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Converting TravelTrailer lead-acid to LiON (w/ Solar, AC/DC & DC/DC)

ekarlson

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I am looking to buy a travel trailer this month (hopefully) and am planning on replacing the lead-acid battery with LiON (I have ordered a SOK 206AH battery based on Will Prowse's breakdown of that battery - of course, it hasn't been shipped yet). While I understand the basic issues that need to be addressed, I am finding it incredibly difficult to get accurate information about all the pieces and some of what I am finding is either conflicting or does not make sense to me given basic electronics. What I am looking for here is any information to fill in the blanks in what I am finding (or, more precisely, not finding) as well as confirmation of the assumptions I am making.

What is known:
My TV is a 2019 V6 Toyota Highlander to which I have added a 7-way RV plug (it came factory-wired with a 4-way plug). This provides an un-switched 12V circuit running from the TV battery to the 7-way plug. This wiring is 10 gauge copper from end-to-end with a 40A auto-reset circuit breaker near the TV battery on the positive wire.

What is maybe known:
I have had difficulty finding a definitive statement as to what sort of alternator is in the Highlander, but I have found some material that suggests it is a 150A alternator (unclear whether this is a "smart" alternator or not).

General claims about LiON batteries I've found in various places:
I have found several places that claim that LiON batteries do not require a "float" stage and that if one's charge controller has a "float" setting that it is pretty much irrelevant, but that setting it to 13.6V is acceptable.

I have also found conflicting claims about the "bulk" stage. Some places indicate that in this stage, the charger will deliver as much current as it can (which means that the output voltage will adjust to whatever level is necessary, given the battery's internal resistance, to reach the charger's maximal output current. But I've seen other places that specify a voltage for the "bulk" stage, which contradicts the notion that the charger will source as much current as it can during the "bulk" stage.

What I have found relating to charging a LiON battery from a TV's alternator:
There are multiple references that I've found that tying a TV's alternator directly to a LiON battery is a bad, bad idea for at least two reasons. First, the standard alternator does not generate the voltages required to properly charge a LiON. Second, the current draw of a LiON battery can burn out one's alternator.

The solution is to use a isolated DC/DC converter, which will both boost the voltage to the levels required to charge a LiON battery as well as regulate the current draw on the TV's alternator. After doing some research I'm looking at Victron Energy's Orion-Tr Smart DC-DC Isolated Charger primarily because it uses the voltage seen from the TV to decide whether the TV's engine is running or not (so no need to find a way to run a wire from the TV's ignition switch back to the trailer to control the DC/DC converter). There are two models of interest - the 12/12-18 and 12/12-30. Since the 10 gauge wire that runs from the TV through the 7-way plug is rated to carry 30A, the 12-12/30 is tempting, but it would seem to be right at the limit of what a 10-guage wire could safely carry, so I am opting for the Orion-Tr 12/12-18 model.

What I have found in downloaded manuals:
The trailer is solar-equipped with a 100W solar panel. The solar charge converter would appear to be a Jaboni 30A MPPT controller that apparently can charge multiple types of batteries, including lead-acid and LiON. The manual claims it supports a bulk/absorption/float cycle for LiON of 14.4V/14.4V/13.6V, but the "bulk" rating is highlighted in a different color than the rest of the chart, which makes me wonder if there really is a "bulk" mode that is distinct from the "absorption".

The AC/DC panel is a WFCO 8900-series unit. The WFCO AC/DC converter only supports lead-acid batteries. However there is a Progressive Dynamics PD4635 AC/DC converter that apparently can be swapped into the WFCO panel to replace the WFCO converter. The PD4635 is supposed to support both lead-acid and LiON batteries. However, the manual was a bit vague regarding what the stages are with LiON. So I wrote to the company and the email I received back indicated that there is a switch to toggle between lead-acid mode and LiON mode. In lead-acid mode there are 3 stages with varying output voltages. In LiON mode it outputs a steady 14.6V at all times - i.e. it is a single-stage charger.

The few pieces of information I can find on the SOK 206AH battery are a bit confusing. I was able to find a document on SOK's Facebook page that claims that the recommended amperage to charge the battery is 40A (yikes) a "recommended" charge voltage of 14.6V and a "balancing voltage" of <14.4V.

