More MPPT Confusion and Question

[BFL13]

Still trying to understand my MPPT set-up and what would be the effect of changing things a bit.

My 230w (30.0 Vmp x 7.7 Imp) poly panel has a Vmp of 30.0v with a measurement tolerance of +/- 3%
So actual Vmp could be anywhere from 29.1 to 30.9

My wiring zoo from panel to controller, measured in action this summer showed panel at EG 29.3 and at controller input 29.0

Voltage drop was therefore 0.3 at about ??7.7a Imp?? (Rated Isc is 8.3a and was getting that panel disconnected)

(I have the controller output wattage then at 203w (12.7Vbat x 16a) but that is after controller efficiency loss and whatever else)

So the question is, where it is trying to stay at Vmp at the "knee" of the IV curve, how bad is that 0.3 voltage drop? What would my controller output be if I cut that voltage drop in half say?

I am thinking (backwards? getting mixed up here) that the controller will "see" the panel voltage as 0.3v lower than it is. But on the IV curve that lower voltage would make the amps higher but if it is lower then amps are way down the knee drop off

Rating max is 230w (30 x 7.7) I measure 29.3v at the panel which is where the controller has set the voltage thinking that is the MPP?

But the controller input voltage is 29, not 29.3 so does that mean the controller's idea of MPP is 0.3 low or 0.3 high?

The IV curve where Vmp is at the knee which has a huge drop off in amps so a small change in voltage must have a big effect, but which way?

I am trying to calculate how much I can improve my wattage as seen at the controller output by reducing that 0.3v loss to the input by x amount and see if it is "worth it." But I am all mixed up 😞

[ken_white]

BFL, look at the top power curve.

The current is constant to nearly 15 VDC and then starts to gradually taper until it hits about 27 VDC.

The current falls off quickly above 30 VDC, so the controller keeps adjusting its input impedance (voltage) while monitoring its input current for maximum power.

Increase the voltage too much and power falls off quickly due to its steep slope...

Lower the voltage and the slope is less steep and almost flat when optimized.

Since energy cannot be created or destroyed, just moved around, the power at the input terminals will "always" be equal to the panel power minus the power losses in the cabling.

The controller runs the zoo...

[Snowman9000]

BFL13 wrote:
Ok Ken that almost made sense 🙂 Now the Tracker picks a voltage from its input that gets it the most power at the controller (one way is to fiddle with some R to find the best voltage).

So you go out and measure 29.3v at the panel and 29.0v at the controller input.

What would happen to the output power of the controller if there was only half that voltage drop? ie 29.15v at the controller instead of 29.0?

When the controller picks 29v at its input that turns into 29.3 at the panel so if 29 was the right answer and the panel ends up at 29.3 doesn't that put it out farther to right on the IV curve and drop way down on the knee for amps and so power? (never mind ??? that the rated Vmp is 30.0v)

I can't figure out who is doing what to whom. 29.3 is closer to 30 than 29 but if the controller chose 30 would that put the panel at 30.3 and down the knee a bunch?

Well, the controller won't pick that number, unless it yields the most watts. Which it won't. So it won't pick it. The controller has a very simple mission. It just tries voltages looking for the most watts. When it finds "it", that is the MPP of the moment.

[BFL13]

Ken the panel has the IV curve with its knee, not the controller. If the controller can't "see" the right voltages at its input due to voltage drop, how can it get the panel to do its best?

With a voltage drop, the controller has lower voltage than the panel--at least that is what I get with the multimeter.

But then they say that the MPPT makes the panel voltage to be Vmp which is why I get 29.3v at the panel instead of Voc of 34v I guess? Or with PWM when panel voltage = battery voltage allowing for voltage drop. But the panel is rated at best watts at 30v and 29.3 is closer than 29.

So it seems like the controller will "make the panel voltage" wrong by the amount of voltage drop and screw up the wattage? Meanwhile the voltage drop is wrt the current on that wire pair. So the IV curve says what the current will be at that voltage, but the voltage determines how much current that is on the panel's IV curve.

Who is running this zoo?

[ken_white]

BFL13 wrote:
Ok that almost made sense 🙂 Now the Tracker picks a voltage from its input that gets it the most power at the controller (one way is to fiddle with some R to find the best voltage).

So you go out and measure 29.3v at the panel and 29.0v at the controller input.

What would happen to the output power of the controller if there was only half that voltage drop? ie 29.15v at the controller instead of 29.0?

