Solar voltage drop calculations, new question

Voltage drop is a function of current and wire impedance. If you have a current to the battery of 1.5 amps for example, the voltage drop for a given wire size will be the same regardless of the controller output voltage. It's the old V=IxR equation. So if you are calculating the voltage drop, forget the starting voltage. Just measure the amps, find the R value in a table and multiply.
Before I get blasted, note that I am talking about voltage drop through the wire, not the endpoint/battery voltage.

Thanks Ed.

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

It is not really the voltage drop itself that matters as a chunk out of the voltage, but what happens to the current you want to get through the battery as battery voltage goes from 13 to 14.8.

The voltage you use is the compromise voltage between the battery and panel when connected as they try to "middle" their voltages. You have the R of the wire which is fixed. V=IR so with voltage held and R fixed it is all down to current how much voltage drop there is. More I, more drop.

At first you have say your 0.5v drop with 13 at battery and 13.5 at panel. With the current through the battery, chemical reaction, battery voltage rises. As battery nears full current acceptance drops so I is less, so voltage drop is less, say 0.25 now and battery is 14.5 and panel is 14.75.

Meanwhile the diff between battery v and panel v is less than before so current is less from that too at some point.

Meanwhile the panel's own current output gets less as seen in that IV curve as battery voltage rises.

You are trying to pick a wire R that is low enough to keep I going as V rises within that range 13-15 volts where I=V/R

Now also this is solar, so you need enough I to get it done before dark too. Lower R helps keep I up and keep the battery chemical reaction going to raise its V sooner

BFL13 wrote:
this calculator shows "the voltage at the end"
http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=12&phase=dc&noofconductor=1&distance=60&distanceunit=feet&eres=8&x=42&y=5

Yeah but does it apply here? It seems like there are volts to lose when the panels can put out 22v and you only need 13-15 at the controller. I know you don't want to fritter the extra volts away but I'm interested in how to figure out the true effects on the bottom line.

red31 wrote:
After a few days of a good charge profile (via a solar), you go home and hook up the WFCO!!

I tried here

http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/27606122.cfm

OK, yes, you are asking the same question. It sure seems like there is a lot of room for voltage drop.

this calculator shows "the voltage at the end"
http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=12&phase=dc&noofconductor=1&distance=60&distanceunit=feet&amperes=8&x=42&y=5

After a few days of a good charge profile (via a solar), you go home and hook up the WFCO!!

I tried here

http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/27606122.cfm

Let's revisit this.

If the panels CAN put out 22v, but are drawn down to 13 during charging, is the voltage drop really based on 13v???

Let's say a voltage drop calculation for 13v for a particular setup shows a drop of 0.5v. Well, the actual voltage at the controller input is still going to be 13, not 12.5, right? Or not? I'm not wording it well, but I hope you get what I'm asking. Put another way, if the battery is accepting 13v and X amps, it's not like it will drop to 12.5 due to the supposed voltage drop. ??

Ah, that might be right. Yes I was measuring at the input side of my dead ebay controller. OK, sorry.