Renogy 20A dc to DC report

[pianotuna]

Hi All,

Regarding the Renogy 20A DC to DC Battery Charger 12V, the unit puts out 20 amps, but draws 30 amps when doing so. This is the result of bench testing.

[otrfun]

theoldwizard1 wrote:

otrfun wrote:
The 3-4a drop in output current at idle vs. driving would concern me. This potentially indicates the alternator is unable to sustain the proper current/voltage at idle. In other words, the dc to dc charger may be overloading the alternator at idle. Every dc to dc charger I've installed has exhibited negligible to no drop in input current to the dc to dc charger when the engine is idling at, say, 750rpm vs. driving at 2000 rpm.

It is not an amperage issues.

As long as the alternator can sustain the MINIMUM input voltage to the DC-DC charger, it is a non-issue. Most of these chargers have a way to determine if the engine is running or not to prevent discharging the vehicle starting battery OR they need to be wired to a KEY-ON circuit.

An adjustable minimum input voltage might be useful.

The Renogy ignition trigger lead and/or commonly used battery isolator accomplishes would you're describing. If the Renogy ignition trigger lead detects a "nominal" 12vdc, the charger turns on. If the 12vdc goes away, it turns the charger off. A battery isolator operates in a similar fashion, but it's much more voltage selective. It closes its relay (providing power to the Renogy dc to dc charger) when voltage reaches approx. 13.3v (alternator active) and then opens it (releases) when voltage drops to approx. 12.8v (alternator offline). These are simple on/off devices.

What pianotuna is describing is a 3-4a drop in output charge current at idle (vs. driving). Neither the Renogy 12vdc ignition trigger lead, nor a battery isolator would cause this. They simply turn the Renogy dc to dc charger on or off. Nothing more, nothing less. They do not interact with the Renogy's internal voltage/current regulation circuitry in any way.

[theoldwizard1]

otrfun wrote:
The 3-4a drop in output current at idle vs. driving would concern me. This potentially indicates the alternator is unable to sustain the proper current/voltage at idle. In other words, the dc to dc charger may be overloading the alternator at idle. Every dc to dc charger I've installed has exhibited negligible to no drop in input current to the dc to dc charger when the engine is idling at, say, 750rpm vs. driving at 2000 rpm.

It is not an amperage issues.

As long as the alternator can sustain the MINIMUM input voltage to the DC-DC charger, it is a non-issue. Most of these chargers have a way to determine if the engine is running or not to prevent discharging the vehicle starting battery OR they need to be wired to a KEY-ON circuit.

An adjustable minimum input voltage might be useful.

[otrfun]

pianotuna wrote:
otrfun,

Out put voltage is set for 14.7 and input is about 13.2.

Which dc to DC unit are you using?

40a Renogy.

[otrfun]

pianotuna wrote:
. . . the Renogy 20A DC to DC Battery Charger 12V, the unit puts out 20 amps, but draws 30 amps when doing so. This is the result of bench testing.

pianotuna wrote:
. . . I don't have an easy way to measure input amps to the unit . . .

I'm confused. Explanation?

Current (along with voltage) measurements at the alternator/battery (power source for the dc to dc charger) and the input terminals of the dc to dc charger are, for us, 2 of the 4 critical data points necessary to determine the overall quality and efficiency of any given dc to dc charger installation. The other 2 are the output terminals and battery terminals on the charge side.

pianotuna wrote:
. . . I have seen 16 amps going to the house bank at idle. The highest reading noted was 19.6 amps, when driving. I don't have an easy way to measure input amps to the unit . . .

The 3-4a drop in output current at idle vs. driving would concern me. This potentially indicates the alternator is unable to sustain the proper current/voltage at idle. In other words, the dc to dc charger may be overloading the alternator at idle. Every dc to dc charger I've installed has exhibited negligible to no drop/rise in output charge current when the engine is idling at, say, 750rpm vs. 2000 rpm.

[pianotuna]

otrfun,

Out put voltage is set for 14.7 and input is about 13.2.

Which dc to DC unit are you using?

[otrfun]

pianotuna wrote:
otrfun,

The bench testing used #4 wire input and the length was inches.

30 amps in and 20 out is not a measurement of efficiency--because we are dealing with mppt boosting the voltage, i.e. swapping amps for volts.

