Unconventional DC-DC Charger Use

[0rion]

Hello, I have a 23' travel trailer and have been preparing it for boondocking - a 200w solar suitcase and Victron shunt, MPPT, and inverter with a 100ah Lead Acid Marine battery for now (hope to get 2x GC2's or Lifepo4 soon). We're light power users.

I'm wondering if it's possible to use a DC-DC charger a bit differently. We leave home fully charged so I'm less concerned about charging the house battery when driving - is it possible to connect the DC-DC to the truck battery only when needed to top off the batteries if the solar can't keep up (basically in lieu of having a generator)? So rather than a hardwired line in the truck, a temporary wire I could connect via Anderson plug or battery clamps as needed?

Also, my trailer has the stock WFCO 8955 converter that is known for only staying in absorption mode, which makes me concerned for the health of my lead acid battery, and any future 6v pair or Lithium. Could we not just use the same DC-DC charger here as well, to regulate the converter output into something healthier for the house batteries (basically in lieu of purchasing a better quality converter like Progressive Dynamics)?

TL;DR:
When connected to shore power, existing converter feeds DC-DC, which takes better care of the battery (battery chemistry agnostic).
When boondocking, can connect the tow vehicle to DC-DC to top up the charge when solar can't keep up. Saves the cost of buying a generator, gas, upkeep, etc.

The only downside I see would be the inefficiency of the DC-DC while on shore, but I'd be willing to lose some efficiency if it means a healthier, smarter charge for the battery. Has anybody done this? It just makes so much sense to me, one DC-DC charger could replace a generator and an upgraded converter. I must be missing something...

EDIT - proposed schematic diagram added below.

 

[snoobler]

Welcome to the forum, oh great hunter.

Your WFCO8955 is an AC-DC inverter capable of supplying 55A of 12V system power. It should bulk to 14.4, absorb at 13.6 and float at 13.2. If it behaves like this, that's great. I'm not familiar with any inherent problems with them. If it's hanging at 14.4V for a long time, bad. If it's hanging at 13.6V for a long time. Cool. Important that you directly measure the battery terminals rather than trust any gauge or measurement at the converter.

You could do what you describe; however, it's crazy inefficient. A 50A DC-DC pulls about 1hp to charge your batteries. You're idling an entire truck engine to pull off 1 hp of charging. GROSSLY inefficient. I suspect a small portable generator can be had for not much more than the price of a DC-DC charger especially when you consider running your truck for a few hours to charge your battery a handful of times.

Using the DC-DC as a "conditioner" for your battery assumes the converter is not performing as it should and the DC-DC can be tailored to better match your battery.

If you're just looking for a solution for while you're parked, just flip the truck around and attach jumper cables to the house battery.

 

[grizzzman]

Hello, I have a 23' travel trailer and have been preparing it for boondocking - a 200w solar suitcase and Victron shunt, MPPT, and inverter with a 100ah Lead Acid Marine battery for now (hope to get 2x GC2's or Lifepo4 soon). We're light power users.

I'm wondering if it's possible to use a DC-DC charger a bit differently. We leave home fully charged so I'm less concerned about charging the house battery when driving - is it possible to connect the DC-DC to the truck battery only when needed to top off the batteries if the solar can't keep up (basically in lieu of having a generator)? So rather than a hardwired line in the truck, a temporary wire I could connect via Anderson plug or battery clamps as needed?

Also, my trailer has the stock WFCO 8955 converter that is known for only staying in absorption mode, which makes me concerned for the health of my lead acid battery, and any future 6v pair or Lithium. Could we not just use the same DC-DC charger here as well, to regulate the converter output into something healthier for the house batteries (basically in lieu of purchasing a better quality converter like Progressive Dynamics)?

TL;DR:
When connected to shore power, existing converter feeds DC-DC, which takes better care of the battery (battery chemistry agnostic).
When boondocking, can connect the tow vehicle to DC-DC to top up the charge when solar can't keep up. Saves the cost of buying a generator, gas, upkeep, etc.

The only downside I see would be the inefficiency of the DC-DC while on shore, but I'd be willing to lose some efficiency if it means a healthier, smarter charge for the battery. Has anybody done this? It just makes so much sense to me, one DC-DC charger could replace a generator and an upgraded converter. I must be missing something...

