Inverter Powering Converter No Loop Test UPDATE

[BFL13]

Updated here in OP with results of same test using 100amp PowerMax with variable voltage.
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Simulated external box with batteries and inverter, RV shore power cord into box's inverter, no batts in the RV, use converter to make the 12v you need

Test in trailer with batt disconnect open, Trimetric to measure amps.

First amps is 7355 converter at 13.8v/ then PM converter at 13.6v.

A. inverter on, no 120v loads, no 12v in rig--0.7a/ 0.6a
B. turn on converter, 12v in rig, fridge on propane-5.2a/6.1a
C. add 120v CFL lamp -6.1a/7.0a
D. add double 12v lamp-9.7a/11.0a
E. add rangehood fan-12.3a/13.6a
F. add turn on tap, water pump on-17.9a/19.8a

I suppose you could use a lower voltage converter than the 13.8v 7355. Rig 12v only needs maybe 11.5v to run things? Of course the amps come from the batts in the external box and that would have solar on it during the day.
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I tried the PowerMax for B. above, at three converter voltage levels, setting the pot to get that voltage:
1. 13.6v--6.1a
2. 12.0v--5.7a
3. 14.7v--6.4a
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I previously posted the amps test results for when you forget and leave the converter on and there is a loop (don't do that! 🙂 )

I am not proposing doing things that way, but wanted to know how it would work. ( I would likely have a battery in the rig for the 12v stuff, leave the converter off, and just get my 120v from the external box. I would use my 120v CFL lamps instead of 12v for lighting too.)

[pianotuna]

Hi mena,

Yes please do post results--but do voltage and amps (so I can transliterate to watts).

[mena661]

No watts here! Amps only! 🙂

[BFL13]

I think the idea is to see if there is a sort of standing draw on the external power battery bank from the converter being on with little 12v load, how much in amps NOT WATTS :), and then what happens with increased load.

Could better compare results with the other converters tested so far if tested the same way. Also confirms those tests if your results are in same ball park.

(There is some evidence the standing draw part increases a bit with heavier 12v but is still "affordable" in the scenario IMO.)

[mena661]

I just remembered you wanted me to try this test with my setup. I think I'll have some time this Friday to try this out. Should I just try out different 12V loads while running the converter from inverter and post here?

[pianotuna]

Hi BFL13,

Not only can it be done--but once, by accident I ran my fridge on the converter, while doing no battery charging for several days. I was driving fairly long distances, and somehow I managed to flip the battery disconnect to off.

The fridge worked well on 12 volts @ 13.6 which was the default setting in my "pre wizard" days. It does draw just about the same amps on 120 volt and 12 volt--but the extra voltage seemed to help.

[BFL13]

Thanks for the info using a WFCO 55amper. It does seem safe to say with three different converters now, that there is an extra draw in amps for making the 12v besides the load's draw, but not "too much" so it can't be handled by the set-up.

The scenario where there is no battery in the RV, just 120v items, including the converter to make 12v, would not be very common of course. But now we know it could be done.

[12thgenusa]

I elaborated a little on your test. I still don’t understand how or why you might want to do this, but I learned a few things about my system by the exercise.

I have a WFCO 8955 converter, ProWatt 2000 (PSW) inverter and four GC2 batteries. The converter has been moved from the power distribution panel at the aft end of the trailer to a location right next to the batteries at the fwd end of the trailer. I was able to simulate the external power source by removing one end of the wire that connects the converter to the battery bank. This allowed the bank to power the inverter only. The converter could then power the systems, but not charge the batteries. The solar power could charge the batteries but not power the 12-volt systems. The external power cord was plugged directly into the inverter with the manual transfer switch set to shore power.

I performed the test with four variations for comparison.
1. Normal operation with link connected with solar
2. Normal operation with link connected w/o solar
3. External power with link disconnected with solar
4. External power with link disconnected w/o solar
The bank was allowed to recharge between the tests.

Since the battery bank was fully charged and the sky intermittently cloudy, the solar contributed very little except to hold the battery voltage slightly higher until the demand exceeded the output of the panels. The results of those two tests were eliminated from the chart below.

