Challenges with Solar on boats

[john61ct]

The 75/15 will be fine with say 2x 35Voc in series and 4x 5A lsc in parallel, for a real life maximum input of around 1200w.

Say you were on a desert island with only on controller but a pallet of panels, such over-panelling would mean getting

[john61ct]

The 75/15 will be fine with say 2x 35Voc in series and 4x 5A lsc in parallel, for a real life maximum input of around 1200w.

Say you were on a desert island with only on controller but a pallet of panels, such over-panelling would mean getting the max 15A even on cloudy days, and probably for all daylight hours minus three.

Silly example I know, but over-panelling with only a 290w panel makes very good sense **to me**.

Anything below 220w doesn't make any sense **to me**, unless space/shape issues interfered.

And it also doesn't make sense **to me** to argue about those preferences anymore.

[StuM]

Originally Posted by foggysail

John, You're getting better The spec's Isc has ZERO to do with anything that the controller will see.

Precisely

The problem lies with the concept of "output voltage of 0" and "output current" (to quote your earlier post).

Isc is the result of the photovoltaic cells' light generated current , i.e. the movement of electrons within the cells when irradiated and really has nothing to do with Voltage, 0 or otherwise.

It is not current from the panel at 0 Volts, Effectively it is the upper limit to the current that the photovoltaic cells in the panel can generate. It is worth noting that Isc is a function of a number of factors including spectrum and light intensity. If you expose a panel with a defined Isc of 20Amps to a light source with a different spectrum or a higher irradiation that 1000W/m², then Isc will be greater that 20A

As soon as put anything across the connections in a panel and current flows through that connection, you no longer have V=0 or Isc.

It's worth noting that at both Voc and Isc power output is 0 and there is nothing to measure

[noelex 77]

Originally Posted by rabbi

The 75/15 never exceed 15A output current even when more energy was available.

Yes, when the panel is capable of producing more current into the batteries, the controller will limit the current to 15A to protect its internal mechanism. So you will not see more than 15A.

There are two problems with this. The first is the question of the life and reliability of the controller. The controller will be running at 100% of its rated output for long periods. It will be relying on his internal protection mechanism to stay within the current limit where it will not be damaged. The second is the loss of output. We had 330W of solar on our last boat and saw over 20A into the batteries for long periods of time. With 350W I would expect brief periods of a 25A output. The Victron 75/15 will limit this to a maximium of 15A. This will significantly reduce the overall power output.

Buying a 15A controller for 350W of solar with a 12v battery system is not a wise choice.

[noelex 77]

Originally Posted by john61ct

The 75/15 will be fine with say 2x 35Voc in series and 4x 5A lsc in parallel, for a real life maximum input of around 1200w.

The input will be a maximum of around 1200w, but the maximum output will be around 220w.

[rabbi]

Originally Posted by foggysail

That is meaningless!!!

Isc is the short circuit solar panel current which means the output V is 0.

Yes, Isc shows on the A V P curves and one can assume that a panel that has an Isc of 20 amperes but the output voltage is 0 under that condition. No controller can have an input current greater than the output current, PERIOD!

Isc is the uppwer maximum current, that is real life current will always be less than Isc.


BTW: Of course SC can have input currents higher than output currents.
MPPT controllers capable of Step-up conversion convert 14V /10A input into 28V / 5A to charge a 24v battery bank. But true boost / buck MPPT controllers are a rare thing.

[foggysail]

Originally Posted by StuM

Precisely

The problem lies with the concept of "output voltage of 0" and "output current" (to quote your earlier post).

Isc is the result of the photovoltaic cells' light generated current , i.e. the movement of electrons within the cells when irradiated and really has nothing to do with Voltage, 0 or otherwise.

It is not current from the panel at 0 Volts, Effectively it is the upper limit to the current that the photovoltaic cells in the panel can generate. It is worth noting that Isc is a function of a number of factors including spectrum and light intensity. If you expose a panel with a defined Isc of 20Amps to a light source with a different spectrum or a higher irradiation that 1000W/m², then Isc will be greater that 20A

As soon as put anything across the connections in a panel and current flows through that connection, you no longer have V=0 or Isc.

It's worth noting that at both Voc and Isc power output is 0 and there is nothing to measure

Isc is the short circuit current from the panel WHETHER YOU LIKE IT OR NOT!

LEARN BASIC CIRCUIT THEORY BEFORE YOU PREACH TO ME AS A KNOW NOTHING

[rabbi]

Originally Posted by noelex 77

Yes, when the panel is capable of producing more current into the batteries, the controller will limit the current to 15A to protect its internal mechanism. So you will not see more than 15A.

There are two problems with this. The first is the question of the life and reliability of the controller. The controller will be running at 100% of its rated output for long periods. It will be relying on his internal protection mechanism to stay within the current limit where it will not be damaged. The second is the loss of output. We had 330W of solar on our last boat and saw over 20A into the batteries for long periods of time. With 350W I would expect brief periods of a 25A output. The Victron 75/15 will limit this to a maximium of 15A. This will significantly reduce the overall power output.

Buying a 15A controller for 350W of solar with a 12v battery system is not a wise choice.

OK, lets assume you have a 100W panel on a 75/15. The battery is full, the SC doesn't need more than 1A @ 13.5v for float.
So what does it do? I varies its impedance so that the operating point of the panel is so bad it just enough power for the SC needs to make 1A@13.5v. Thats what its designed for.

So what happens if the SC is connected to 350w panels (keeping maximum Isc and Voc in mind)? It will just move the operating point so much not to overload its circuits, using the exact same mechnism as it uses when limiting the input power for a fully charged battery.