When the controller picks 29v at its input that turns into 29.3 at the panel so if 29 was the right answer and the panel ends up at 29.3 doesn't that put it out farther to right on the IV curve and drop way down on the knee for amps and so power? (never mind ??? that the rated Vmp is 30.0v)

I can't figure out who is doing what to whom. 29.3 is closer to 30 than 29 but if the controller chose 30 would that put the panel at 30.3 and down the knee a bunch?

If in MPPT mode, the controller only optimizes the values at its input terminals.

The controller has no idea what is being delivered or lost while being delivered.

[BFL13]

Salvo wrote:
The irradiance during the test probably wasn't 1000W/m^2 but somewhat lower. It's difficult to determine irradiance. But you can see from this graph the greater the irradiance the greater Vmp becomes.

Salvo that is part of the big puzzle to me. The Isc was over the rated Isc of 8.3 so to me that means the panel is at least at STC for irradiance, allowing for the high temperature raising Isc but lowering Voc.

I just figure if the Isc is over its rating that means the insolation is ok, and any losses are from something else. No?

[BFL13]

Ok Ken that almost made sense 🙂 Now the Tracker picks a voltage from its input that gets it the most power at the controller (one way is to fiddle with some R to find the best voltage).

So you go out and measure 29.3v at the panel and 29.0v at the controller input.

What would happen to the output power of the controller if there was only half that voltage drop? ie 29.15v at the controller instead of 29.0?

When the controller picks 29v at its input that turns into 29.3 at the panel so if 29 was the right answer and the panel ends up at 29.3 doesn't that put it out farther to right on the IV curve and drop way down on the knee for amps and so power? (never mind ??? that the rated Vmp is 30.0v)

I can't figure out who is doing what to whom. 29.3 is closer to 30 than 29 but if the controller chose 30 would that put the panel at 30.3 and down the knee a bunch?

[Salvo]

The irradiance during the test probably wasn't 1000W/m^2 but somewhat lower. It's difficult to determine irradiance. But you can see from this graph the greater the irradiance the greater Vmp becomes.

[ken_white]

BFL13 wrote:
...I still don't get how the MPPT works around the voltage drop...

The controller determines how much current to supply to the battery based on the batteries state of charge and controller design.

The MPPT function only kicks in when the PWM functions can not deliver this power demand - desired voltage at desired amps.

The controller kicks into MPPT mode when the panel can not deliver enough power due to either limited irradiance or its rating is just not big enough.

So the controller maximizes the voltage and current delivered at its input terminals - maximum input power, and then delivers maximum output power minus its losses.

The controller has no idea what the panel voltage or current is, only what is optimized for maximum power at its input terminals.

[BFL13]

I still don't get how the MPPT works around the voltage drop, but I will just treat it as a "black box" and not try to understand it. There is nothing I can do with my set-up to affect that it seems.

The 25w from heat and the mystery 25w is total 50 watts gone from my supposed 230w. 50/13.5= 3.7 amps and that is 44AH in 12 hours.

So the cost effective thing to catch up is fire up the Honda and the PowerMax 100 amper and run that for 26 minutes 🙂

[JiminDenver]

BFL13 wrote:
It's all JiminDenver's fault--he's the guy who camps at 10,000 ft.

It's not my fault that I happen to camp in the best place for solar in all of North America. Maybe there is a really big hill you could try it on. 😉

[mena661]

Just put up another panel and those losses will go away. 🙂

[ken_white]

BFL13 wrote:
...Solar with MPPT loses power when out in the sun, so from now on, I will use it at night!

That is what I do...

Seriously, if you do a cost benefit analysis associated with minimizing the power losses, you will see you are probably right where you need to be, or are close enough to not loose any sleep over it.

Enjoy the outdoors and let the systems chug along at their own pace and if they fail you, do an upgrade to fill in the void.

🙂

[BFL13]

Ok, mena is channelling Salvo 🙂 Actually there must be some but not that bad.

PT found say 10 of my 25 watts so let's say 15w to go on that. Must be getting within the margin of error here.

Yes the controller is out of the sun at ambient (25C while under the tipped up panel the IR gun shows 51C) I even tried cooling the controller with ice packs and got its temp down from 25 but no change in amps at the battery, so that wasn't it.

Solar with MPPT loses power when out in the sun, so from now on, I will use it at night!

[pianotuna]

Hi BFL13,

96% is what Blue Sky claims, so that is 9.2 watts on 230 under perfect conditions.

[jrnymn7]

Is the mppt being kept cool enough, and could that be a cause for lost power?