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]

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.

[BFL13]

I will try a new test with better input.

Watching this somewhat confusing video (WP again 🙂 ) He is using skinny wires for his testing, and first gets 201w in and 174w out for 86% efficiency.

He then tries a higher load on the output and gets 658w in and 519w out, which is 79% He said the output then was 40 amps at 12.7v battery so that is 508w. He did not say what the input watts components were for that 658w. At 14v truck battery that would be 658/14 = 47 amps and at 13v truck that would be 51 amps.

With the fat wire test I did, it was
Input watts 12.2 x 30 = 366w Output watts 14.38 x 20 = 288w
288/366 = 78.7% so his 79% compares.

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

https://www.youtube.com/watch?v=EcVjPap9dkY

[StirCrazy]

otrfun wrote:
Took these current/voltage readings right after we installed a 40a Renogy dc to dc charger in our truck camper (truck has a 220a alternator). Used ~23 ft. of 2 gauge cable to fabricate the positive run from the TV battery to the Renogy located in our truck camper (via several 175a Anderson-type connectors). Used approx. 5 ft of 2 gauge for the negative run (truck frame just below the truck bed to the Renogy in the truck camper). Remaining negative run (from the truck's battery to the front of the truck bed) was via the truck's frame and OEM negative cabling. All readings are approx.

1. 14.1v 43.5a measured where the 2 gauge cable terminated at the TV battery terminal (alternator online)

2. 13.8v 42.2a at the Renogy's input terminal

3. 40a at Renogy output terminal (current remained steady at approx. 40a while charging LifePo4 battery from 15% SOC to ~97% SOC)

4. 39.8a at LifePo4 battery terminal (~10 ft run of 2/0 cable from the Renogy to the LifePo4 battery; voltage drop <.1v)

You can drop these current readings by 50% if you want to get a rough idea of how a 20a Renogy would have performed (installed on our TC and truck). This would be based on using one cable size larger than what Renogy recommends for the 20a Renogy for our install. Renogy recommended 4 gauge for our 40a Renogy install; we used 2 gauge.

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

[StirCrazy]

BFL13 wrote:


You can install yours and see what you get. The owner's manual says input amps will be 50% more than output amps at max amps, so expect 60 when yours is doing 40. It does say the unit will "perform better" with less voltage drop on the input side. It does not say that better performance will mean fewer amps.

remember it actualy says Up to 50% more. so that implys the more you reduce the loss the less it will be, but seing the min recomended size and that you were trying to tun it through the charging circut I dont think you could have dropped it down, nothing to defend at all, you were just trying to make it work with what you had.

BFL13 wrote:
There is also heat. The unit's fan comes on. Some watts are involved there.

yup, that would be acounted for in the efficency rating.

BFL13 wrote:
A lesson learned was to by-pass the 7-pin connectors for both pos and neg. Those connectors work for the signal lights but sure have high R for the 12v charging wire pins. If we still had the 5er, I would look at that even with no Renogy involved, where 7-pin charging is so lame.

I dont use that charging for anything. I disconcect it in the rv side of the plug, just because I have had batteries overcharged on long trips and such. I will have the same issue trying to run some big wires under the truck and I am not looking forward to running them, but at least I only have to do it once haha

[otrfun]

pianotuna wrote:
. . .
Now if someone would connect bypassing the Renogy....and compare that to Ren in, Ren out.

Bypassing the dc to dc charger when you need a really quick, fast charge would be nice option to have.

We used 2 gauge for our install. I'd be concerned about current ramping up too high, overloading our 220a alternator. Our total run from TV battery to the 200ah LifePo4 battery in our truck camper would provide approx. 4.5% voltage drop at 60a, ~6% at 80a, ~7% at 100a. Not sure if that's enough (drop/resistance) to keep the current at reasonable levels or not.

[BFL13]

Thanks for the readings. My tests indicate that my wiring and connections are not nearly that good on the input side. My fat wire test did not go far enough to replace the whole input circuit apparently.

Good news for those who choose to go fatter wire on the input!

I have trouble getting wires down from the truck battery and back without getting too close to the hot exhaust. Have to do some more with that. It is easier to improve the neg side via the frame behind the cab to the camper, which definitely helps.

[otrfun]

Took these current/voltage readings right after we installed a 40a Renogy dc to dc charger in our truck camper (truck has a 220a alternator). Used ~23 ft. of 2 gauge cable to fabricate the positive run from the TV battery to the Renogy located in our truck camper (via several 175a Anderson-type connectors). Used approx. 5 ft of 2 gauge for the negative run (truck frame just below the truck bed to the Renogy in the truck camper). Remaining negative run (from the truck's battery to the front of the truck bed) was via the truck's frame and OEM negative cabling. All readings are approx.

1. 14.1v 43.5a measured where the 2 gauge cable terminated at the TV battery terminal (alternator online)

2. 13.8v 42.2a at the Renogy's input terminal

3. 40a at Renogy output terminal (current remained steady at approx. 40a while charging LifePo4 battery from 15% SOC to ~97% SOC)

4. 39.8a at LifePo4 battery terminal (~10 ft run of 2/0 cable from the Renogy to the LifePo4 battery; voltage drop <.1v)

You can drop these current readings by 50% if you want to get a rough idea of how a 20a Renogy would have performed (installed on our TC and truck). This would be based on using one cable size larger than what Renogy recommends for the 20a Renogy for our install. Renogy recommended 4 gauge for our 40a Renogy install; we used 2 gauge.

