Updates 23 June--new test with Renogy input exposed for voltage readingsUpdate 25 June more readingsUpdate 4 - 27 June-- fewer amps with fat wire were noted this time!-----I have the 20 amper Renogy with a 105a alternator in the 2003 truck. The Renogy is in the truck camper near the house battery bank (200AH) and input to the Renogy is via (thin) 7-pin wiring for pos path and neg path.To run the test I inserted an analogue 60 amp ammeter in the pos path from truck to camper close to where the 7-pin connects to the camper's 7-pin. Camper batts at 50% SOC so lots of time to stay at constant amps in Bulk to run the test.Truck engine bay heated up during the test (idling with hood up) and truck engine battery voltage came down over time. Noted in test results.Trimetric monitor for camper batt readings and digital meter for truck input readingsTrimetric showed 12.2v and -00.3a before test. I will give output amps as what the Tri said plus the .3 so 18.8 + .3 = 19.1 Renogy output to battery bank, eg.Truck and Renogy on at first with 7-pin. Truck batt 13.94 after a few minutes (14.5 start), voltage tapered more as warmed up over 45 minutes.19.1 amps out at first, then tapering as truck warmed up. (bad--I will have to do some work on that) When Renogy turned on, output amps ramp up so not instant full amps. (good IMO)Results:Truck batt 13.72v, at ammeter 9.88v, 30 amps Tri and 12.8v/ 16.8a tapering amps.Then 13.63/9.86, 30a and 12.8/15.8,Then 13.55/9.83, 30a and 12.8/15.2Renogy unit stayed cool to the touch for whole 45 minute test.So now the big question is--What happens if you use fatter wiring between truck batt and Renogy compared with the 7-pin????Using a 25 ft fat wire set of jumper cables (not sure of AWG--ISTR #2 cu-al) Pos path change only, neg path still via 7-pin:13.32/11.84 (big jump there!) and Tri showing 12.8/19.1 (big jump in amps there!) still 30 amps on ammeter.Added neg path half of jumper cables13.53/13.01 (ha!) 12.9 (battery coming up by now) /19.1 a no tapering.Checked Renogy output at the unit 13.00v, and Tri says 12.9 so that's ok.So there you have it for what happens with fatter wire from truck to Renogy No change in Renogy draw still 30 amps, but way better output amps. Adding neg path no change in output, so there is a voltage drop that is not worth improving looks like.

Thanks BFL13I think a table might help with understanding the results. I know they are pain to do here.

Frame ground note FWIW: Some time ago I tested the wiring to my gen using a 80A load. The frame ground voltage drop was less than the 1/O copper cable voltage drop but not by much. IE Steel certainly has higher resistance than copper but there is a lot more steel cross section area. For a trailer to TV using frame grounds plus the frame to frame wire/connector should provide very acceptable voltage loss.

However you do it, it seems the Renogy did not perform so well with only about 10v input, and it did well with closer to 12v input. It was pulling 30 amps whatever else was going on in my test, so idea of making it easier on the alternator by using fatter wire would not work in my case anyway--just way better output. Some kind of conversion efficiency thing perhaps.

Something doesn't seem right with the DC-DC charger or the measurements. In all cases it calculates out to being about 65% efficient. That is terrible efficiency, and in that case it should have been getting hot as the charger would be dissipating about 135W of waste heat.

FWC wrote:Something doesn't seem right with the DC-DC charger or the measurements. In all cases it calculates out to being about 65% efficient. That is terrible efficiency, and in that case it should have been getting hot as the charger would be dissipating about 135W of waste heat.Agree on the watts in vs watts out which didn't come out as I expected. However, those were the numbers I got the way I got them. Measurements have some "internal consistency" so IMO they are ok. eg the way the engine battery voltage tapered with higher engine bay temps.I carried on after the test and recharged the batts with the converter and the Tri was working right, and I compared Tri voltage with meter voltage and came out ok. Don't know what else could be skewing things.Somebody else with a Renogy could try his numbers for a comparison. A guy in the other thread said his input amps draw was lower with fatter wire, but I did not see that at all. He has a clamp ammeter while I have an old time car dash type, but it showed 30 amps and wasn't stuck needled. Beats me.

Renogy DC-DC Test Results-UPDATE 4 (Better!) | Good Sam Community

pianotuna
Nomad III
Jun 22, 2021
Thanks BFL13

I think a table might help with understanding the results. I know they are pain to do here.
CA_Traveler
Explorer III
Jun 22, 2021
Frame ground note FWIW: Some time ago I tested the wiring to my gen using a 80A load. The frame ground voltage drop was less than the 1/O copper cable voltage drop but not by much.

IE Steel certainly has higher resistance than copper but there is a lot more steel cross section area.

For a trailer to TV using frame grounds plus the frame to frame wire/connector should provide very acceptable voltage loss.
BFL13
Explorer II
Jun 22, 2021
However you do it, it seems the Renogy did not perform so well with only about 10v input, and it did well with closer to 12v input.

It was pulling 30 amps whatever else was going on in my test, so idea of making it easier on the alternator by using fatter wire would not work in my case anyway--just way better output. Some kind of conversion efficiency thing perhaps.
FWC
Explorer
Jun 22, 2021
Something doesn't seem right with the DC-DC charger or the measurements. In all cases it calculates out to being about 65% efficient. That is terrible efficiency, and in that case it should have been getting hot as the charger would be dissipating about 135W of waste heat.
BFL13
Explorer II
Jun 22, 2021
FWC wrote:
Something doesn't seem right with the DC-DC charger or the measurements. In all cases it calculates out to being about 65% efficient. That is terrible efficiency, and in that case it should have been getting hot as the charger would be dissipating about 135W of waste heat.