What I cannot find:
I cannot tell if there is any sort of isolator in the trailer between the 12V wire coming from the TV and the trailer's battery to prevent depleting the TV's battery. There is certainly no isolator in the TV.
.
What I am guessing at:
I am assuming that the output of the AC/DC converter, the Jaboni MPPT charge controller and the 12V un-switched circuit from the 7-way plug are all wired in parallel to the trailer's lead-acid battery and the internal 12V circuits. This means that any or all of the 3 possible charging sources (the AC/DC converter, the MPPT controller and the 12V wire from the TV) are driving the battery and internal 12V circuits in parallel.

I am assuming that when the BMS in the LiON battery cuts off the charger to avoid over-charging, that this is done using a diode, so that the battery can still source electricity to the trailer to operate the internal 12V circuits rather than leaving that entirely to the chargers (in general, having a charger directly drive a load instead of charging a battery seems to be a bad idea). I guess the AC/DC converter could drive the 12V loads directly since it is a simple, steady-state converter, but it seems like trying to directly drive a load from an MPPT controller or a DC/DC charger is a bad idea as they are trying to alter voltage levels based on the perceived charge of a battery.

Since the 12V circuit from the TV is incompatible with a LiON battery, I would have to interpose the DC/DC isolated charger between the 12V wire from the TV and the inputs to the LiON battery. So irrespective of whether there is an isolator already installed in the trailer, I will have to add my own to support the LiON battery.

I am assuming that the Orion-Tr 12/12-18 would not draw more than about 23A from the 12V wire from the TV (max continuous output is 18A at 15V with an 87% efficiency and an input activation voltage of no less than 13.3V -> (18A * 15V / 87%) / 13.3V = 23.3A). This value seems to be well within the range that a 10 gauge wire should be able to safely carry. The specs in the manuals do not actually indicate this parameter, so I am inferring it from the other parameters that are stated in the docs.

My questions:
1) The SOK battery docs that I can find talk about "recommended" and "balancing" voltages. It is not clear to me how this relates to the concepts of bulk/absorption/float found in the settings/specs of most chargers. I think I would set the "absorption" voltage to the "recommended" voltage value and the "float" voltage to the "balancing" voltage, but I'm not certain about this.

2) The Orion-Tr DC/DC charger seems to have bona fide bulk/absorption/float stages. It is not clear that the Jaboni has a bona fide "bulk" stage (meaning, variable voltage in order to achieve maximal current). The AC/DC converter in the PD4635 seems to be a simple, constant voltage output. Given that the charging profile of these three units are all slightly different form each other, is there a problem with having them wired in parallel to the trailer's LiON battery? I don't think that the minor differences between the Jaboni and Victron units would matter. The piece that I'm uncertain about is whether the constant voltage output of the PD4635 would cause a problem for the LiON battery.

3) The 100W solar panel would generate no more than 6.8A at 14.6 volts, and the Orion-Tr would cap out at 18A. This means that at best, while towing the vehicle, I would get 24.8A to charge the LiON battery, when the recommended amperage is 40A. When only on solar, I am limited to about 6.8A. I assume this simply means that my battery would charge slowly but would not otherwise reduce the battery's lifespan or otherwise damage the battery.

4) Regarding the single-stage output of the Progressive Dynamics PD4635 modules, one option I've considered is to wire the output of the AC/DC converter into a second DC/DC converter (specifically, a Renogy DCC50S) and then wire the output of the DC/DC converter to the LiON battery et al. This would provide for a bona fide 3-stage charger when power is coming from the shore line. Question is, does this make sense or is it over-engineering?
 
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Are you aware that WFCO now has their own Lithium battery charger for many of their converters? If the Progressive ones are constant output voltage, that doesn't sound good, the WFCO one I looked at was not CV.

I have a similar situation, wanting to upgrade an RV to Lithium.
 
Yes, the "Lithium Switch" power centers - just very difficult to find. I did finally find an RV dealer who was able to special order one (WF-8735Lis).

