Renogy DC-DC Test Results-UPDATE 4 (Better!)

[BFL13]

Updates 23 June--new test with Renogy input exposed for voltage readings
Update 25 June more readings
Update 4 - 27 June-- fewer amps with fat wire were noted this time!
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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 readings

Trimetric 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 12.8/15.2

Renogy 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 cables

13.53/13.01 (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.

[otrfun]

oops. Wrong post. Please delete.

[BFL13]

S Davis wrote:
So I just retook the readings for my 50 amp Redarc

Batteries at 80% 12.5 +-

13.8 volts truck idling
28 amp draw at under hood breaker

25’ 1/0 DLO both positive and negative to Redarc

13.6 volts into Redarc

14.4 volts out to batteries @ 24 amps

I remember in a past test how the truck's voltage fell off once I reached a certain demand on the truck battery by increasing the load on the inverter clamped to the truck battery (truck idling). It held its voltage near 14v up to a point, then the dash voltage needle started to tremble and then it fell off to the left. The truck's system can only take so much! I expect it is not linear.

I wonder what the numbers would be when output is 50 amps at 14.4v. What about the higher demand pulling the truck's battery voltage down where that may not be linear?

I am thinking the 24 vs 28 amps at half rating might not be just 50 vs 58.3 amps. You would need the house batts down to more like 50% instead of 80% going in to test for that.

The input watts use the voltage at the DC-DC end after voltage drop from the truck's end, but that truck voltage comes down too.

We have been looking at the efficiency of the DC-DC, but there is also the non-linear ? efficiency of the alternator. Efficiency of these electronic things seems to be somewhere on an "efficiency curve". What if the curves for the DC-DC don't match the shape of the curves for the alternator, so you don't get what you expect just from calculating voltage drop etc for the DC-DC unit?

Guys who know these things will be able to confirm or deny this whole notion and perhaps explain more how it actually works.

[BFL13]

It is true that efficiency is watts in vs watts out. But here we are just looking at amps in vs amps out where we want to keep the amps in from being too high for the alternator, at max amps out.

So the measure of success is with the amps, whatever the efficiency happens to be. When doing the upgrade wiring you know you are there when you get the input amps right.

[CA_Traveler]

Measuring efficiency with just amps has no meaning. Take a piece of wire for example: 10A in and 10A out - therefore 10/10 = 100% efficient and no loss? Not a chance, it's power in vs power out and that's watts or both volts and amps.

Efficiency of a converter is the same because what you want to know is the power loss.

[BFL13]

Hard to say what value the "efficiency" has, and whether that or how much of that is internal to the DC-DC.

If the object is to get full constant amps out and keep the draw on the input low enough to not over-task the alternator, then all you need to know is how much is too much for the alternator.

If you can meet that spec with 80% efficiency, why go to 90%? How much gas do you save while driving the truck for a few hours by having 90% vs 80%?

Comparing efficiency using watts as in above posts, with just using amps is sort of interesting. EG, in my fat wire test it was 78% at best, but 20/24 (if 24 is valid!) using amps is 83%.

The 91% above was with 24/28 amps = 86% So if the purpose of the exercise is about the amps then using watts might be less useful as an indicator. Too many factors in motion all at the same time!

It does seem like you can over-do it beyond what you need to meet the objective though. Do the easy stuff first and see if you are already there before doing the hard parts.

[S_Davis]

When I have some time I have a Blue Sea ARC relay that I am going to install in parallel to the Redarc to see what a direct charge from the alternator would be, I am going to make sure there is a alternator available before the test though:-)

[S_Davis]

The readings above where taken with a Fluke 376 clamp meter.

