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 😞

[Snowman9000]

My understanding of MPPT is that the controller will not let it freefall over the knee. The reason being that there would be fewer watts there than higher up on the curve. The MPPT controller's job is to find the voltage that yields the highest watts, then buck that down and "create" amps. The controller will be doing so with whatever net V it gets, post V-drop. It has no idea what the panel MPP is; it just works with whatever it gets, and determines the "system" MPP accordingly. It can't (won't) let the panel V drop into the knee because the system MPP will not be found there no matter what.

Chasing V drop might still be worthwhile. But not as drastically important as you are thinking. Just IMO!

[mena661]

The 25W could be measurement error. :B

[jrnymn7]

I posted this in the 'how you heatin' thread, but I think it applies here as well:

http://www.solimpeks.com/wp-content/uploads/2012/06/pvt_presentation_en.pdf

Heat is definitely a big issue. In summer you could simply blow the heat into the environment, using a low draw fan and heat exchanger. Obviously, overall Ah gain would increase with a larger array.

[BFL13]

Yes I have accounted for panel heating and wire loss but am still missing 25 watts at the batteries. I can't believe the controller is that inefficient, so where did the 25 watts go?

At 13Vbat that would be worth 25/13 = 2 amps. I already lose 2 amps between Spring and Summer due to temperature (15.5a vs 13.5a) so if I could find those missing 25w, I could get back to at least as good as what PWM would do (14.5a)

Some dirty rat ate my amps! It probably has to do with those watt things. 😞

[pianotuna]

Hi BFL13,

I think Salvo is saying that the losses are due to heating of the panel(s).

The greatest amperage I ever saw was in June of 2009 with 21 amps on a day when the ambient temperatures were 40 C (104 f). The panels had been installed the month before--and they had not "settled in" yet. I now see a maximum of 17 amps.

I was near the US border in North Dakota I think at about 48.345833, -100.404999

[BFL13]

You can use 7-pin type connectors too with just two wires at pin #4 and ground. For 5ers that can also let you connect to the 7-pin cord as a way to get to the batteries inside.

I just use the clamps (clamp to clamp) on my string of heavy jumper cables out to the portable panel. With 1% voltage drop at 35 ft one way, the ugly hodge-podge using jumper cables is doing ok it seems.

[pianotuna]

Hi,

When making the adapter use the ground wire for one of the connections. That way if someone gets "helpful" and plugs in the panels to 120 volts nothing will happen.

pnichols wrote:
By the way, for those who want to mount their controller for portable panels back in the RV to be close to the batteries - there's a very easy way to carry along a long and flexible, heavy gage, fairly inexpensive cable to run between the panels located aways from the RV:

Use a contractor-type 10 or 12 gage extension cord available at hardware stores. Make an adapter for it's female end so that the panels can plug in using a regular 120V plug.

[BFL13]

So I am just doomed by having MPPT in the summer temperatures instead of PWM like I used to have. 😞 It's all JiminDenver's fault--he's the guy who camps at 10,000 ft.

The trouble remains that the stealers here are charging $2/w for 12s but $1/w for 24s and you can get an Eco-W for $102, not that much more than a decent PWM controller.

[Salvo]

Panel power reduces by 0.48%/C. If your panel is at 50C, then you got a 25C rise above spec.

Imp at 50C is:

Imp = 7.7A * (1 - .0048 * 25) = 6.8A

[pnichols]

By the way, for those who want to mount their controller for portable panels back in the RV to be close to the batteries - there's a very easy way to carry along a long and flexible, heavy gage, fairly inexpensive cable to run between the panels located aways from the RV:

Use a contractor-type 10 or 12 gage extension cord available at hardware stores. Make an adapter for it's female end so that the panels can plug in using a regular 120V plug. Plug the other male end into a common 120V receptacle that you wire to the controller's input back at the RV. Use the two hot lines in the extension cord, the 120V plug, and the 120V receptacle to carry the power from the remote portable panels.

We carry a 50 foot heavy gage extension cord along in the RV all time anyway (for rare 15amp/20amp hookups), so I'll use this for the 2nd purpose of running out to a set of two portable panels hinged together for battery charging when drycamping.

[BFL13]

That is the missing information for sure, at the controller input. I have taken voltage there but never the amps. I have taken Isc there and got higher than panel rating 8.3 so I thought that meant insolation was ok.

Isc is not what the controller uses for Imp and I still have the missing watts.

So if 230w is supposed to be 30 x 7.7 and panel voltage measures 29.3 and watts is maybe 200, then 200/29.3 = 6.8a I cannot understand how Imp could be so low when Isc is at its rating. Isc does go up a bit with temperature but not that much.

I keep thinking I am farther down that nasty IV curve knee than I should be.

[MrWizard]

Do remember....That the absolute stated values are at laboratory conditions of solar irradiance
Which is greater than your backyard or normal local
To approximate lab conditions you need to be at 10,000 ft mountain top in June

Your loss could be no more than location losses
You can put a Turnigy watt meter in the panel feed before controller
Measure power into controller.instead of doing all the math
Verify input power

[BFL13]

Thanks all, so I can keep my wiring from panel to controller as is. I can work out the penalty for moving the controller farther away from the batteries depending on the wiring there.

I would still like to know where my missing 25 watts went.

[Salvo]

If you have a wire drop of 0.3V at 7.7A then your wire resistance is 0.3V/7.7A = 39 mohm.

The cable power loss is:

P = 0.3V * 7.7A = 2.3W.

If the battery is at 12.5V then charging current is reduced by:

I = 2.3W / 12.5V = 0.2A

For a thirsty battery, that could be equivalent to about 1 AH per day loss.

[BFL13]

I understand that the controller will still be tracking for MPP but was thinking there is less power at MPP. I was looking at that knee drop off at Vmp where a small change in V makes a big change in A and thought that 0.3v would have an exaggerated effect if the V the controller is "seeing" is higher by 0.3v. Except it measures lower at 29v at the controller vs 29.3v at the panel--I am confused which direction the controller is "seeing."

I was separating that maybe big effect from the straight 1% loss on the wire, which is pretty straight forward. If that 1% goes to the output then 15a would lose 0.15a ?

I am seeing 180 watts at the output when panel temperature is 50C. That 50C vs 25C is supposed to create a 10% power loss or 23w. So I should see 207w - 1% or 2w = 205w less controller efficiency? I am still missing 25w somewhere (205-180) The controller is supposed to be quite efficient.