Had to drill an extra hole in the camper to pull extra wires from the 7-pin cable up to the DC-DC charger in the forward storage compartment (tried a Renogy DCC30S - for a variety of reasons I was unimpressed - am currently switching over to Victron Energy Orion Smart Tr 18 and SmartSolar 75/15). Had to go under the trailer and re-wire the 12V feed from the 7-pin to bring it directly up to the DC-DC charger. I suspect that I am going to have to add a second panel in series with the existing 100W unit to get the voltage high enough to make the MPPT controller effective for lithium (the Renogy DCC30S is limited to 25V from the PV, which precludes wiring two 100W panels in series - this is part of the reason I am returning it).
 
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I'm still looking for someone that can order one (different model) for me.

In the interim, I've thought about intercepting the output from the non-Lithium-supporting AC converter that I have, and running it to the Victron (same 18A unit you mentioned) that will be powered by the 7-pin. At least my truck is old enough to have a decent 7-pin amperage, without rewiring it. That would only give about half the max current on AC that my current 40A converter puts out... I intended to do some current measurements today, to see if 18A suffices, or what restrictions that would imply, but got interrupted by a different time-consuming project. So maybe tomorrow. Of course the big potential consumer of power, the refrigerator, would run on AC while plugged in to shore power. Couple fans, water pump, LED lights, don't sound like they'd draw 18A.

Although it would stretch out the recharge time for the house battery, I'm wondering if, with the Victron, I'll be able to run the refrigerator on DC while driving. A little safer than using the propane, and less likely to blow out. With the lead-acid, and no DC-DC converter, it seems the current draw dropped the voltage in the RV low enough to not charge the battery at all, in fact, it discharged the battery when I tried that once.
 
I am looking at installing a 500VA inverter to power the fridge while I am towing (same concern - don't want to be running propane while towing). I have a Dometic DM2672 and when I put a kilowatt meter on the shoreline, it seems to be drawing about 340W of power (note - most of the power draw is for heating the ammonia-based absorption cooling system, not the fans, etc). There is some sort of reactance in the AC circuit as the VA rating is not the same as the watts - so I am using the observed watts to size the inverter. Working backwards through a 90% efficient inverter to a 14.4V lithium battery means a draw of about 26A. The DC-DC charger should be able to source 18A and a pair of 100W panels should be able to provide about 11A, which in theory would power the fridge and leave about 3A to recharge the trailer's battery.

Currently my biggest challenge is trying to figure out how to run the AC wires from an inverter in my forward storage compartment back to the refrigerator...
 
I would probably run the AC wires from the forward compartment\inverter back to the WFCO control panel. That way it can power all 110V devices (including the fridge).
 
If running a supply wire the WFCO transfer switch can mount directly to the back of the converter, might make for a neater install. You do have to disconnect or shut off the charging when on inverter, Go Power has a schematic somewhere on there site that shows 1 way to do it.
I don't know how hot it gets where you are but we haven't left our fridge on while travelling for more than 30 years-nothing thawed yet....
 
@Whinny Multiple reasons I am not running over to the power panel. (1) the inverter would barely have enough juice to power the fridge, let alone the rest of the trailer's systems, (2) requires an ATS and rewiring of the power panel's AC/DC converter to not run off the inverter, (3) running wires to the power panel is even harder than getting to the fridge & (4) unsure that there is sufficient space behind the power panel to install an ATS there.

Don't know how you've been able to avoid thaw in your freezer while towing. Just did a trip this weekend and after a mere 4 hours the freezer went from -2F to 34F and items started to thaw. Outside temperature was in the high 60's to low 70's. Hence why I am looking at the fridge-specific inverter setup.
 
Are you aware that WFCO now has their own Lithium battery charger for many of their converters? If the Progressive ones are constant output voltage, that doesn't sound good, the WFCO one I looked at was not CV.

I have a similar situation, wanting to upgrade an RV to Lithium.
and eklarson -- I contacted WFCO support and they said to get this --

WFCO WF-8950L2-MBA Replacement RV Power Converter Lithium-Ion Main Board​

and change out the converter main board ( 2 screws and 5 wires ) 15 min job you can find most anywhere -- on Amazon

 
Yes, the "Lithium Switch" power centers - just very difficult to find. I did finally find an RV dealer who was able to special order one (WF-8735Lis).

