what voltage to make panel?

[nerdyguy]

I currently have 3 amorphous panels running at 23.5volts. I am building my own panel and plan to make it 18v. My question is, can I connect the 23.5v and 18v panels together to the same charge controller (it is rated to handle the additional current) or will that cause problems? What issues can the two different voltages cause? Should I plan to build my panel so that it is 23.5v instead? Is there a standard voltage in which to aim for when building panels? This is for 12v output from the charge controller to charge deep cycle batteries.

[Mike90250]

The voltage of the "system" will be dragged to the lowest voltage, about 16V, at the input of the charge controller. 18V with a moderate load, is a good voltage to deliver to the charge controller. Any power over 18V, will be wasted. So your 23V panels, will be pulled down to about 17 or 18 V, and their power at that voltage, is 20% less than at 23V.

Now "Voltage Max Power" of 23.5V is sort of odd, sure thats not "Volts Open Circuit" ?? What's the sticker on the panels say ?

[nerdyguy]

Hi, yes you are right these panels are 23.5v open circuit. So, if I don't want to sacrifice power from the existing panels then I should plan on building my custom panel so that it produces 23.5v open circuit? Either that or run two charge controllers on the same battery?

Is there a way to predict what the loaded voltage of a panel will be? Do I need to determine the resistance of the charge controller panel input to calculate this or is there something more involved, like will the voltage fall further once the controller is connected to the battery?

Thanks!

[Sunking]

Well what kind of panel are you building? I know it cannot be amorphous (thin film). That being the case you cannot build a panel to match them using factory reject monocrystaline cells.

[a1bigtuna]

Voltage selection

Is there an advantage in adding some "extra" voltage to a panel like 3 or so extra volts on a 21V solar panel to keep the amperage up if clouds diminish the sun light? Or, is the extra voltage just wasted, and not used by the controller.

[greenHouse]

Tuna,

Current is purely a function of the amount of silicon facing the sun, taking into account that two cells in series don't add up the current, only the volts.

So, no, making a series string "longer" to boost the volts doesn't add amps.

[a1bigtuna]

Panel voltage

I understood the current max would be the same, just was thinking if the sun light was cloudy and the voltage output lowered per cell, that the extra voltage would be used by the charge controller would be able to force a little more current into a battery than with the lower voltage level. Realizing the voltage would be lowered on all the cells, just thought there would be some added "push". So, no advantage at all?

[greenHouse]

If you have an MPPT charge controller you will get some benefit, but only because the voltage will be higher, even if the current is lower.

The voltage output of a solar cell is fairly constant for different amounts of sunlight. It's the current that changes. So in a low-light condition, what you see is the same voltage as in a normal light condition, but reduced current.

[a1bigtuna]

Panel voltage

Thanks, that is what I was thinking. Do you think the addition of 4 6x6" 8 amp cells to one panel would add even an amp with reduced sunlight? Not concerned about the size or cost, just need to have some power at a fishing camp site.