All it does is use a less-than-optimal point in the I-V curve to produce less power. I doubt this can do any harm, as long as the panel's Voc and Isc stay within the limits of the controller.

Only the current passing through the controller can cause an issue, not the 100s Watt of unused panel power.


Of course the SC runs at 100% capacity for longer periods than with say a 220W panel. Can this limit the life of the controller? maybe, I don't know but if it does it can only be due to high temperatures resulting from the 15A passing through.
My MPPT controllers have never been hot to touch, only warm so I think they are using conservative specs.

Victron explicitly say in their datasheets that higher Isc or Voc may damage their controllers. In the same datasheet they say that panel power above their suggested 220W will just cause the SC to limit the input energy, not mentioning any potential damage.


So how often does the controller really run at 100%?
Oversized panels produce more current in the early morning hours, so my batteries are full at 11am.
So when conditions are good enough for 15A (midday) the batteries are full enough not to accept any significant current, so the SC doesn't often do 15A.
I really rarely see 15A going into the battery bank, only if there are additional heavy loads.


In my book buying a 15A controller with oversized panels can be a wise choice, as it's making best use of the available budget and real estate.

[foggysail]

Originally Posted by rabbi

Isc is the uppwer maximum current, that is real life current will always be less than Isc.


BTW: Of course SC can have input currents higher than output currents.
MPPT controllers capable of Step-up conversion convert 14V /10A input into 28V / 5A to charge a 24v battery bank. But true boost / buck MPPT controllers are a rare thing.

Where are you people coming from??? In order for current to flow, there must be a completed circuit. It does not flow outward into the ether. The V-I curves pertain to the panel's output connected into a load that varies from an open circuit to a short circuit. Real life current.....LIKE IT OR NOT is shown on the V-I curve.


Input current greater than output current.....REALLY??? Gees, another whack a mole!

The converter's input voltage must be greater than the battery's by several volts. Converters such as Victron's operating efficiency exceeds 98%. Within reason, the input current cannot be greater than the output. DO THE MATH.

[rabbi]

Originally Posted by foggysail

Where are you people coming from??? In order for current to flow, there must be a completed circuit. It does not flow outward into the ether. The V-I curves pertain to the panel's output connected into a load that varies from an open circuit to a short circuit. Real life current.....LIKE IT OR NOT is shown on the V-I curve.

So Isc is what in your opinion, an imaginary number?
Its what it says, the shortcut current.

Real life current depends on the impedance of the SC, which an MPPT controller actively controls within limits (thats why Victron specifies safe operating limits Voc and Isc).
The key is: real life current is always less than Isc, and real life voltage is always less than Voc.

Originally Posted by foggysail

Input current greater than output current.....REALLY??? Gees, another whack a mole!

The converter's input voltage must be greater than the battery's by several volts. Converters such as Victron's operating efficiency exceeds 98%. Within reason, the input current cannot be greater than the output. DO THE MATH.

You are completely mistaken. Please google "MPPT boost converter" and take the time to read.

If you want to buy a product: Genasun
From the specs: Minimum voltage fr charging a 12v bank: 5V

Pls note: The Victron MPPT is not a boost converter, and at least I have never said so. I mentioned that MPPT controllers with boost converters do exist, not that Victron's MPPT is an implementation of such a thing.

Victron's claimed 98% efficiency has nothing to do with this.. I just means 100W in, 98W out. It does no say anything about voltages or currents.

[john61ct]

OK, the (theoretical short-circuit rated panel's allowable maximum) current input is greater than the (actual charging maximum SC's) current output.

Does that resolve the conflict?

[john61ct]

Originally Posted by noelex 77

The input will be a maximum of around 1200w, but the maximum output will be around 220w.

Yes, exactly what I said, along with the fact it was a thought exercise, too wasteful to actually implement.

I'd say between 200w and 400w might be a reasonable range, the exact limit for each a personal choice.

[StuM]

Originally Posted by foggysail

Isc is the short circuit current from the panel WHETHER YOU LIKE IT OR NOT!

LEARN BASIC CIRCUIT THEORY BEFORE YOU PREACH TO ME AS A KNOW NOTHING

Getting a bit hot under the collar there?

Basic circuit theory should tell you that at 0 Volts, there is no current from the panel.
Current in a circuit is driven by potential differences.

It may be a good idea to study some physics and how photovoltaism and diodes work.

Isc is by definition the current through an irradiated solar cell when the voltage across the cell is 0.

I repeat - there is no current from the panel when voltage across the panel is 0
(whether you like it or not. ).

[StuM]

Originally Posted by foggysail

Where are you people coming from??? In order for current to flow, there must be a completed circuit. It does not flow outward into the ether. The V-I curves pertain to the panel's output connected into a load that varies from an open circuit to a short circuit. Real life current.....LIKE IT OR NOT is shown on the V-I curve.

Input current greater than output current.....REALLY??? Gees, another whack a mole!

The converter's input voltage must be greater than the battery's by several volts. Converters such as Victron's operating efficiency exceeds 98%. Within reason, the input current cannot be greater than the output. DO THE MATH.

There are so many misconceptions and incorrect statements there that it is hard to know where to begin.

But two things you may like to learn about.

1. The Ideal Diode Law and "light generated current" (IL) in a diode. (and the definition of Isc)
2. Buck and Boost converters.

[john61ct]

I have say a big Thank You for those calmly and rationally helping people less knowledgeable than themselves.

In the end, if we are willing to be mindful that our knowledge is imperfect, we all end up learning from each other.

And that, to me, is what such online discussions are all about.

For any newcomers or infrequent readers, this thread is a perfect case study in learning to judge who are the real topic authorities. Following their contributions back through historical threads can be a great shortcut to a deep education in DC electrics.