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Lwiddis

Cambria, California area

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Posted: 06/28/22 06:39pm Link  |  Quote  |  Print  |  Notify Moderator

That’s terrible efficiency! Thank you for posting.


Winnebago 2101DS TT & 2020 Chevy Silverado 1500 LTZ Z71, WindyNation 300 watt solar-Lossigy 200 AH Lithium battery. Prefer boondocking, USFS, COE, BLM, NPS, TVA, state camps. Bicyclist. 14 yr. Army -11B40 then 11A - (MOS 1542 & 1560) IOBC & IOAC grad


BFL13

Victoria, BC

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Posted: 06/28/22 07:48pm Link  |  Quote  |  Print  |  Notify Moderator

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.


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theoldwizard1

SE MI

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Posted: 06/28/22 07:54pm Link  |  Quote  |  Print  |  Notify Moderator

CA Traveler wrote:

Not very efficient. [emoticon]

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.

Gdetrailer

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Posted: 06/28/22 08:46pm Link  |  Quote  |  Print  |  Notify Moderator

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?

pianotuna

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Posted: 06/28/22 09:31pm Link  |  Quote  |  Print  |  Notify Moderator

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).


Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.

BFL13

Victoria, BC

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Posted: 06/28/22 10:17pm Link  |  Quote  |  Print  |  Notify Moderator

"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

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Posted: 06/28/22 10:56pm Link  |  Quote  |  Print  |  Notify Moderator

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).

theoldwizard1

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Posted: 06/29/22 05:59am Link  |  Quote  |  Print  |  Notify Moderator

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.

theoldwizard1

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Posted: 06/29/22 06:01am Link  |  Quote  |  Print  |  Notify Moderator

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.

Gdetrailer

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Posted: 06/29/22 06:54am Link  |  Quote  |  Print  |  Notify Moderator

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..

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