Had to drill an extra hole in the camper to pull extra wires from the 7-pin cable up to the DC-DC charger in the forward storage compartment (tried a Renogy DCC30S - for a variety of reasons I was unimpressed - am currently switching over to Victron Energy Orion Smart Tr 18 and SmartSolar 75/15). Had to go under the trailer and re-wire the 12V feed from the 7-pin to bring it directly up to the DC-DC charger. I suspect that I am going to have to add a second panel in series with the existing 100W unit to get the voltage high enough to make the MPPT controller effective for lithium (the Renogy DCC30S is limited to 25V from the PV, which precludes wiring two 100W panels in series - this is part of the reason I am returning it).
I contacted WFCO support and they said to get this --

WFCO WF-8950L2-MBA Replacement RV Power Converter Lithium-Ion Main Board​

and change out the converter main board ( 2 screws and 5 wires ) 15 min job you can find most anywhere -- on Amazon

 
@IDMT The power center in my trailer is the WF-8735. That replacement module only fits into the 8900 series power centers. My only choice is to replace the entire power panel with a WF-8735LiS or to install a completely separate deck charger unit and disable the AC/DC converter in the power panel. I have gone the route of replacing the entire panel as the cost is only around $200.
 
My fridge is 3-way, so has a DC option. Always more efficient to avoid conversions, but that'd be a big-ticket item to replace. Indeed, the main power draw would be the heating element, would think that to be mostly resistive, so surprised you are seeing reactance, but I didn't design the circuit! Gives me something else to measure on mine tomorrow, for comparison... I've got one of those Kill-a-Watt units.

Refrigerator is likely on its own circuit... click the rest off, and run the inverter to the main AC line? Sure is easier to run wires before they put the paneling on!

You might have an issue with your amperage calculations if you have a cloudy day, or happen to drive at night.
@IDMT The power center in my trailer is the WF-8735. That replacement module only fits into the 8900 series power centers. My only choice is to replace the entire power panel with a WF-8735LiS or to install a completely separate deck charger unit and disable the AC/DC converter in the power panel. I have gone the route of replacing the entire panel as the cost is only around $200.
@ekarlson can you share your source for the WF-8735LiS? Maybe they'd get me an WF-8740LiS.
@IDMT When I contacted WFCO they said any RV parts store could order it, but I haven't found one that is willing to, as yet.
 
@G4-L8 Yes, my calculations are assuming driving during a sunny day, which would be the normal situation for me. Driving under sub-optimal lighting conditions would require that I either allow my battery to supplement the power to the fridge or that I simply forego running the fridge and let things warm up. My plan is to put in a SOK 206Ah battery so I would have a fair amount of battery capacity to run the fridge, but it would still represent a significant draw on the battery (and I am still waiting for it to be delivered - 2 months and counting). I am also trying to not pay attention to how much I am spending on "improving" my brand-new trailer as, well, it might ruin some of the fun... ;-)

I am assuming there is a reactance given the disparity between the VA and watts reported by the kilowatt meter, but that is just an assumption. Maybe some sort of coil in the heater assembly?

I actually considered replacing the fridge with a 3-way, but opted not to because (1) expense, (2) need to run a special DC circuit to the 3-way that could carry the required current and (3) labor - trying to rip out and then install a new fridge (that may not be the same size), would be a lot of work.

The people who were able to order the WF-8735LiS for me were Noah's RV Center: https://www.nohrsrv.com/
 
My fridge is 3-way, so has a DC option. Always more efficient to avoid conversions, but that'd be a big-ticket item to replace. Indeed, the main power draw would be the heating element, would think that to be mostly resistive, so surprised you are seeing reactance, but I didn't design the circuit! Gives me something else to measure on mine tomorrow, for comparison... I've got one of those Kill-a-Watt units.

Refrigerator is likely on its own circuit... click the rest off, and run the inverter to the main AC line? Sure is easier to run wires before they put the paneling on!

You might have an issue with your amperage calculations if you have a cloudy day, or happen to drive at night.

