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Old 03-20-2023, 12:02 PM   #1
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Solar Controller as DC/DC charger?

I guess a bit of a thought experiment. For our upcoming GT 25FBT with rooftop solar in addition to lithium I'm planning on adding a second Victron 100/30 MPPT controller for external plug-in solar panels. As discussed in a number of places in the AS forums it's desirable to add a DC/DC converter from the TV 7-pin to charge the batteries with a lithium profile while connected to the TV. But I'm wondering why not route the 7-pin charging to the solar controller instead of a separate DC/DC charger? The controller doesn't care where the electricity is coming from and will charge the battery appropriately. There's probably a gotcha, I seem to remember the solar controller requires a minimum voltage, but I didn't find it in the Victron specs.
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Old 03-20-2023, 12:17 PM   #2
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The MPPT charger by Victron will not activate until the incoming voltage is 5 volts over the battery's voltage. A DC to DC charger will use incoming voltage down to 8 volts and elevate it to the profile required for your battery chemistry. Additionally, the DC to DC charge can detect either thru software or via an DC input when your tow vehicle is running and will only turn on when that is active. You cannot use a MPPT charger in place of a DC to DC charger.
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Old 03-20-2023, 12:33 PM   #3
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Thanks! This will be a short thread then. For the sake of argument if the battery voltage dropped below 7V or so then it might work, but then the BMS doesn't allow the batteries to go below 11V or so. Sigh, so much for saving $180 or so.
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Old 03-20-2023, 12:45 PM   #4
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Renogy has a controller that does both solar and dc/dc charging.
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Old 03-20-2023, 05:40 PM   #5
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I used a voltage doubler in my last tow vehicle (for a SOB trailer) and hooked the output to the 30A line running to the charge line on the 7-pin connector. In the trailer, I had a 24VDC relay that would disconnect the solar power and connect the charge line to the MPPT controller. Worked well for the 2x 6V trojan batteries that were in the trailer.
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Old 03-20-2023, 07:06 PM   #6
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Another reason to not do it is that an MPPT controller tries to maximize power by varying the load on the source. Since your TV source is relatively low impedance it would have a difficult time trying to maximize power via voltage control with your solar panels connected. Best to have separate controllers with their own purpose. Even if the Renogy is capable of doing both you still want to use a separate controller/charger for the two sources.
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Old 03-21-2023, 09:23 AM   #7
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Hi

To get a bit techie: The typical DC/DC converter used on the 7 pin is a "buck/boost" converter. If you put in 7V, you get out 14 ( = it boosts the voltage). If you put in 15V, you get out 14V (= it bucks the voltage). Solar converters are "one way only" devices.

The next issue is current limiting. It is likely that your solar converter is quite happy to pull way more than the 7 pin can supply. For your DC/DC, you want a device with a well defined current limit. If the current limit is too high, the system "burps" along (turns on, voltage drops, shuts down, recycles) and you don't get much to the battery.

A reasonable DC/DC can be in the $60 range. Fancy ones are more. You likely will spend a chunk of that money trying to switch the solar around.

As you are rumbling down the road, the sun *is* likely to shine some of the time. You very much do want that solar setup online. It's free power and it adds to whatever the 7 pin is giving you.

Hard wiring things in place ( DC/DC goes here ...) is less failure prone than switching stuff around. The "failure" might just be "I forgot to throw the switch". Even so, it still is something that will stop things from working correctly.

Lots of issues .....

Bob
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Old 03-21-2023, 09:09 PM   #8
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Victron

Quote:
Originally Posted by JeffKim View Post
I guess a bit of a thought experiment. For our upcoming GT 25FBT with rooftop solar in addition to lithium I'm planning on adding a second Victron 100/30 MPPT controller for external plug-in solar panels. As discussed in a number of places in the AS forums it's desirable to add a DC/DC converter from the TV 7-pin to charge the batteries with a lithium profile while connected to the TV. But I'm wondering why not route the 7-pin charging to the solar controller instead of a separate DC/DC charger? The controller doesn't care where the electricity is coming from and will charge the battery appropriately. There's probably a gotcha, I seem to remember the solar controller requires a minimum voltage, but I didn't find it in the Victron specs.
Just buy the Victron 30 DC-DC ours works flawless. I will swap it out when the new 60 is released and then when the new 100 which is in trial testing
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Old 03-22-2023, 09:42 AM   #9
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Thanks for the comments!

I will go with a separate DC/DC charger, but from Bob's and OC's comments, my knowledge about how the MPPT charge controller works is lacking. I was thinking the solar input is what it is (varying depending on solar exposure) and the charge controller takes that and then optimizes the voltage/amperage for charging purposes.