Agree on the watts in vs watts out which didn't come out as I expected. However, those were the numbers I got the way I got them. Measurements have some "internal consistency" so IMO they are ok. eg the way the engine battery voltage tapered with higher engine bay temps.

I carried on after the test and recharged the batts with the converter and the Tri was working right, and I compared Tri voltage with meter voltage and came out ok. Don't know what else could be skewing things.

Somebody else with a Renogy could try his numbers for a comparison. A guy in the other thread said his input amps draw was lower with fatter wire, but I did not see that at all. He has a clamp ammeter while I have an old time car dash type, but it showed 30 amps and wasn't stuck needled. Beats me.
otrfun
Explorer II
Jun 22, 2021
BFL13 wrote:
FWC wrote:
Something doesn't seem right with the DC-DC charger or the measurements. In all cases it calculates out to being about 65% efficient. That is terrible efficiency, and in that case it should have been getting hot as the charger would be dissipating about 135W of waste heat.
Agree on the watts in vs watts out which didn't come out as I expected. However, those were the numbers I got the way I got them. Measurements have some "internal consistency" so IMO they are ok. eg the way the engine battery voltage tapered with higher engine bay temps.

I carried on after the test and recharged the batts with the converter and the Tri was working right, and I compared Tri voltage with meter voltage and came out ok. Don't know what else could be skewing things.

Somebody else with a Renogy could try his numbers for a comparison. A guy in the other thread said his input amps draw was lower with fatter wire, but I did not see that at all. He has a clamp ammeter while I have an old time car dash type, but it showed 30 amps and wasn't stuck needled. Beats me.
I agree with FWC, something is wrong with your readings and/or your installation if your 20a Renogy dc to dc charger continues to draw 30a regardless what size cable you use. 30a is the absolute max current the input of a 20a Renogy dc to dc charger can pull---worst case. 30a tells me you have a very large voltage drop on the input to your 20a Renogy dc to dc charger. I'd guess-estimate you have at least a 1-2v voltage drop to force 30a. If you're willing to get that voltage drop down to less than .3v, you will see the input current to your dc to dc charger drop to less than 22-23a (with 20a on the output). Of course, if you're happy with 50% conversion efficiency you can certainly leave things the way they are.
BFL13
Explorer II
Jun 22, 2021
Might be this. I was not able to take the input voltage directly from the Renogy's terminals (no access to that end of the Renogy), but took that voltage outside the camper where I had the ammeter inserted. So that left a bit of 7-pin wire still from there to the Renogy and the ammeter.

So there could have been a voltage drop from the ammeter to the Renogy, making it still do 30 amps, but then how to explain the big improvement in output amps I got by using the fatter wire from the engine batt to the ammeter?

You use the total R on both paths in the circuit and each path can have sections of higher or lower R. It would reduce the R on that part of the path with the fatter wire as shown by the reduced voltage drop as seen on that part. Also when I did the neg path got even less drop by reducing total R even more.

Hard to believe that remaining bit of 7-pin and the 7-pin connectors could have so much R, but I have not measured that. I did get way better output amps though, so I am not convinced that was it, but can't prove anything with no measurement on that part of the pos path.

If the total R was still enough to make the Renogy pull 30 amps, how did it notice a higher input voltage so it then did its 19.x output amps?
FWC
Explorer
Jun 22, 2021
It sounds like the Renogy is current limited on the input to 30A. So it will pull 30A * However many volts it can, then convert that (with some inefficiency) to the output power. You most likely had enough voltage drop in the remaining wire that the limiting factor in all cases was the input current limit - thus it could never get to the full 20A on the output.

To make this test meaningful you actually need to measure the voltage at the input terminals to the DC-DC converter.
CA_Traveler
Explorer III
Jun 22, 2021
Charging at 15A with initial wiring to 19A with larger wires. Individuals will need to decide if that increase is worth the larger wire installation.

I was also wondering why the charging never got up to float level, ie it was 12.8V. Isn’t the unit a charger that should do bulk/absorb/float? I’m wondering if the unit isn’t designed for this application but rather higher solar panel voltage input? Or maybe it’s related to the higher efficiency losses in the setup?

MPPT solar controllers generally use the higher panel voltage with it’s amps and convert to lower voltage and higher amps. Wondering if this unit cannot convert to both higher voltage and higher amps.
BFL13
Explorer II
Jun 23, 2021
The 200AH bank at 50% SOC takes a long time to get up to Vabs at 14.7v. so it only got to 12.9v during the 45 min test with 15-20 amps of charging. If left on for hours and hours it would reach the 14.7 set on the Renogy.

After the test I turned off the Renogy and turned on the 55 amp converter set to 14.7v, which got it all done after a few hours.

The Renogy is buck/boost. An MPPT is buck only.

I didn't like the amps tapering down to 15 (could have gone lower if left running like that, don't know. ) Tapering was much slower by the time it got down to 15 so might have reached a bottom soon, don't know.

Also engine bay heat was climbing so truck voltage was also tapering. Might be related where input voltage got below the range the Renogy can convert (boost)

I could pull out the Renogy so I can access the input terminals, but would still need to install the ammeter somehow. I think the ammeter adds some R to the path, which I could measure across it (as taught here by Salvo back in the day), since you can see what the amps are.

It was out-putting max amps of 20 as rated showing 19.1 at the camper battery if you allow for some voltage drop on the wires from Renogy to the battery bank.

I don't care about the 30 amp draw as such for my scenario as long as I get the 20 amps out, but I need to run fatter wire to get that as seen in the test.

Those who want to reduce the amps draw on the alternator/engine battery by using fatter wire and still get rated output charging amps will not be able to use the OP test to see how that would play. Not sure I can re-arrange things to answer that. Have some other chores to get to meanwhile, so maybe somebody else can show that from actual results.