@ekarlson can you share your source for the WF-8735LiS? Maybe they'd get me an WF-8740LiS.
@IDMT When I contacted WFCO they said any RV parts store could order it, but I haven't found one that is willing to, as yet.
@G4-L8

Just looking I found 4 places ---

https://parts.unitedrv.com/products...ry-converter-w-switch-55-amp-wf-8955pec-b-lis

https://www.walmart.com/ip/WFCO-WF-...er-11-Circuits-Lithium-Switch-Black/391523379

https://www.boatandrvaccessories.co...ution-center-11-circuits-lithium-switch-black

https://www.camperid.com/wfco/wf-89...6-ac-11-dc-branches-mpn-wf-8955pec-b-lis.html

https://www.ebay.com/itm/WFCO-WF-89...Output-W-Distribution-Center-11-/324317508688
 
@IDMT None of those are the WF-8740LiS.
Happly Nohr's RV that @ekarlson found is willing to order what I need, and ship it to me.

So I did do my electrical measurements today. I think my refrigerator is a bit smaller than yours, @ekarlson.

In case it helps anyone else, here is what and how I measured. I was already plugged in to AC to trickle charge, but I disconnected and inserted the Kill-a-Watt device into the AC line to get measurements. I also used two DVM, one for DC voltage measurements and one (a clamp meter) for DC amperage measurements.

First I measured the AC "baseline". I think the only things consuming power were the battery charge maintenance, and the gas detector.

From the Kill-a-Watt: 0.19 AC amps, 10.5 watts, 23.2 voltamps, 0.45 power factor. From the DVM, DC voltage 13.05 I didn't measure the DC amps at this point, as I was focusing on AC consumption at first.

Speculations: the converter probably has a switching power supply to manufacture DC from AC. Switching power supplies have a power factor, because they only consume power at the more extreme voltages, on the upslope of the AC sine wave, rather than during the downslope or the near-zero voltages. In any case, power consumption for these two uses is minimal.

I turned on the refrigerator, in AC mode.

1.8 AC amps, 190 watts, 190 voltamps, 0.99 power factor, DC voltage 13.05

What this tells me is that there is very little, if any, reactance in my refrigerator, running on AC. The slight power factor is probably a result of the baseline load and DC converter, as speculated above, with the refrigerator itself being a resistive load (as I would expect, from a heating element).

Then I turned the refrigerator to DC mode (but still connected to shore power). The converter immediately turned its fan on, seeing the increased demand for DC power.

2.94 AC amps, 257 watts, 350 voltamps, 0.73 power factor, DC voltage 13.05, DC amps for refrigerator heating element: 17.15

The DC voltage staying steady (measured at the converter) indicates that the 40Amp converter is not being stressed by this load.

The power factor going up confirms my speculation that it is the converter that is the source of the reactive load, not the refrigerator. The watts going up indicates that the converter is not 100% efficient (no surprise there). A rough estimate of its efficiency would be 190 / 257 = 74%. Not super great, but probably in the expected range for an inexpensive unit.

The DC consumption for the refrigerator indicates that there would be precious little power left from an 18Amp DC-DC converter from the 7-pin to charge the battery when running the refrigerator on DC at max cooling. I should probably do some more measurements with different cooling settings.

I turned off the refrigerator, disconnected from AC shore power, and turned my attention to DC loads in the RV. I put the clamp meter on the DC neutral wire where all loads return to the converter DC fuse panel.

The background (probably just the propane detector) DC amp draw was 0.29 amps, Voltage still sitting at 13.05.

I turned the refrigerator on DC. Amps: 15.04 DC Voltage 12.16. Refrigerator DC heating element must be roughly 0.75 - 0.8 Ohms. from the different volt/amp rations from the DC operation on AC and battery power sources. Actually, since the DC-DC converter produces 14.5V for lithium charging, that heating element , as a pure resistive load, would probably consume ALL the output of the 18A DC-DC converter when running on max cooling. More refrigerator measurements with different cooling settings seem imperative.

I turned the refrigerator off, and looked at other devices.

I have two fantastic fans. I turned one on, and measured the amps for each of its three speeds:

low: 1.44 A 12.12 V
med: 1.95 A (missed this one) V
high: 2.6 A 12.05 V

Turned that off, and turned on 3 of the 8 LED room lights: 1.39 A
Tured those off, and turned on the light and fan on the range hood: 1.92 A.