I've set the unit to 14.7 volts output to maximize charging.

In the RV the wire length is about 12 feet due to the dc to DC having to be located under the passenger's seat. It is #8 wire which is more than adequate for 30 amps input (about 6 feet on input side).

I have seen 16 amps going to the house bank at idle. The highest reading noted was 19.6 amps, when driving. I don't have an easy way to measure input amps to the unit.

I'm more than satisfied with these results and I'd recommend dc to DC charging.

On a disappointing note, Victron, who make excellent equipment, in general, chose to not have a cooling fan on their dc to DC charger. Some reports say it gets too hot to touch after operating for an hour.

The renogy does have a cooling fan.

Yes, input and output voltage delta of a typical dc to dc charger is a factor. However, the delta is not enough to substantially change the final calculation based on current alone. For general discussion it's more than accurate.

The typical dc to dc charger install will have less than a 10% voltage delta. If it's a quality install, it will be significantly lower.

In our case, our 40a dc to dc charger had an input voltage from the alternator of 14.2v with the output charge voltage set to 14.4v (voltage increased from 13.6v to 14.4v during the charge process). Max input current was 44a, max output current 40a. ~90% efficiency even with the voltage delta taken into consideration.

[pianotuna]

otrfun,

The bench testing used #4 wire input and the length was inches.

30 amps in and 20 out is not a measurement of efficiency--because we are dealing with mppt boosting the voltage, i.e. swapping amps for volts.

I've set the unit to 14.7 volts output to maximize charging.

In the RV the wire length is about 12 feet due to the dc to DC having to be located under the passenger's seat. It is #8 wire which is more than adequate for 30 amps input (about 6 feet on input side).

I have seen 16 amps going to the house bank at idle. The highest reading noted was 19.6 amps, when driving. I don't have an easy way to measure input amps to the unit.

I'm more than satisfied with these results and I'd recommend dc to DC charging.

On a disappointing note, Victron, who make excellent equipment, in general, chose to not have a cooling fan on their dc to DC charger. Some reports say it gets too hot to touch after operating for an hour.

The renogy does have a cooling fan.

[otrfun]

pianotuna wrote:
Hi All,

Regarding the Renogy 20A DC to DC Battery Charger 12V, the unit puts out 20 amps, but draws 30 amps when doing so. This is the result of bench testing.

If you use large enough gauge cable, and your alternator voltage is high enough (relative to charge voltage), the Renogy dc to dc chargers can be almost 90% efficient. A 90% efficient 20a Renogy dc to dc charger would draw 22a from the alternator while producing 20a of charge current. A 40a Renogy dc to dc charger, 44a. A 60a Renogy, 66a.

Renogy's cable recommendations generally result in 70-80% efficiency. If you step up one gauge larger (physical size), one should get very close to 90%.

We have an approx. 20 ft run of 2 gauge cable from our alternator to our 40a Renogy dc to dc charger in our truck camper. It draws 44a from the alternator while producing 40a of charge current.

[StirCrazy]

pianotuna wrote:
BFL13,

I'm delighted with the results from the dc to DC charger. So far the highest "return" has been 19.6 out of a possible 20 amps.

did you ever check to see what the extra draw is on the altanator? I am still trying to decide which size to order for my dc to dc.

Steve

[pianotuna]

BFL13,

I'm delighted with the results from the dc to DC charger. So far the highest "return" has been 19.6 out of a possible 20 amps.

[BFL13]

PT the alternator should be supplying enough to run the V-10 etc, plus do some charging to the house. So if the front is drawing 8 amps from the house to keep up, what is going on?

And if you isolate the house from the front, how will it find that 8 amps it needs?

I don't think I have the story straight since you know all that.

[StirCrazy]

pianotuna wrote:
Hi All,

Regarding the Renogy 20A DC to DC Battery Charger 12V, the unit puts out 20 amps, but draws 30 amps when doing so. This is the result of bench testing.

Didnt BF test his out with different wire sizes and found that excess drops with bigger wires feeding it?

[pianotuna]

Hi BFL13,

My cousin did the bench test. I was not there. He said it provided 20 amps out and drew about 30 amps to do so. He did not mention the voltage, and I did not think to ask him. But if it was pushing out 20 amps I think it probably was set for the 14.7 output voltage.