I have a Renolgy 40 Amp DC to DC that is set up with 20' 4 AWG jumper cable's 6' on one end and 14' out the other. A fuse and holder and fuse is how is turned on and off. Its coiled up in a pelican case when not in use. This set up works well.
In my opinion the WFCO gets a bit of a bad rap. 1. The RV manufacturers (not all but most) run long 12 volt small guage wires to the batteries.(This keeps the unit from going into boost. 2. Proper heat venting is ignored.(causing overheating when the consumer over discharges the battery.)

 

[0rion]

Welcome to the forum, oh great hunter.

Your WFCO8955 is an AC-DC inverter capable of supplying 55A of 12V system power. It should bulk to 14.4, absorb at 13.6 and float at 13.2. If it behaves like this, that's great. I'm not familiar with any inherent problems with them. If it's hanging at 14.4V for a long time, bad. If it's hanging at 13.6V for a long time. Cool. Important that you directly measure the battery terminals rather than trust any gauge or measurement at the converter.

You could do what you describe; however, it's crazy inefficient. A 50A DC-DC pulls about 1hp to charge your batteries. You're idling an entire truck engine to pull off 1 hp of charging. GROSSLY inefficient. I suspect a small portable generator can be had for not much more than the price of a DC-DC charger especially when you consider running your truck for a few hours to charge your battery a handful of times.

Using the DC-DC as a "conditioner" for your battery assumes the converter is not performing as it should and the DC-DC can be tailored to better match your battery.

If you're just looking for a solution for while you're parked, just flip the truck around and attach jumper cables to the house battery.

Thanks for the welcome, snoobler...nice to know that honey badger DO care! Been lurking for a while after discovering and devouring Will's YouTube channel.

Yes the 8955 is advertised as a 3-stage charger - it's problem is that it's often installed at the midpoint or aft of the trailer, far from the battery, and wired with inadequate cabling. As a result due to the wire resistance and internal battery resistance an inaccurate SOC is read by the charger and it hardly ever goes into bulk mode. When it does, it does so for a max of 4 hours. The fix is usually to run thicker wires or upgrade or relocate the charger. As we may be selling our camper in the next couple years I wanted a solution that could keep it closest to stock, while still taking better care of the battery.

Since we have modest power needs I plan on having an 'adequate' battery (280ah lischen maybe?) and more solar - hoping that the need for a generator would be very rare.

I always envisioned that in the rare need for supplemental power the truck would be turned around with the hood up for an hour or two, and the inefficient loss of gas and runaway horses would still be less than the cost of a generator (tow vehicle is a Durango, so the only 'outdoor' place to transport it would be the trailer tongue, another factor why I'd like to avoid needing one).

The DC-DC solution seems like it could kill both birds.

 

[0rion]

I have a Renolgy 40 Amp DC to DC that is set up with 20' 4 AWG jumper cable's 6' on one end and 14' out the other. A fuse and holder and fuse is how is turned on and off. Its coiled up in a pelican case when not in use. This set up works well.
In my opinion the WFCO gets a bit of a bad rap. 1. The RV manufacturers (not all but most) run long 12 volt small guage wires to the batteries.(This keeps the unit from going into boost. 2. Proper heat venting is ignored.(causing overheating when the consumer over discharges the battery.)

So I understand, you use this when you're parked as needed, not while driving? What's the logic behind the cable lengths and where is the Renogy when you're using it?

Yes, the WFCO is probably doing the best it can with what it has to work with.

 

[grizzzman]

It is just used as a back up power source. As an extended family of campers i felt that the ability to use as you you would jumper cables. ( Have you ever had to jump a car in a driveway) Sometimes a tow vehicle may need a jump as an example. It sits on top of the Pelican case when in use. I have no desire to pack fuel, gen, bla etc. heck my chain saw is electric. Charged off of solar.

 

[snoobler]

Thanks for the welcome, snoobler...nice to know that honey badger DO care! Been lurking for a while after discovering and devouring Will's YouTube channel.