I couldn't get a good reading on the water pump, because the RV is still winterized.

After turning everything off, the battery voltage (measured at the converter) was 12.35. I didn't run anything long enough to seriously discharge the (current, flooded lead-acid) battery.

I hooked back up to shore power, and did other stuff until writing up this report and analysis.
 
Unsurprisingly, if the various "coldness" settings have any effect, it must be on cycle time, rather than operational amperage.

Someone with better knowledge of absorption refrigerators than I could probably confirm that they only work in a particular temperature range, to make the coolant cycle work properly, and so "coldness" settings vary the "on" time duration.

Since I was starting with an "outdoor temperature" (can't say "room temperature") when I did my tests on each coldness setting, and since they were brief, but all came up with (almost) exactly the amperage draws of the "coldest", that convinced me that the "coldness" setting affects cycle time only.

Which, when considering charging the battery with "leftover" power from an 18A DC-DC converterter, the battery would only get any significant charge current when the refrigerator would cycle off. I have no idea if this would result in adverse effects on the lithium battery.

@ekarlson 's solution of having some solar to augment the DC-DC, which is almost imperative for his larger refrigerator and use of a lossy inverter, may also be appropriate for me, to allow running the refrigerator from DC. Another alternative for me would be to run a bigger gauge wire from the the alternator, and use a higher amperage DC-DC converter, and place more demands on the alternator (which could be upgraded if necessary).

Anyone know what current draw would burn up an alternator, or where to find such specs? My understanding is that they can put out plenty of current, but if such current is demanded too long at slow RPMs, that they can burn out. At higher RPMs, the internal fan in the alternator can keep it cool enough to put out plenty of current without burning out.
 
Regarding my fridge, it is a Dometic DM2672. According to the product page (https://www.dometic.com/en-ca/ca/pr...ators/dometic-dm-2672-_-183085#specifications) it draws 2.7A @ 120V, which actually lines up very closely to what I measured. The wattage came in at about 325W once I had let the batteries full charge (about 20W draw with charged batteries and everything off, about 345W when I turned the fridge on).

In terms of "coldness", I did run an experiment where I montoring power usage over 24 hours, which gave the fridge plenty of time to cool down and possibly start cycling. I did not see any significant change in power consumption when the fridge had completely cooled down, so I am unsure how it regulates its temperature.

In terms of how much current one's alternator can source, I think the Highlander's alternator is rated at 150A, but really the issue is what happens when there is a high current draw when the engine is idling. There is a video at Victron Energy that discusses this: https://www.victronenergy.com/blog/2019/10/07/careful-alternator-charging-lithium/. The answer seems to be "it depends". Some alternators apparently have logic to reduce output when they start heating up, some do not. So I think one would have to figure out what your specific alternator does.

In terms of installing an inverter, I found a piece of paneling near the power panel that was held in place with screws, so that I could actually open it non-destructively. There is a fairly large open space around the back of the power panel, assuming I can bundle up some of the wires strung all over the place. I think I can replace the removable panel with an air vent cover to provide some ventilation and install a surge protector, ATS and inverter back there, and it seems like there is 6 AWG wire running from the battery to the power panel. So this might actually work.
 
Interesting. I sort of expect that if you have "coldness" set to max, that it runs continuously, never cycling. I don't think they have any sort of thermostatic control, or feedback mechanism (at least, the 5 I have experienced, some in the big Class As and long trailers claim to have "residential style" refrigerators, and those might have thermostats). My refrigerator, in moderate ambient temperatures, is quite happy to freeze everything on max "coldness". I keep a wireless themometer in it, to monitor when to set it to less or more than what it is. Not sure what you had it set on for your 24 hour test, but that is interesting.

Glad you found some space.