[BFL13]

Notes from back in Feb on 7-pin vs DC-DC:

7-pin starting at 13.2 amps and after a minute was 10 amps and after four minutes was 4.4 amps.

Renogy does its 20 amps in Bulk tapers only in Absorption.

I got the thing as an emergency battery charger for the TC which has no generator, that DW can operate if I am not there, if the camper batts get too low and no sunshine. (Turn on truck and let it idle, turn on Renogy switch in camper)

Also I did not want to mix the house and engine batteries in parallel with their different chemistries (but Mex et al say that is not a real problem)

Probably never get used, but it beats getting somebody else at a dog event to help out with jumper cables like you see some people needing at times. (Some folks are good at dog stuff, but shaky on how their RV's work)

Renogy will add its amps with solar amps with the same Vabs settings too. You can't set a 7-pin Vabs so you lose some " alternator charging" with the solar on with its higher voltage.

I don't need it for the Class C in our scenarios, but if I did, I think it could be swapped back and forth using quick connects. For the price, probably just get another one for the C.

[pianotuna]

The best news is that it works to boost the output charging amps. Thanks for your efforts BFL13.

I think BFL13 chose the perfect size to protect an alternator.

Now if someone would connect bypassing the Renology....and compare that to Ren in, Ren out.

[BFL13]

With 11.x input voltage compared with truck battery voltage of near 14, that shows a considerable amperage flow on that circuit. When I had the ammeter in there it said it was 30 amps, but of course I can't say what it is without the ammeter in there Still a big voltage drop, so got to be lots of amps, ammeter or not.

I don't have to defend anything since I did make clear the input watts situation did not come out as expected, and I don't know why.

You can install yours and see what you get. The owner's manual says input amps will be 50% more than output amps at max amps, so expect 60 when yours is doing 40. It does say the unit will "perform better" with less voltage drop on the input side. It does not say that better performance will mean fewer amps.

There is also heat. The unit's fan comes on. Some watts are involved there. FWC mentioned a possible current limit for input, but AFAIK that should be applied to the truck battery end not the receiving end at the Renogy --not too clear on that.

A lesson learned was to by-pass the 7-pin connectors for both pos and neg. Those connectors work for the signal lights but sure have high R for the 12v charging wire pins. If we still had the 5er, I would look at that even with no Renogy involved, where 7-pin charging is so lame.

[StirCrazy]

BFL13 wrote:

Input voltage was 10.42, now it is 11.28v. No change in output no way to know the draw (assumed still 30 amps)

this is the main issue here, your making assumptions. untill you have a way to measure input and putput voltages and currents at the same time you will never be able to get a factual and defendable result.

it only stands to reason if your output current and voltage stays the same but you raise you input current by almost one volt that your input current is going to reduce.

power in = power out + losses is a standard and your losses should be remaining the same or what we can consider the same if your output isnt changing.

the problem is to do it properly you would need shunts that have external power and two voltage meters not triametrics that use powr themselves and are "close"

I also expect like other that the ground set up is playing a roll, the manual specifies a 6ga wire for the positive and ground wires min, for your run it probably should follow the 4ga recomendation from your truck battery to the charger depending where the batteries are.

Steve

[BFL13]

CA Traveler wrote:
It is for sure that larer wire reduces voltage loss and reduces power for a given amps. The resulting power for the output (with device losses) is available at the output with different amps and voltage.

Typo? Reducing voltage drop means higher input volts and so higher watts with the same amps. Output watts is battery voltage at the time (rising while being charged) x amps

Output "battery voltage" will be lower with loads so not clear to me what voltage to use for output watts when not just a battery being charged.

Watts are a PITA! 🙂

[CA_Traveler]

It is for sure that larer wire reduces voltage loss and reduces power for a given amps. The resulting power for the output (with device losses) is available at the output with different amps and voltage.

[BFL13]

I got the input voltage to go higher but AFAIK the amps are still 30 amps so input watts went up with the higher voltage if amps the same. Output is the same.

You would think that can't be right. I don't have a clamp ammeter to measure the input amps. The clamp type does not add any R to the circuit like the one I used in the test seems to.

So it is not for sure using fatter input wire to reduce voltage drop does not reduce the input amps from my results. Manual says 20 amps out means 50% higher draw (30 amps) Could be more going on than meets the eye! Whatever, mine works. 🙂

EDIT--I did have 12.82v input at the Renogy terminals with fat wire before, and the ammeter still said 30 amps, so now with 11.28v input, I don't imagine amps could be less than 30.

I am putting it all away again so no more access to the input terminals. I am happy with the way it is now (11.x input volts and no tapering of the 20 output amps)

[CA_Traveler]

BFL, Thanks for some recent observations and input. For me a input of 30A and 20A output with the updated voltages seems reasonable. Input 10.4V and output 20A 14.0V reguires 26A input. Add some wiring/Renogy losses and it seems reasonable.

Apparently some of the initial wiring configurations weren't obvious as to their impact but regardless thanks for the clarification.