCC. My SCC is about 94% efficient in this conversion (34.8 input volts to 14.5 Volts "CV" into my LFP battery string), so the batteries can only receive 420 watts. At 14.5V, that's a bit less than 29A, and I have tuned down my "30A - Capable" Controller to limit itself at 29A.
- - -
It all works like a dream, as long as 420 Watts is sufficient charging energy for your battery bank. If your battery bank is configured at "24V", this still runs great - just set your SCC output for "24V" batteries; the MPPT Input Voltage is plenty high enough to charge such a string (although the maximum MPPT output current must be cut in half.) By switching on the fly, and only while already in motion at "adequate" RPMs, you avoid stressing the Alternator. You can tow other Trailers, just by leaving the "enable" switch turned off. And your Trailer sets itself back to default mode ("Trailer Battery Charge" being connected into the normal "12V" system, and the "genuine Solar panels reconnected to the Solar Controller) as soon as the Bargman is disconnected, OR the "enable" switch is turned off, or the SUV/truck ignition is turned off.

Best of all - no additional cable. You already have the Bargman connected for brake lights, brake power, turn signals, and running lights - you simply use the TBC wire within the Bargman cable differently, and more efficiently.

My existing Thread - written less well, and with a funny title (but with diagrams and pictures): https://diysolarforum.com/threads/r...rs-of-mppt-charge-solar-in-the-trailer.20730/
 
Not vehicle mounted, but I'd like to be able to charge a 12v LFE from my SUV in a pinch. Is a DC-DC charger the best way? Sounds better than using my inverter on the car battery to charge.
 
At least in the truck world, the three letter acronym TBC would stand for Trailer Brake Controller, not trailer battery charge.

Rick, I've read through your posts and you've provided a very good description of your system. I understand it at a high level, but it's not something I would implement. I would rather run a dedicated larger wire than implement more complexity. Then again, I don't need charging from the tow vehicle at this time, so I don't have an appreciation for your solution.
 
I have 2015 Roadtrek express 3500 gas chassis with dedicated under hood alternator 270amp for the 4 Lifepo4 12v battery packs at 272AH each. Nations Alternator says I have to covert alternator from internal voltage regulator to external and use Balmer Regulator with custom harness from them for $400. Can I bypass this and just use a DC to DC charger?
 
I have 2015 Roadtrek express 3500 gas chassis with dedicated under hood alternator 270amp for the 4 Lifepo4 12v battery packs at 272AH each. Nations Alternator says I have to covert alternator from internal voltage regulator to external and use Balmer Regulator with custom harness from them for $400. Can I bypass this and just use a DC to DC charger?
Yes
 
At least in the truck world, the three letter acronym TBC would stand for Trailer Brake Controller, not trailer battery charge.

Rick, I've read through your posts and you've provided a very good description of your system. I understand it at a high level, but it's not something I would implement. I would rather run a dedicated larger wire than implement more complexity. Then again, I don't need charging from the tow vehicle at this time, so I don't have an appreciation for your solution.
In my case, I need to charge a 24v bank from 12v. With 24v, I don't need a huge wire running from DC to DC charger. I will mount the DC to DC charger as close to the engine bay as I can or in it, this allows a much smaller cable to be used after the charger.
 
In my case, I need to charge a 24v bank from 12v. With 24v, I don't need a huge wire running from DC to DC charger. I will mount the DC to DC charger as close to the engine bay as I can or in it, this allows a much smaller cable to be used after the charger.
That can work great in your special case, with the DC-->DC "boost" charger battery Voltage determined via Trailer Battery SOC, determined over the (smaller) long wire. The same could also work in a 12V-> 12V battery bank system, as long as the wire was big enough.
 
In my case, I need to charge a 24v bank from 12v. With 24v, I don't need a huge wire running from DC to DC charger. I will mount the DC to DC charger as close to the engine bay as I can or in it, this allows a much smaller cable to be used after the charger.

There is close to zero room in the engine bay of my F-350. If I stuck the DC-DC charger in the engine bay I would put some heat shield between the charger and any heat sources. From what I read, the chargers de-rate when they heat up.
 
There is close to zero room in the engine bay of my F-350. If I stuck the DC-DC charger in the engine bay I would put some heat shield between the charger and any heat sources. From what I read, the chargers de-rate when they heat up.
Like I said, as close to the engine bay as I can or in it.......
 
In my case, I need to charge a 24v bank from 12v. With 24v, I don't need a huge wire running from DC to DC charger. I will mount the DC to DC charger as close to the engine bay as I can or in it, this allows a much smaller cable to be used after the charger.
What do you know about voltage drop. It is my opinion you have this backwards in your head.
 
What do you know about voltage drop. It is my opinion you have this backwards in your head.
I doubt I have it backwards. Running the same watts, smaller voltage drop on 24v compared to 12v. Amps are amps, that sizes your wire. Watts on the other hand depend on the voltage.
 
