Solar voltage drop calculations, new question

Snowman9000 wrote:
Using a calculator:
I like this one

When figuring wire size from panels to controller for "12v" panels in parallel and PWM controller, we should use 20v as our input, right??? I see writeups where people use 12v or maybe 14v in their calculations. That doesn't seem right. I have 21v give or take at the input side of my controller. The open circuit voltage in other words.

For MPPT controller I would think 18v would be the proper input into the calculator. Correct me if I'm wrong. Of course from the controller to batteries you still use 14v.

The benefit of using the higher voltage is that you find less loss for a given wire size. Maybe you don't need as big of a wire. Etc.

Edit: Guess I was wrong, see below. Well, it's good to learn. :)
2nd edit: See below.

Snowman,

For PWM, The voltage at the panel is going to be typically just a little bit higher than the battery voltage. And the voltage drop from Panel to Controller is not the most critical issue.

PWM gory discussion:
With PWM the Panel voltage will be pulled down very close to the battery voltage. That's the way PWM works when charging, it essentially connects the panel to the battery during "bulk charging".

Assume full sunshine, typical panel with Voc = 20V
Vp = Voltage at Panel
Vc = Voltage at Controller which should be almost the same as the battery voltage.
Vdrop = voltage drop between Panel and controller.

Vp = Vc + Vdrop
What this says is the Voltage at the panel is going to be a function of the battery voltage primarily.

Vdrop = Isc * Rwire

Even if Vdrop is huge, say 2V, it won't really matter for PWM controller in full sun.

So if the battery is at 14V, and the Vp is trying to be 20V (but is held down to 16V, you are fine.

With PWM, you leave some power "on the table" because you can't operate the panel at Vmp. And the Voltage drop (wire loss) is not going to be greater than the difference between Vmp and Vc.

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Now for MPPT, the panels Vmp is a good value to use as the panel voltage for computing drop between panel and controller.


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Yes, thicker wires always helps. But the critical part for solar charging is the voltage drop between the controller and the battery must be kept very small. That is where you run your shortest fattest wires.

Basically, the charge controller (whether PWM or MPPT) will output optimal voltage (as best it can) to charge your battery. The controller optimizes the voltage on the output side, and in doing so, can hide some level of voltage drop between panel and controller.

Ed