I was surprised at the flat voltage output of the converter without a battery in the system. With no loads the voltage was 13.69. With a nearly 100 amp combined load on the external battery bank (20 amp DC through the converter) it was still at 13.66. The voltage was 13.26 measured at the distribution panel so I have a 400 millivolt loss in the long wire from the converter to the panel at 20 amps.

To make the voltage variation a little more visible I cropped the chart so the high amp draw isn’t shown when I turned on the toaster. The amp draw when wired as a normal system was 86.7. When wired as an external power supply, it was 96.8 amps so the “cost” of powering the DC through the converter at that high total demand through the inverter was 10.1 amps.

[mena661]

BFL13 wrote:
At least that is two converters now telling the same story, so that's good to know. Need more!--- 🙂

Not sure when I'm getting the rig back they aren't done yet but will do when I get it.

[BFL13]

Mena I may have posted it in a confusing way. The PM amps were a touch higher as to be expected with the 100amper vs the 55amper, but only a little. I don't have the 7355 draw with no DC connection to compare with the PMs--should be a bit less there too I expect.

At least that is two converters now telling the same story, so that's good to know. Need more!--- 🙂

[mena661]

Much lower power draw. Be interesting to see what the IOTA does. I'm speculating it will be on the high side.

[BFL13]

Updated OP with PowerMax results

[BFL13]

The inverter choice can matter for this testing exercise, but not for the real thing I think.

---I could not get the 12v battery supply to the rig cut off completely until I remembered that my 1000w inverter has something going on in it where the ground prong of its receptacles allows a path for the rig's 12v neg side to complete via the shore cable ground wire back to the power centre which is frame grounded. (I do not have this inverter chassis grounded to the rig frame either)

It is all about how your battery disconnect switch is mounted, pos or neg. Mine is on the neg, so the pos is still live and all it needs is a neg path somewhere.

In the real remote box scenario, the battery bank will not have a connection to the RV, so this shore cord path would not matter.

(Almot had this same model of inverter but returned it when he discovered this about it--I have had no problems in real life using this inverter for "whole house" since I have the 12v on anyway, so don't care about the extra path along the shore cord)

So I had to plug the shore cable into the 2000w inverter, which has a kind of GFCI receptacle set with what ISTR Wayne called "floating ground" Anyway that made the 12v go away, so the test could continue.

---The remote power supply is not quite like pedestal power because that has the bonded neutral. This raises the old chestnut about the requirement for that with an RV. Perhaps (without going into all that again in this thread please!) it is situational for the design and location of the remote set-up you would have, don't know.

(for testing like I did, it doesn't matter, since it is with all the same equipment in use already, however you have that arranged)

----I tried my 100amp PowerMax converter with the variable voltage unique feature, just plugged in but not with the DC output wires connected (will try that later)

It draws about 5amps on start up surge then quickly settles at about 2 amps steady at 13.7v. Changing the voltage up and down makes a minor diff in draw like 1.8a to 2.2a but need to do more tests. Anyway, that is half what the 7355 was reported at above but this is with nothing connected on the DC side. I will repeat the OP test later with the PowerMax instead of the 7355

Still hoping some will try theirs for this easy test and we can get comparable figures for other brands and sizes of converter.

---Another thing is your Trimetric or other monitor for the remote battery bank so you can see it in the Rv and in the case of the Trimetric, be able to push its buttons. You would not have the wire to the remote box most likely. Might be a wireless way to take care of that? No idea.

[mena661]

BFL13 wrote:

One item for Mena in Death Valley would be some way to keep the inverter in the remote vehicle cool enough --which would cost some AH from the bank on top of everything else.

Speaking of Death Valley, it was 115F/46C on Sunday at my house (the actual Death Valley was at 129F/54C :)). It's 106F/41C here right now. My rig is in cooler temps (was only 103F/39C on Sunday) where it's being worked on. The heat really "soaks in" much above 105, not sure fans would make any difference as even the wind is hot.

[HiTech]

I'm dabbling with ways to mount the inverter and AGMs under the back seat of the SuperCrew. The cab can get warm from the greenhouse but it stays pretty cool under the seat.

Jim