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.

[theoldwizard1]

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.

[theoldwizard1]

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.

[pianotuna]

Hi BFL13,

As the #8 is already in place both for input and output, moving up in size before I try it seems a bit silly. I'll certainly let you know (after Cdn Thanksgiving).

[BFL13]

"Rated Max power is 250 watts."

Yes but what is that rating based on? 20 amps output means 250/20 = 12.5 volts.

You can choose a charging profile that is 14.7v at 20 amps = 294w.

That is what it is supposed to do, never mind its "rating" of 250w. I have no idea why they have that in their specs. It is like those converters that are "rated" for when they are at 13.6v and not when they are at 14.4v. Some kind of "Sales" stuff maybe, who knows?

Meanwhile you can install it and see if it holds 20 amps steady when set at 14.7v. If it does, you are good. If not, you will have to use fatter input wiring or use the frame until it holds its 20a.

Also when they specify the output wiring they seem to forget that the batteries also have some resistance, not just the wiring. So you might have too much voltage drop between Renogy output and battery post voltage when you measure each in real life. IMO use your #8 for the output and #4 for the input and hope for the best.

You won't really know until you install it and try it out. Measure everything and be ready to adjust the installation as required.

[pianotuna]

The owners manual suggests #8 wire is good for up to 20 feet for the 20 amp dc to DC unit. Fuse at 30 amps. ANL type

Output wire is #12 for up to 10 feet. Fuse at 25 amps. ANL type.

Input voltage is between 8 and 16 volts.

Rated Max power is 250 watts.

Efficiency is 90% (excellent).

[Gdetrailer]

Lwiddis wrote:
That’s terrible efficiency! Thank you for posting.

Re read my post.

It looks bad, but in reality it isn't as bad as you may think.

If you use watts as part of the conversion you will notice that watts will be pretty much the same other than the switching power supply efficiency which can be as low as 85% and as high as 92%.

for instance 12.0V at 30A is 360W

If you convert to 14.0V at 360W you get 25.7A at the output not including the switching power supply loss of say 15% which would be about 3.9A, the result is 21.8A available at 14V...

Converting the voltage up drops the amps of the output but the watts stay the same. It is the principle behind why utility companies use massive transformers (which waste a lot of energy) to boost the power plant voltages up for transmission then convert the voltage down to your household voltages.. Without that conversion they would need to string even bigger more massive wires on the poles..

Does that help clear it up a bit?

[theoldwizard1]

CA Traveler wrote:
Not very efficient. 😞

Wonder if it varies by voltage or amps.

I am betting it does !

Comparing just amps is not fair. You have to look at power which is watts which is amps TIMES voltage.

[BFL13]

Lwiddis wrote:
That’s terrible efficiency! Thank you for posting.

85% is typical for converters and chargers so not "terrible" at all.

Plus the "efficiency" of a solar controller varies with how much buck or boost it is doing, so I am guessing it is the same with the DC-DC units of whatever brand. Although solar controllers are in the 90s % not the 80s like converters.

[Lwiddis]

That’s terrible efficiency! Thank you for posting.

[BFL13]

"The whole idea of the dc to DC is to boost the voltage going into the battery bank. Therefore voltage drop to the charger doesn't matter--so long as the ampacity of the wire size is not exceeded."

Yes it does matter.

The unit is "buck/boost" and the output voltage will be higher than the input's in many situations so it has to boost. The voltage drop to the charger is vital to keeping the input watts high enough to keep the watts up. It also affects the amps draw on the alternator, which you want to keep as low as possible.

I had a problem where my output amps fell off from 20 and it turned out the input voltage was too low. The unit has a boost spec from about 8 volts but not quite so in real life. Anyway, you improve your input voltage "somehow" until the unit can sustain its full output amps at the chosen output voltage. You cannot control the input amps directly.

I am using the thin and long positive wire in the 7-pin as input so to get the whole circuit better, I had to swap over to using the frame instead of the neg 7-pin long thin wire. If that had not worked, I would have had to run a fatter positive wire too.

[pianotuna]

BFL13,

#8 has a voltage drop of 2.46% at 10 feet (more than I need). It can carry 55 amps.

The whole idea of the dc to DC is to boost the voltage going into the battery bank. Therefore voltage drop to the charger doesn't matter--so long as the ampacity of the wire size is not exceeded.

On the Output side I might have about a 4 foot run. Voltage drop about 0.76% with #8 wire at 20 amps. Pretty much I think that can be ignored.

I must look up the charging voltage on the telcom jars that have served me so very well.

Thanks for the thought provoking comments.

[wa8yxm]

Watts out = watts in plus overhead and conversion loss.

A DC=DC converter usually is an inverted/charger paired. Power in is chopped into Square wave A/C and then processed. This is about 90% efficient going in and another 10% is lost going out that's best case. Few are that good.

Now.. some interesting things.. Because it's not converting to 60Hz AC but usually a much higher frequency. the processing is easier and if it uses a transformer that's much lighter.. But basically you have an inverter and a converter in one box.

[BFL13]

#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. You can tell by if it can't hold that 20 amps output, that you need more input (ie, less voltage drop)

I can't remember the unit's efficiency, but for instance:

14.7 x 20 = 294w and 85% efficiency (eg) needs 346w input.

If input voltage is 13v after v drop from engine batt, 346/13 = 26.6 amps. If you could keep input voltage at 14, then 346/14 = 25 amps

You are supposed to get input from the engine battery, not from the alternator. You need a common ground like it shows in the manual. With a Class C that would be the frame

The unit turns on and off using the little wire that connects to pos input. I have a switch on that little wire in my set-up, and just ran that wire over to the pos input terminal.

You must not also have the OEM alternator charging to house connected like in a MH, or you will add to the amps demand on the alternator, which the DC-DC is supposed to limit.

Pick the nearest charging profile on the Renogy to what your batt spec is (14.7 on mine) and then set your solar controller to match that voltage so they add their amps best while driving down the road.

[CA_Traveler]

Gdetrailer - Thanks for the reminder of watts.

[pianotuna]

Hi All,

I did not do the bench test--my cousin did, so I don't have answers to your questions. I thought it might be useful to others.

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.

I'll have the ability to switch the dc to DC off.

I'll still have a "poor man's boost ability, too.