[BFL13]

S Davis wrote:
So I just retook the readings for my 50 amp Redarc

Batteries at 80% 12.5 +-

13.8 volts truck idling
28 amp draw at under hood breaker

25’ 1/0 DLO both positive and negative to Redarc

13.6 volts into Redarc

14.4 volts out to batteries @ 24 amps

13.6 x 28 = 381w, 14.4 x 24 = 346w/381 = 91%

[S_Davis]

So I just retook the readings for my 50 amp Redarc

Batteries at 80% 12.5 +-

13.8 volts truck idling
28 amp draw at under hood breaker

25’ 1/0 DLO both positive and negative to Redarc

13.6 volts into Redarc

14.4 volts out to batteries @ 24 amps

[S_Davis]

noteven wrote:
You guys are connecting the DC-DC supply cables directly to the starting battery on the vehicle with a fused connection at the battery, right?

I was thinking size large booster cables would make a good conductor.

Marine grade trolling motor 3 pole connectors would be convenient for the cables and the signal wire... these connectors are not that cheep...

That is how I did mine, off the aux battery in the truck. A 120 amp fuse and then 120 amp breaker so I can shut off the charge circuit. For the charge wire I used two 25’ runs of 1/0 DLO cable.

[noteven]

You guys are connecting the DC-DC supply cables directly to the starting battery on the vehicle with a fused connection at the battery, right?

I was thinking size large booster cables would make a good conductor.

Marine grade trolling motor 3 pole connectors would be convenient for the cables and the signal wire... these connectors are not that cheep...

[StirCrazy]

otrfun wrote:

StirCrazy wrote:
thats what I wanted to see. do you think there would be any benifit of going larger than 2ga or do you think at that point its not worth the extra money?

Steve

You mentioned in a previous post you wanted to keep your voltage drop down to less than 1%. 1% for our install (23 ft.) would have required 4/0 cable. We're experiencing a ~2.5% voltage drop using 2 gauge cable (~10% overall conversion loss w/40a Renogy). For our install, 2 gauge provided us max. bang for the buck (cost, installation hassle, voltage drop, etc). If our install had required a ~30 ft. cable run, like many 5r/TT installs require, we'd probably bumped up to a 1 gauge cable to keep our conversion loss at <=10%.

That's just our preference. Nothing wrong with going with an extremely low or high (<=50%) conversion loss as long as you understand the implications.

Hope this helps.

yup it does. I will be running an total new wire run so I'll go as fat as is afordable and that I can find a nice plug type conection for.

Steve

[BFL13]

So forced into a new test by otrfun, this time all fat wire pos and neg input and with and without the ammeter inserted:

No ammeter:
Input- 13.62v x ? amps
Output- 12.60v x 20 amps = 252w

With ammeter (grads are 0, 10, 20, 30 etc to 60a with fat needle small space between 10 amps marks) also truck engine bay getting hot so lower engine batt voltage too:

Input-12.82 x 26ish amps = 333w
Output-12.56 x 20 = 251w

251/333 = 75%

So that looks more like what to expect within margins of error. Thanks all for keeping me at it, so the results make more sense.

Just how much installation upgrades are "worth it" is hard to say, once you get max output steady. Some wiring improvements are easy and worth it, while some are harder and maybe not. It also depends on your whole set-up for opportunities for wiring routes etc.

EDIT-- using Salvo's methods from way back, I have tried to get the amps with the first case where there was no ammeter.

First get the R of the connection with the ammeter showing 26 amps and voltage drop of .96 from truck to Renogy, R = 0.96/26 = 0.03692, so with voltage drop no ammeter at 0.88, and same R, amps would be

0.88/0.03692 = 23.8 amps which seems about right, and

13.62 x 23.8 = 324w, and 252/324 = 78%, which also works.

All numbers are approx but IMO within the ball park for showing how the Renogy operates.

[otrfun]

BFL13 wrote:
. . .
otrfun got 13.8 x 42.2 = 582w in and 40 x ? 13.4 ? = 536 out = 92% . . .

Charge current remained steady at 40a for nearly the entire charge cycle; however, the charge voltage steadily increased from ~13.7v to ~14.4v as the LifePo4 charged from 15% SOC to ~97% SOC. I didn't take current readings at the Renogy input when charge voltage approached 14.4v. Odds are the Renogy's input current starting going up slightly once the Renogy's output charge voltage exceeded the alternator's 14.1v output voltage.