What solar panel specs are used for calculating voltage loss

I like smkettner's calculation. It reduces the chance of error. Be sure to really go overboard with corrosion proofing the battery end of the controller to battery leg.

With PWM, panel voltage = battery voltage plus line loss, so howinheck can the controller input be at 16v?

That's not true
I can measure 17v from panels at the controller any time there is good Sun
And 14+ volts out of the controller
You are confusing shunt controller with PWM
shunt controller connects directly to battery
aka switch close..switch open
Maybe some of these cheap Chinese PWM are really shunt controllers
Those that don't allow voltage settings ??

Well that needs to be clarified. The panel's IV curves have battery voltage as the V.

If the 12v panel is at 17v that is way past the 15v knee on the IV curve and you would not get any amps

There is no "working voltage" with PWM---that is MPPT language meaning the chosen Vmp.

smk is saying the controller is what reduces the voltage to battery voltage but the PWM controller does not have a buck converter in it, so it can't do that.

OTOH, an MPPT controller in its PWM mode does still have that buck converter in it and it still works that way, but now the chosen voltage for input is higher than Vmp was and nearer to Voc.

A PWM controller is not the same as an MPPT controller in PWM mode.

right now the sun is NOT quite STC
i am under partial shade, sun only on front panels
other panels fully or partly shaded (leafy shade NOT deep shade)
i just measured 18.88 amps @ 14.31v as controller output
input voltage measured at 15.45v

we don't know how the IP curves were created, but unless it was on a known calibrated load that we can compare too (which is not stated)
I think it was probably measured on a short circuit or directly connected to a battery

My battery and panel V are the same until abs.
Panel V rises in abs cuz the meter is reading an ave of batt and VOC as the on/off duty cycle changes. Very shaded.

BFL13 wrote:
With PWM, panel voltage = battery voltage plus line loss, so howinheck can the controller input be at 16v?

Yes exactly. Amps increase to Isc causing the voltage drop along the panel IV curve.

Same as any current limited charger or converter. Voltage sags anytime you are at max amps.

The issue is to have the potential to get the voltage and amps to the controller so that the wire does not create the limit but rather the battery and controller create the limits. This allows the panel to run at the maximum.

The calculation above was to present information on the needed wire so as not to cause resistance induced limits. The calculation was not intended to describe the actual voltages along the wire.

MrWizard wrote:
right now the sun is NOT quite STC
i am under partial shade, sun only on front panels
other panels fully or partly shaded (leafy shade NOT deep shade)
i just measured 18.88 amps @ 14.31v as controller output
input voltage measured at 15.45v

we don't know how the IP curves were created, but unless it was on a known calibrated load that we can compare too (which is not stated)
I think it was probably measured on a short circuit or directly connected to a battery

Very interesting! Would you call that 1.14 volt drop in the controller "line loss" except not in the wire paths to and from it?

The controller will have some sort of R

^^^ what is missing is the controller set point assuming PWM.

smkettner wrote:
^^^ what is missing is the controller set point assuming PWM.

I don't have a PWM set-up anymore to test with. However I do still have the manual for my old ASC controller that has a "field test" in it that may be what smk is talking about there.

"Put a solar panel in full sun. Remove any connections. The Voc needs to be 17 volts or higher to do this test.

With all ASC connections removed, connect panel + to the ASC's Array+ and panel - to the ASC's Array - (no battery connection

Measure voltage at BATT+ and BATT- terminals on the ASC. The reading should be between 14 and 15 volts (units with temp comp and adjustable set-points may vary beyond this range.

A very high reading 16-20 volts would indicate an open FET or very low reading 2-5 volt would indicate a shorted FET (open FET means will overcharge. Shorted FET will not charge a battery)


Another thing is a note I have from playing with that controller, where I had it on a battery with no array. Batt terminal v was BattV of 12.53 and there was 1.39 volts at the array terminals.