Yes the 8955 is advertised as a 3-stage charger - it's problem is that it's often installed at the midpoint or aft of the trailer, far from the battery, and wired with inadequate cabling. As a result due to the wire resistance and internal battery resistance an inaccurate SOC is read by the charger and it hardly ever goes into bulk mode. When it does, it does so for a max of 4 hours. The fix is usually to run thicker wires or upgrade or relocate the charger. As we may be selling our camper in the next couple years I wanted a solution that could keep it closest to stock, while still taking better care of the battery.

Since we have modest power needs I plan on having an 'adequate' battery (280ah lischen maybe?) and more solar - hoping that the need for a generator would be very rare.

I always envisioned that in the rare need for supplemental power the truck would be turned around with the hood up for an hour or two, and the inefficient loss of gas and runaway horses would still be less than the cost of a generator (tow vehicle is a Durango, so the only 'outdoor' place to transport it would be the trailer tongue, another factor why I'd like to avoid needing one).

The DC-DC solution seems like it could kill both birds.

Ah yes. It actually does go into bulk. It just happens so fast, you never see it on the voltage readout. When I upgraded to a 100A converter with the puny wires, exactly the same thing on my Powermax, except it lasts for about 3 minutes.

I wouldn't worry about using that charger. Worst case, you don't get to 100% SoC, but I bet you get 80%+ after several hours.

 

[0rion]

I realized that I've been approaching this question backwards so please allow me to restate it - what I really wanted to know is if occasionally charging from the alternator through a DC-DC charger was valid (while parked, not driving) versus needing a generator. I googled 'alternator vs generator rv' and found several sources, such as...

...that endorsed this possibility.

And a value-added benefit to having the DC-DC charger attached to the battery is that it could be used, as has been said, to condition the power from the converter tailored to my FLA's needs - and I could go with dual GC2's, AGM, or lithium in the future (I'm still undecided, I may even upgrade to a new trailer) and still keep it somewhat stock with the existing WFCO converter.

Ah yes. It actually does go into bulk. It just happens so fast, you never see it on the voltage readout. When I upgraded to a 100A converter with the puny wires, exactly the same thing on my Powermax, except it lasts for about 3 minutes.

For a battery with 50% SOC, shouldn't it be in Bulk longer than the blink of an eye? I tested mine and this is what I saw. This is why I wouldn't trust the WFCO with dual GC2's - it would likely take forever to bring them even to 80% with only absorption in effect. As I said, we're still in the preparing stage of boondocking so I'd like the flexibility that my theorized setup would give us, as we get to know what our needs are and grow the system accordingly - that is if the theory is workable...

 

[grizzzman]

How far away is the battery wiring (wire distance) from the WFCO? What size is the wire? How large of a DC to DC charger going to be?

 

[0rion]

How far away is the battery wiring (wire distance) from the WFCO? What size is the wire? How large of a DC to DC charger going to be?

In the range of 30-40 feet, round trip from battery to converter. 8 gauge wire, with the chassis as ground. I'm considering keeping it in the Victron family and going with the Orion 12/12-30 (I like the name too )

 

[shortyjacobs]

Hey Orion, did you ever do this? I’m thinking of an identical use case to you with one extra of getting a dual use charger (MPPT solar + B2B dc-dc charging). I figure I run my WFCO 8955 into the dc-dc charger to condition power for lithium charging, have the ability to use my tow vehicle as a super crappy “generator” with jumper cables if I need a few amps while boondocking, and have the ability to add in solar in the future without buying more controllers.

So, did you ever do your upgrade?

 

[0rion]

Not yet. I found someone who used their WFCO 8955 to charge their LiFePo4's directly, and said it wouldn't damage them - it just wouldn't charge them fully. To get around this, he had a panel on the roof with a MPPT controller set to charge lithium, so the converter would bring (and keep) the batteries at ~60% (or whatever), and the solar would take them to 100%. Even if the solar performance is poor, he said the lower charge from shore would be healthier for the batteries anyway.

Up to learning this, I wasn't aware that this was an option. I thought a lithium profile charger was a must-have when on shore power.

That all said, I haven't pulled the trigger one way or the other yet. While this config would be good while my RV is in our driveway with full sun or travelling, we tend to camp in shady sites, so still not sure if I'll buy the 200w solar panel or the DC-DC charger.