I am installing Victron for most everything so if I get the fancy pants Smart Victron Smart 12/12-18 then it's $170 or so, but I'm not sure if the Bluetooth will tell me much so I could save $40 for the Blue version.
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Old 03-23-2023, 06:46 AM   #10
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Quote:
Originally Posted by JeffKim View Post
Thanks for the comments!

I will go with a separate DC/DC charger, but from Bob's and OC's comments, my knowledge about how the MPPT charge controller works is lacking. I was thinking the solar input is what it is (varying depending on solar exposure) and the charge controller takes that and then optimizes the voltage/amperage for charging purposes.

I am installing Victron for most everything so if I get the fancy pants Smart Victron Smart 12/12-18 then it's $170 or so, but I'm not sure if the Bluetooth will tell me much so I could save $40 for the Blue version.
There's more to the victron DC to DC than simply seeing it via bluetooth. First, you will only 'see' it when your tow vehicle powers its input. You'd think the since its hooked to your battery, it would be persistently visible. However, there's an even more important aspect. Victron allows their shunt and charging sources to communicate via a network to optimize charging. This is done via a Cerbo if you have one, or via bluetooth if you don't. The shunt, DC to DC charger and solar controller coordinate to optimize the charging of your battery. Stay with Victron. (I do this stuff)
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Old 03-23-2023, 07:56 AM   #11
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I want to add a dc-dc charger my tow vehicle has a 220 amp alternator, setup is 2 Battleborn, Victron Shunt, 90 watt on the roof and 120 watt Zamp portable. My question is what size Victron dc-dc unit should I be looking toward ? Help appreciated.
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Old 03-23-2023, 08:48 AM   #12
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I want to add a dc-dc charger my tow vehicle has a 220 amp alternator, setup is 2 Battleborn, Victron Shunt, 90 watt on the roof and 120 watt Zamp portable. My question is what size Victron dc-dc unit should I be looking toward ? Help appreciated.
IMO, no more than 30a, if you want to run new fat wires in the TV. 18amp if you want to use the existing charge line.
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Old 03-23-2023, 09:43 AM   #13
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IMO, no more than 30a, if you want to run new fat wires in the TV. 18amp if you want to use the existing charge line.
I should say, assuming the existing charge line is 10 gauge.
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Old 03-23-2023, 09:53 AM   #14
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I should say, assuming the existing charge line is 10 gauge.


Thank You!
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Old 03-23-2023, 10:01 AM   #15
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Quote:
Victron allows their shunt and charging sources to communicate via a network to optimize charging. This is done via a Cerbo if you have one, or via bluetooth if you don't. The shunt, DC to DC charger and solar controller coordinate to optimize the charging of your battery. Stay with Victron. (I do this stuff)
I was vaguely aware of this capability and found much more detail here: https://www.victronenergy.com/media/.../index-en.html. I did see however that the Blue MPPT controller installed by Airstream does not play even with the dongle (Section 4): "Uses received sense data to optimise charging. Synchronised charging via Bluetooth is not possible." Also, the Orion Smart DC/DC charger is "Not yet supported, may be compatible in the future.". So, sounds like I might need to replace the Blue controller with a Smart controller (not exactly cheap) and I can get the Smart DC/DC but with no immediate benefit over the Blue other than I can monitor the charging from my phone.
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Old 03-26-2023, 09:26 AM   #16
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DC-DC chargers

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Originally Posted by rfequity1 View Post
Renogy has a controller that does both solar and dc/dc charging.
I went with RedArc as camping in the desert others would just overheat. It also has an input for solar so during the day while driving down the road, less load is on the alternator.
https://www.redarcelectronics.com/us...attery-charger

Curious about which model did you use for Renogy? How has it been working for you?
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Old 03-26-2023, 03:36 PM   #17
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IMO, no more than 30a, if you want to run new fat wires in the TV. 18amp if you want to use the existing charge line.
Confirming what dznf0g said. Get the 18amp version, independent of what the alternator can produce or what the fuse says on your tow vehicle fuse panel. Unless, you are willing to run a new, dedicated fat-ass wire (yes, that's a highly technical term) between your tow vehicle battery terminal and your trailer DC-DC unit. After the fat-ass wire installation, you can upgrade the DC-DC to 30amps. Or run multiple 30A DC-DC's in parallel. Victron says that multiple parallel DC-DC's are ok and some folks with huge alternators have done this.

My Cayenne has a 30amp fuse, but I can only pull a fraction of that through the existing factory wiring harness without excessive voltage drop causing the DC-DC to keep cutting out and rebooting.

Also, you will definitely want the bluetooth version of the DC-DC. You will need to fiddle with the settings quite a bit and since the DC-DC is only powered when the tow vehicle is set up and running, it's a real pain to try and optimize the settings without using the victron VRM app on your phone in order to communicate with the DC-DC via bluetooth.

Get the bluetooth version. You will thank me later, LOL.
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