AC vs. DC for solar powered fridge

[dwh]

Just do it like that dongle in the link. Have the female on the solar. Then it wouldn't be possible to plug the solar or extension cord into a power outlet.

Doing it that way you could also use a regular shore power plug at the vehicle end:

But hauling around a 3 conductor cable when you only need two, and 3 #10s to boot... 100' twist lock extension cords are heavy

Well, have fun with that. Better you than me.

 

[Bbasso]

Why not use this?
https://www.amazon.com/gp/aw/d/B00K..._SY340_QL65&keywords=mc4+cable+100+feet&psc=1

 

[dwh]

Why not use this?
https://www.amazon.com/gp/aw/d/B00K..._SY340_QL65&keywords=mc4+cable+100+feet&psc=1

It's a single cable, so he'd need two. From the image it looks like a low strand count, probably not very flexible and certainly not intended to be rolled/unrolled a bunch of times.

 

[Rando]

There is a lot of misinformation in this thread. Yes there is voltage drop on the wires between a solar panel and the battery/charge controller, no it is not a big issue, yes it does slightly impact the amount of power harvested by the panel, no you shouldn't use an inverter.

As others have pointed out, ohms law is not waived for wires attached to solar panels - the voltage drop across the wires is still equal to the current * the resistance of the wire. However this does not really matter so much as the solar panel is essentially a current source until you get close to Voc, and the voltage is controlled by the battery. The battery will be at the voltage the battery is at (determined by SOC and to a lesser degree by charge current), and the voltage drop in the wires will lead to a higher voltage at the solar panel itself. For example, if your battery is at 13V, and the combined voltage drop across both your wires is 1V, your solar panel voltage will be 14V.

This voltage drop causes the operating point on the solar panel I-V curve to move to the right (see curve below) resulting in a lower current out of the solar panel and less power to your batteries. Unless you are operating right at the maximum power point of your panel which is right at the 'knee' of the I-V curve (which you aren't if you are using a PWM controller), the curve is pretty flat, so you are not really loosing much power - maybe a few %. This is likely less than the losses in the inverter and power supply in your fridge, so you still come out ahead with the longer DC wires.

 

[dwh]

What part of, "it is a circle" is so hard to understand?

The voltage does not drop between one non-existent end point and some other non-existent end point.

When there is voltage drop it happens all the way around the circle. Which does happen when extracting energy from the circle, and it drags down the voltage of the circle.

Injecting energy into the circle at a higher potential is a different situation, it drives the voltage up - all the way around the circle.

You cannot both drag the voltage down and drive it up at the same time. Either the extraction will win, in which case the voltage will drop, or the injection will win, in which case the voltage will rise.

While injecting energy into the circle, the resistance of the wire will not drag down the voltage, it will limit the rate at which the voltage rises, exactly the same as the battery does. It adds to the limiting effect of the battery's resistance.

 

[dwh]

And as to Ohm's Law...

"The I–V curves of four devices: Two resistors, a diode, and a battery. The two resistors follow Ohm's law: The plot is a straight line through the origin. The other two devices do not follow Ohm's law."

https://en.m.wikipedia.org/wiki/Ohm's_law


The diode curve shown above is for a p-n junction. A solar cell is a p-n junction.

There is a circle (circuit). Two parts of the circle obey Ohm's Law (the wires), and two parts don't (the solar cells and the battery).

While charging, you've got one device (solar, shore power charger or alternator) injecting energy and acting to drive the voltage up. Another device acting to limit the rate of rise (the battery, which does that, by limiting the flow of current).*

In that situation, as I said, the resistance of the wires adds to the resistance of the battery, further limiting the rate at which the voltage rises by further limiting the flow of current.

But the resistance of the wire is not dropping the voltage of the circle.



*(The battery limits the rate at which the voltage of the circle rises, until the voltage of the circle reaches the point at which the charge controller takes over limiting the rate of voltage rise, which it does just like the battery, by limiting the flow of current.)

 

[Rando]

OK then... I was under the impression the question was about voltage drop across long wires between a solar panel and a battery/fridge. Based on your circle there can never be such a thing as "voltage drop" because in all circuits the sum of the voltages across all the elements in that circuit is always equal to zero (a truism known as Kirchhoff's second law). However I assure you, if you measure the voltage drop across those long wires, it will be proportional to the current times the resistance of those wires.

 

[e60ral]

What part of, "it is a circle" is so hard to understand?

The voltage does not drop between one non-existent end point and some other non-existent end point.

When there is voltage drop it happens all the way around the circle. Which does happen when extracting energy from the circle, and it drags down the voltage of the circle.

Injecting energy into the circle at a higher potential is a different situation, it drives the voltage up - all the way around the circle.

You cannot both drag the voltage down and drive it up at the same time. Either the extraction will win, in which case the voltage will drop, or the injection will win, in which case the voltage will rise.

While injecting energy into the circle, the resistance of the wire will not drag down the voltage, it will limit the rate at which the voltage rises, exactly the same as the battery does. It adds to the limiting effect of the battery's resistance.