So to answer your question, even if I go with a DC-DC charger I likely won't be using it in the 'creative' way I suggested in my original post. I am curious how it all would work though, so if I do try I'll report back

 

[Jasgeer]

I have a Renogy 12V-12V 20 Amp Charger and have been using it for a couple of years in my 2016 28ft travel trailer equipped with the WFCO converter. I want to be polite but I never saw the three step voltage output from the WFCO do anything but maintain a constant voltage. Even if the batteries were dead it would not charge them properly. I installed the Renogy and wired it so that it could charge from the output of the WFCO or the power from my truck. Notice I used a dual battery switch. My batteries now properly charge and I have had no problems. In hindsight perhaps I should have purchased a Renogy DC-DC with higher current.

 

[shortyjacobs]

I have a Renogy 12V-12V 20 Amp Charger and have been using it for a couple of years in my 2016 28ft travel trailer equipped with the WFCO converter. I want to be polite but I never saw the three step voltage output from the WFCO do anything but maintain a constant voltage. Even if the batteries were dead it would not charge them properly. I installed the Renogy and wired it so that it could charge from the output of the WFCO or the power from my truck. Notice I used a dual battery switch. My batteries now properly charge and I have had no problems. In hindsight perhaps I should have purchased a Renogy DC-DC with higher current.

Good info, thanks!

 

[Jasgeer]

I have a Renolgy 40 Amp DC to DC that is set up with 20' 4 AWG jumper cable's 6' on one end and 14' out the other. A fuse and holder and fuse is how is turned on and off. Its coiled up in a pelican case when not in use. This set up works well.
In my opinion the WFCO gets a bit of a bad rap. 1. The RV manufacturers (not all but most) run long 12 volt small guage wires to the batteries.(This keeps the unit from going into boost. 2. Proper heat venting is ignored.(causing overheating when the consumer over discharges the battery.)

Grizzman,
From my experiece the WFCO is cheaply designed and performs poorly. There are also many reports of failures in the trailer forums. Mine is a 2016 and is still working as a power supply but has never charged the batteries beyond float. My WFCO is connected to the batteries with two 8 or 10awg in parallel. The Renogy DC to DC does an excellent job in comparision.

 

[INCATFAN]

I have a Renogy 12V-12V 20 Amp Charger and have been using it for a couple of years in my 2016 28ft travel trailer equipped with the WFCO converter. I want to be polite but I never saw the three step voltage output from the WFCO do anything but maintain a constant voltage. Even if the batteries were dead it would not charge them properly. I installed the Renogy and wired it so that it could charge from the output of the WFCO or the power from my truck. Notice I used a dual battery switch. My batteries now properly charge and I have had no problems. In hindsight perhaps I should have purchased a Renogy DC-DC with higher current.

Just joined the forum, and I realize this thread is old, but I just bought a fifth wheel and I was thinking about upgrading the battery to lithium and I had this same idea to use a DC-DC between the WFCO and the battery. My question is, what gauge wire do you have running from the tow vehicle? Will the DC-DC charger pull too many amps on the small wire run from the tow vehicle to charge the batteries?

 

[0rion]

Hi @INCATFAN - since first posting this question I went ahead and got a LiFePO4 battery (200ah) - decided to keep the WFCO as stock and forego the DC-DC charger. It did fine - with the distance between the truck battery and the 7-pin there was enough voltage drop to not be an issue with high amps, most I saw while driving was about 10amps peak going to the lithium battery. If you wanted to get serious about charging from your alternator through a DC-DC charger, I'd recommend checking a wire size guide for proper gauge and checking some install videos like All About RV's or RV with Tito DIY.

That said, this question of using a DC-DC between the battery and WFCO has always stuck with me. The DC-DC would 'bypass' the WFCO charge to the battery, and condition the charge for the Lithium. Any reason why this wouldn't work?

Pros:
- Allows for alternator charging as well as shore charging house battery while saving on the expense of a replacement LiFePO4 Converter/dedicated charger
- The Converter could be connected to the tow vehicle to charge the truck starter battery if in a pinch

Cons:
- Would reduce the RV capacity from 55amps from the WFCO to 30amps from the Victron Orion (as configured)
- ?