I made a comment earlier about running the inverter into the main breaker, and someone else commented that you would need to turn off the converter if you did that.... a very wise comment. I also note that my refrigerator has an AC plug in the "equipment compartment" area, plugged into a socket there, so if you do figure a way to run AC wires back there, you could put in a switch there if you want. I also noticed that my RV has a circuit for "refrigerator and GFCI" So there is little else (only optional, probably not-in-use-while-traveling) on that circuit. So you might be able to intercept that standard feed for that circuit behind the main panel, and run a more easily accessible switch to switch that circuit from "shore power" to "inverter", and power only that one circuit from the inverter. Depends on what else might be on the circuit in your rig, of course.
 
Yes, if one puts in an inverter you have to re-wire the AC input the the panel's AC/DC converter such that it runs off the shoreline ahead of the ATS.

It is a rather funky space back behind that panel next to the power center, but I think I can build a small platform, move some strung wires to the side and put in the inverter, ATS and surge protection there. (it is kinda weird back there - there are at least two places where there was non-visible paneling installed during the build process which was then crudely cut away to pass all the wires, pipes and tubes around) This would then allow the inverter to supply A/C power to anything (other than the AC/DC converter) - albeit the inverter won't have enough juice to power much beyond the fridge. There are 3 AC circuit breakers in my panel, but nothing is labeled so I do not know what goes where. If I am lucky and the fridge is on its own AC circuit I might put the ATS just on the fridge circuit to avoid the possibility of overloading the inverter by turning on the air conditioning (though it is tempting to put some of the AC outlets onto the inverter as well for powering small electronics chargers - kinda weird to go from DC to AC just to power a charger that converts it back to DC). The wires that carry power from the battery/alternator DC-DC/Solar that pass under the trailer appear to be 6AWG so as long as those wires continue all the way to the power panel I should be able to use them to power the inverter. I think I can also get sufficient ventilation as well by replacing the removable panel with a vent register.

My fridge has a plastic slide in the fridge section that can be slide up or down on the cooling fins in the back of the fridge. Supposedly this adjusts the temperature by changing the vertical position of the thermistor. What that thermistor controls is still a mystery to me.
 
It is quite likely that the air conditioner is on its own circuit... while I don't have one, the RV is wired for one, and has a dedicated circuit for that. My refrigerator circuit is marked as also powering the GFCI, which I assume is the AC power outlet right next to my power panel. On AC power, the refrigerator draws less than a couple amps, so doesn't overload the circuit. It could be yours is also wired with other things... such as outlets. Seriously, most of the built-in stuff in smaller RVs is made to run on DC, so air conditioner, refrigerator, and outlets would be most of the AC needs. My RV has built in 12V and USB outlets, direct from the DC side of things, so I can recharge a variety of stuff without inverting to AC. If you list all the AC-consuming items, it isn't too hard to flip the breakers off one at a time and figure out what each one powers, and it is good to know stuff like that before they flip by themselves! If you only have 3 circuits, I wouldn't be surprised if one is air conditioner, one is power to the converter/DC side, and the other is refrigerator and all the outlets. But maybe one is the main, and the DC is bundled with the refrigerator, or maybe you didn't count the main breaker when you said 3.

I took a picture of my breaker labels, but it turned out fuzzy, and for the life of me I can't figure out what in the RV starts with "M" and 4-6 more letters. Almost looks like "MODULE", but I'll need to go read that label again, and get a better picture., and I sure don't know what the "MODULE" would be, if that is what it says. There is an outside AC outlet...maybe it is that? The readable ones are what I listed above: Main, Air conditioner, GFCI+Refr, 12Volt, so I have one (or two?) more breakers than you, in a smaller RV! I might have to do a little breaker flipping myself, if it isn't clear when I get back to reading that label.

My DC circuits are labeled as #1 - #6, and I can read the fuses for the amperage (4 15s, 1 20, 1 30, and converter protection (2 30s), and I haven't figured out the DC circuit mapping yet.

Interesting. Your bigger refrigerator might be just bigger enough to have some amount of attempt at a feedback loop, with that thermistor measuring temperature at whatever vertical position you set. I would immediately think than lower would be warmer, and higher would be cooler, if the thermistor controls the cycle time, as the bottom would likely stay cooler than the top (hot (well, not quite as cold) air rises). I have a button that cycles through 5 numbered LEDs from less to more cold, assuming it is shorter to longer cycle time, with 5 probably meaning never cycle off.
 
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