So if I put my panel here in Raleigh 8 can run 16g wire to my controller in Bangladesh because it's a circle?

No, you are entirely missing the point

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[dwh]

However I assure you, if you measure the voltage drop across those long wires, it will be proportional to the current times the resistance of those wires.

Theoretically yes, of course it would - if you could measure that - but you can't do it without creating a bypass that isolates part the circuit and taking a measurement of an isolated part of the circuit.

In which case you are only measuring what is happening on the newly created bypass circuit, not what is happening over on the complete circuit without the bypass.


Which is why I say people often fool themselves with their meters. They take readings at various places, each time creating a bypass circuit with their meters and reading what is happening on the bypass circuit.

They see a higher voltage in one place, and a lower voltage somewhere else, which reinforces the false notion that there are end points in the middle of a circle, and that voltage drops between these end points.

Which also reinforces the dogma that wire resistance always drops the voltage regardless of the situation.



I blame the internet solar gurus. They sink their teeth into this idea of voltage drop due to wire resistance, and then apply that idea across the board to any situation that involves a wire. Even when the wire resistance results in a different net effect.

And then that gets parroted endlessly until everyone and his mother thinks it's gospel.

 

[dwh]

So if I put my panel here in Raleigh 8 can run 16g wire to my controller in Bangladesh because it's a circle?

You could.

It wouldn't actually do anything because even at 0.0000000001 amps, the total resistance of the circle would be way more than the potential of the power source can overcome. Electrons just wouldn't flow.

There wouldn't be any voltage drop of course, because there wouldn't be any voltage.

 

[e60ral]

But you realize that voltage drops across resistors, and that words are resistors? Because they just asked a simple question and you keep obfuscating their question by saying things that are technically true but wrong in context

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[dwh]

But you realize that voltage drops across resistors,

Voltage drops around the entire loop containing the resistor, not just from one end of the resistor to the other end of the resistor.

and that words are resistors?

No means no?

Because they just asked a simple question

They who? What question was I asked?

As far as I know, I wasn't answering questions, I was refuting a common myth.

and you keep obfuscating their question by saying things that are technically true but wrong in context

(Irony. Gotta love it.)

Actually, that's backwards. The wire resistance dropping the voltage of the circuit is technically true, and correct in the context of drawing power out of the battery, but wrong in the context of injecting power into the battery.



And, I've been attempting to do the opposite of obfuscating. I've been explaining in layman's terms, so that anyone who reads this today, or in the future, can (hopefully) understand it.

I'm not the one who busted out Ohm's Law and started dragging down the voltage of the explanation by adding the increased resistance of replacing layman's terms with mathematics.

(You do realize that math is a resistor, right?)

 

[e60ral]

Except that if you are trying to get to a controller you don't want that drop, the panel isn't wired to the battery, the controller is. If you have a long wire you have less voltage at your controller that is charging your battery. The question was about trying to avoid that their solution wasn't a good one but none of your responses made sense.

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[dwh]

Except that if you are trying to get to a controller you don't want that drop,

There is no voltage drop. The voltage cannot drop while the panel is driving the voltage up.

What you don't want is the amperage drop. But, like voltage drop, what is a reasonable amount that can be lived with, is contextual.

the panel isn't wired to the battery, the controller is.

If it's a PWM controller (fast acting switch) then the panel certainly is wired to the battery whenever the switch is closed. The voltage is being driven up by the panel, but being limited by the battery. That's a limit, not a drop.

If it's an MPPT controller, the panel is wired to the MPPT function of the controller. The voltage on the solar side is being driven up by the panel, but being limited by the MPPT function. That's a limit, not a drop.

(And the MPPT function limits the voltage on the solar side, by connecting the solar to the battery, through the buck converter.)

If you have a long wire you have less voltage at your controller that is charging your battery.

No, you don't. Not as long as the solar is driving the voltage up.

You would need to run #16 to Bangladesh to create a situation like that. Or start with a panel that only has a Vmp of 15v and then use an extreme length of tiny wire.

But again, that wouldn't be a voltage drop. That would be a limit which prevents voltage rise.

The question was about trying to avoid that their solution wasn't a good one but none of your responses made sense.

My response didn't make sense to you, because you still think that there was some voltage drop that needed to be avoided.

But there isn't. So the OP's solution of trying to eliminate something that wasn't happening in the first place wasn't appropriate.

It was also inappropriate in another way: Trying to run an inverter from a sketchy, variable power source (solar panel), without having something (capacitor or battery) to buffer the input to the inverter and smooth out the power flow into the inverter. That would just end up making the inverter beep and shutdown whenever clouds (or flying cows) passed over.

 

[dwh]

Sure, And touch the ends to your tongue, I dare you !
Wonder how much inducted voltage may be present? Could get your azz zapped.

Please leave the planetary core out of this.

Besides, everyone knows that induction is only for cooking and blowing up iPhones.