DC to DC charger input question

Maple_Geek

I'm trying to figure out if the following input to my Victron 12-12-18 DC-to-DC charger would work. I believe I have seen this on another forum and since I only have one cable available from the front of the camper I'd like to avoid fishing more cables.

For simplicity I've only illustrated the charge (+) wires. 
As you can see, there is one 10 gauge wire coming from the front. At the moment this wire is used to minimally charge the batteries (located at the back) while I drive but it also provides power to the juction box and emergency brakes should the camper gets disonnected from the TV.

I obviously want to continue to provide power to the junction box but I now want to use the same wire to provide input to a DC-to-DC charger to augment the charging capacity when I drive. I also want to avoid phishing a new wire to the front as I don't see an ovious way to do this.

Any thoughts would be appreciated.

There is a 2nd 10 gauge wire available (previously used by the existing MPPT charger when the battery was in the front tub) but I'm currently using this wire for my external solar panel. Worst case I could reuse this wire for the DC-to-DC charger and drill a hole on the passenger side of the camper near the batteries to install a new solar port there. I'd like to avoid doing this if I can help it but that's another option.

Thanks!

Yoshi_TAB

Hi, 

I'm not sure this will directly answer your question, but maybe some food for thought that might help.  

I have a 2021 and moved my batteries from outside to inside w/o drilling any holes. It sounds like you also moved your batteries inside.   I abandoned the battery wires in  the front tub.  This leaves the  two OEM  10 AWG DC battery wires (old positive and old negative) that can be used to get power from the outside front tub to the inside of the TAB w/o drilling any holes.  If you moved them inside,  aren't those unsed wires still available?  (Sounds like you used one).  Is the negative wire from the battery in the front tub available?  Or did you use that for your solar panel?   The negative wire will be tied into the ground in the junction box, but you could move it over to the positive (I  think #6) terminal and use it as a positive wire.   You have to check continuity to make sure you have the correct end.

I also wired my e-brakes directly to the inside battery independant of the batttery switch so I no longer have to double check my battery switch is on for the e-brakes.  I used one of the abandoned battery wires from the WFCO (that went to the junction box #6 position) and tied it directly to the batteries inside.

My wire for the 7 pin pos. wire was left untouched and still goes directly into the junction box.

Horigan

@Maple_Geek
I don't see how that scheme would work.  The baseline charging from the tow vehicle is limited by the wire size through the tow vehicle and trailer, and the output capacity of the alternator.  A DC to DC charger is looking to see that the source DC power (TV battery ) is being charged (some voltage above 13V, user defined).  Once you start drawing current through the tow vehicle the voltage seen by the DC to DC charger will drop and it will stop charging, then the voltage will recover and it will start charging, then the voltage will drop and it will stop charging, etc.  Overall you're trying to extract more power (voltage x current) from the tow vehicle than the current system provides, and that's not really possible.  If you want more charging power from the two vehicle, upsize the wires running back to the 7-pin in the tow vehicle.  You also need to understand how much current the alternator can put out.

Plus, I don't know how the DC to DC charger would respond to having the input and output connected together.  It probably would shutdown.

Where DC to DC chargers shine is raising the voltage above the source power to charge at a higher voltage, say to charge a LiFEO4 battery from a lead acid/AGM battery.

Maple_Geek

Thanks @Yoshi_TAB. You are correct, I did move my battery bank from the front tub to the back (next to the Air8 A/C)

That left 2 pairs of 10 gauge wires that I'm now reusing:
1) One is used today to bring power to the front junction box. It was already going to the front junction box to charge the battery from the WFCO when the battery and power the 12V circuits when it was in the front tub
2) The other is reused to connect to the SAE port on the side of the tub

In my opinion, my proposal above won't work and I believe I need a dedicated wire from the 7 pin as an input for the DC-to-DC charger.

Maple_Geek

@Horigan thank you and I agree. I just saw this proposal somewhere else and wanted to explore it.
The DC-to-DC charger that I have should limit the output to 18Amps so that shouldn't be a problem for my alternator or the 10 gauge wire that I have to the 7 pin connector from my alternator.

rfuss928

I'm working on the same decision on where I would connect a DC to DC converter.
Its only purpose IMO is to enable the TV to more effectively charge LiFePO batteries.  If it is installed in the 7 pin +12v line, it will need to carry the charging load and anything else running on the trailer's 12v system.  The refrigerator in particular comes to mind.  I don't think this is a problem with a 20 amp or higher system.  My TV's has been able to charge the 200ah AGMs I've been using so I would like to think that will still be the case when I switch to LiFePO.   If it would still bulk charge the LiFePO , I'd be okay letting solar or shore power do a top off when available?  In that case I wouldn't bother with the DC to DC at all.
I know how to change the solar controllers settings to Li and I hope the auto detect converter does what it is supposed to. 
I'm following this to see what the community is learning.

KK1L

Horigan said:

@Maple_Geek
I don't see how that scheme would work.  The baseline charging from the tow vehicle is limited by the wire size through the tow vehicle and trailer, and the output capacity of the alternator.  A DC to DC charger is looking to see that the source DC power (TV battery ) is being charged (some voltage above 13V, user defined).  Once you start drawing current through the tow vehicle the voltage seen by the DC to DC charger will drop and it will stop charging, then the voltage will recover and it will start charging, then the voltage will drop and it will stop charging, etc.  Overall you're trying to extract more power (voltage x current) from the tow vehicle than the current system provides, and that's not really possible.  If you want more charging power from the two vehicle, upsize the wires running back to the 7-pin in the tow vehicle.  You also need to understand how much current the alternator can put out.

Plus, I don't know how the DC to DC charger would respond to having the input and output connected together.  It probably would shutdown.

Where DC to DC chargers shine is raising the voltage above the source power to charge at a higher voltage, say to charge a LiFEO4 battery from a lead acid/AGM battery.

I know this is a late reply, but nonetheless...

The Victron DC-DC is not a voltage regulator, but a buck-boost converter. The 12/12-18 will take an input voltage between 8 and 17V and convert it to an adjustable voltage between 10 and 15V with 87% efficiency. Let's say that in @Maple_Geek case he is sourcing 18A to the battery at voltage of 13.8V while charging (max current for the unit). This is 248Watt which would require an input power of 285W. Let's assume 20' of 14ga from the alternator to the DC-DC. This is 0.05 Ohms wiring resistance. The alternator will be supplying something between say 13.8V and 15V or between 19A and 21A to source 285W. This would be as much as 1.05V drop from the alternator to the DC-DC converter. This is 22W dissipated in the wiring to the DC-DC. If the wire is 12ga from the alternator the voltage drop and power dissipated is halved.

Horigan

Thanks for researching the DC to DC specs and doing the math on the voltage drop.  Understood.  I do think connecting input and output on the DC to DC would still not work.

KK1L

Horigan said:

Thanks for researching the DC to DC specs and doing the math on the voltage drop.  Understood.  I do think connecting input and output on the DC to DC would still not work.

Oh agree with that 100%. I am not sure how the unit would react to that, but I don't expect it would be a happy reaction