I'll be traveling to him in late September or early October for the installation.

My reason for adding the dc to DC unit is that currently my "house" bank discharges into the V-10 at about 8 amps when trundling down the road at highway speeds. Some of that loss is made up by the solar--but not all. This started when I moved to the AGM telecom jars.

Since 2009, I've driven the class C about 10,000 miles a year. With higher fuel prices that may drop by 50%.

[BFL13]

theoldwizard1 wrote:

BFL13 wrote:
#8 might not be enough. I had to change my input neg path to use the frame (as a really fat wire) to get my voltage drop small enough to keep input voltage in the 13s.

The whole point of a DC-DC charger is that it will accept low input voltage and still provide the correct output voltage.

Yes, it has "boost" (and "buck" if needed) but you cannot control the input, only the output setting voltage. The 20 amps is built in. Once you set the output voltage, you now have the output watts it needs to "supply"

So now the unit will "demand" the watts it needs for input. You have no control over how many amps that will be. All you can do is tinker with input voltage to try and keep the amps demand from being too high for the alternator. Also you don't control the vehicle's voltage, just the size of the input wiring.

The alternator should not have more than about 1/3 of its rating demanded ("pulled") from it, so you have to know your alternator's rating before you pick your DC-DC "size" in output amps. We have had several threads about that on here.

Some on here have reported how they use really fat wire on their inputs and the truck frame as needed, and have their DC-DC pulling only a few amps more than their rated output amps "size". The Renogy unit says it can pull as much as 30 amps to output 20 amps, but you can use fatter wire to get higher input voltage and thus have fewer amps being pulled to get the same input watts.

Going back to the example output setting of 14.7v and 20 amps for 294w and spec 90% efficiency, it will "demand" 327w input.

If the vehicle is running at 14v after warming up, and you have 13 amps showing at the DC-DC input (1 volt drop) it will pull 25 amps. ( if you did get 14 at the unit, amps would be 23.4.

My truck alternator is rated at 105 amps and 1/3 of that is 35 amps, so I am good with that pull of 25 amps. That leaves a few amps to run the truck too (as was mentioned by one of the guys on here recently)

People with vehicles that have alternators rated at over 200 amps can use the Renogy 40 amper which might pull 60 amps and they will be ok as long as they have fat/short enough wiring for input.

On the output wiring, you can set the unit for your battery spec of say 14.7v for charging. But how many volts reach the battery of that 14.7? You want it all, but if the battery can still get charged with 14.5v (the specs usually give a range of ok voltages eg 14.5-14.8) So you have to use fat/short enough output wire to get 20 amps at 14.5v at the battery posts, allowing a 0.2v drop.

PT--I thought you did a bench test and it was not installed yet. Confusing. Anyway, same thing--try it out and if it does not hold 20 amps at the set output voltage, you must improve the input so it will.

[Gdetrailer]

theoldwizard1 wrote:

pianotuna wrote:
Hi All,
I'll be "feeding" the unit with #8 wire.

I have a choice of powering from the alternator, or powering from the starter battery (which is of course charged by the alternator), but only with the ignition key turned on. Doing the battery route would eliminate surge on the alternator, so I'm leaning in that direction.

You are OVERTHINKING !

#10 wire is more than enough (I always recommend pre-tinned marine wire - no corrosion).

You want an input that turns off when the key is off. Don't worry about surges. The charger will prevent that.

10 ga may be "heavy enough" for the amperage, but depending on the length of the run may not be "heavy enough" due to the resistance of the wire.

When working with "12V" systems even a .1V can make a huge difference between working well and not working at all.

This is one place where you just need to throw away the amperage capacity charts and start looking at the resistance per ft charts.. And while at it toss the "3% voltage drop rule", that only applies to 120V and higher, with "12V systems", 3% voltage drop is way too much loss..

Math.. 12.0V x .03 (that is 3%) = .36V loss...

To get around that loss, one MUST do one of two items, shorten the run and/or use a much heavier gauge wire..

Shortening the run while cost effective may not be in the cards due to no good location exists to allow the items to be moved closer to the power source.

8 Ga in the case the OP is using is a good starting point, but it may still prove not heavy enough to minimize voltage drop to their batteries..