What do you know about voltage drop. It is my opinion you have this backwards in your head.
grizzman, I think Zwy's got it exactly right: Allowing the LONG wire to handle less than half the current (at 27-29V for battery charging), incoparison to pulling more than twice the current at a Voltage below 13.5V for the same amount of power (and it might be dragged down to quite a bit less than 13.5V by the time it reached the Trailer-mounted DC-->DC Converter/Charger in that scheme).

It's winning strategy, if we are safe in assuming that the DC-->DC converter sits in enough airflow to avoid overheating. HRTKD says that he can't put it in his truck, the crowded engine compartment would cook it. And, if my 4Runner were a V8 model, it would probably cook my own 12-->36 Converter as well. But my V6 has plenty of free space and airflow, when it's in motion, and my Converter doesn't even need on onboard fan. Both can work, reducing current on the long wire path to the Trailer.
 
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grizzman, I think Zwy's got it exactly right: Allowing the LONG wire to handle less than half the current (at 27-29V for battery charging), incoparison to pulling more than twice the current at a Voltage below 13.5V for the same amount of power (and it might be dragged down to quite a bit less than 13.5V by the time it reached the Trailer-mounted DC-->DC Converter/Charger in that scheme).

It's winning strategy, if we are safe in assuming that the DC-->DC converter sits in enough airflow to avoid overheating. HRTKD says that he can't put it in his truck, the crowded engine compartment would cook it. And, if my 4Runner were a V8 model, it would probably cook my own 12-->36 Converter as well. But my V6 has plenty of free space and airflow, when it's in motion, and my Converter doesn't even need on onboard fan. Both can work, reducing current on the long wire path to the Trailer.
The problem with large voltage drop is the change of volts that is dependent on the amps drawn. Not a great way of controlling charge input.
 
The problem with large voltage drop is the change of volts that is dependent on the amps drawn. Not a great way of controlling charge input.
You're stretching there in the argument, you stated, " What do you know about voltage drop. It is my opinion you have this backwards in your head."

Nothing in my response said anything about charge input. But I digress, we shall have the discussion on several fronts. First, do you know the most accurate way to measure VD on a wire with no addition/subtraction or any other type of math, no calculator and gives 100% accurate results every time? If you don't, I'd be more than obliging to explain it to you.

Second, charge input is controlled by DC to DC charger which also is a step up converter in this case, 12v to 24v. Input 30a, outputs 15a. Watts in equals watts out minus some loss for the converter but that isn't relevant to the discussion. Output voltage is regulated by the DC to DC charger.

As for your response in the block above, you have voltage drop attempting to control amps from the way you worded it. It can be done, I don't know why you would want to waste energy as heat on a charging circuit but go for it. Take blower motor resistors, the amps drawn by the motor will be less, however the total amp draw with the resistor in the circuit will be the same regardless of speed, note I did not say load. A given load however will increase amps over a lighter loaded condition but still, the circuit with the resistor installed will still draw the same amps on the complete circuit with the increased load, motor will still draw less amps than with the resistor in the circuit. Not a hard concept to understand.

Now, let's look at the DC to DC charger, 12v to 24v step up type and the wiring required. On the 12v side, amps will be double and requires a wire size according to those amps. On the 24v side, the wire size can be 1/2 as only 1/2 the amps will be carried by that wire. Watts across the circuit remain the same, except for converter loss. In the case of a Victron 12/24 DC to DC, input is 12v 30A, output is 24v 15A.

Now. I'll really mess with your head. Take a length of wire and pass amps thru it to power an incandescent light bulb. Record the result. Now, add resistance to the circuit in series at the exact same voltage. Will the circuit draw more,less or the same amps? Correct answers only.

Second, is VD the same on a given length of wire at different voltages? Again, correct answer only.
 
The problem with large voltage drop is the change of volts that is dependent on the amps drawn. Not a great way of controlling charge input.
This is true, but only for a while. "CV" at a high Voltage will start off with high current, and the received Voltage will be low, in comparison to what the DC-->DC battery charger thinks that it is putting at. But the current will drop off towards the end of the charging process, and the accuracy of the DC-->DC "SOC" determination will improve. It might stay in "CV" Absorb mode for a while longer (if its smart), and that is where most of the charging action will occur anyway. (LFP batteries will accept more in float stage than lead-acid batteries will, but most of the job should have been completed earlier.

Less accuracy, but higher efficiency.
 
LiFePO4 batteries are thirsty and can overtax an alternator if connected directly, The DC-DC charger acts as a middleman between the charging system and the lifepo4 battery and will limit the current to some maximum value (which I believe should be output current + inefficiency).
..and do you have to have a VSR as a middleman between the alternator (indirect the starter battery) and the DC-DC to protect the starter battery from draining? Or am I getting something wrong?

So, alternator - starter battery - VSR - DCDC - LFP ?

Sorry for being noob, still trying to figure this out..
 
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