IV Curve with MPPT Charge Controller

[jflorey2]

It doesn't always do the full sweep. Sometimes it just does a small quick perturbation of voltage (and of course current) and then gets back to business.

One of the reasons I like the Midnite Solar controllers is that you can set how the controller does the sweeps. You can set it to do periodic full sweeps or a less aggressive P&O (perturb and observe) which sounds like what you are seeing above.

I've also noticed that the voltage often (usually?) stays constant through short-term changes in sun intensity from clouds passing overhead.

Not surprising. Voltage changes with temperature; current changes with sun intensity (over most of the panel's operating range.)

[BackwoodsEE]

Here is another plot this morning as the sun hits the panels at a significant slant and there is a small amount of shading. At this point in the day, the CC is trying to get as much as it can from the panels and the limiting factor is the solar illumination. That will change by noon when my small 12V 100 Ah VRLA battery gets mostly charged up and the CC goes into absorb mode.

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[SunEagle]

The graphs are still cool to look at and can provide some idea on how the solar panels & CC works

[sensij]

No it is not pointless. Sure you know anything about CC's? The voltage should never dip below Vmp for any length of time in Bulk. Only when it goes into PWM mode aja Absorb and Float will you see voltage go below Vmp.

You do know Vmp changes throughout the day, right? A little with irradiance, more with temperature, and a lot with partial shade. Op is showing how this specific CC finds Vmp under a variety of conditions, so it can operate there. The process of finding Vmp means that for a very brief amount of time, it sweeps across the voltage range to see how power responds, and yes, those sweeps appear to go all the way to battery voltage

Sometimes an mppt controller might even find Vmp to be the same voltage as PWM controller.

[J.P.M.]

You do know Vmp changes throughout the day, right? A little with irradiance, more with temperature, and a lot with partial shade. Op is showing how this specific CC finds Vmp under a variety of conditions, so it can operate there. The process of finding Vmp means that for a very brief amount of time, it sweeps across the voltage range to see how power responds, and yes, those sweeps appear to go all the way to battery voltage

Sometimes an mppt controller might even find Vmp to be the same voltage as PWM controller.

Might be a definitional thing. Vmp vs. Vmpp ? Voltage will change as the logic roots around trying to maximize the power output for the instant. conditions (Vmp ?), and the rated power being Vmpp X Impp @ STC.

[BackwoodsEE]

The graphs are still cool to look at and can provide some idea on how the solar panels & CC works

Thanks! It's been a lot of fun for me as well, and after a few months of looking at the plots I think I've got a lot more intuitive feel for how PV generation works under real-world conditions.

Here's another plot for your viewing pleasure: We have light cirrus clouds right now, and the sun is currently behind one. But the sunlight is still bright enough to cast some shadow on the ground (nothing shading the array right now), and the PV array is operating at at about 1/3 capacity. Note the pink dashed line: That is the current vs. voltage curve from the previous sweep, when there was nearly twice as much production. (I only preserve a copy of the green IV curve from the previous sweep, not the blue power vs. voltage curve.)
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jflorey2 observed that current changes with sun intensity, and you can clearly see that in the positioning of the dashed pink curve (more sunny) vs. the green curve (behind a cloud).

The CC is going all out right now trying to keep up with some heavy loads and a half-empty battery.

[Sunking]

You do know Vmp changes throughout the day, right? A little with irradiance, more with temperature, and a lot with partial shade.

Yes I understand that, have even built a couple prototypes when bored to see if I could improve the wheel. Could not improve it, and less expensive to just buy a CC.

My point is if you are doing a serious test, a battery is not a load you want to use. It has to be dynamic and not fixed. Same with battery testing, you cannot used a Fixed Load to test Amp Hours. Using a battery as a Test Load only gets you Ball Park.

Backwoods is an engineer and should understand what I am talking about. A circuit to act as such is pretty simple. In a properly designed MPPT voltage should only range as low as Vmp, and up to Voc so it behaves like A VOLTAGE SOURCE. A PWM operates from Isc to Voc and behaves like both a Current Source and Voltage Source. That i show you catch the cheater Chi-Coms.

[Sunking]

The CC is going all out right now trying to keep up with some heavy loads and a half-empty battery.

I see you took my advice. You are seeing something different now.

[sensij]

I see you took my advice. You are seeing something different now.

It isn't any different than at any other point in the bulk charging stage. The CC doesn't care whether there are additional loads or not, the battery is taking all the current that can be produced. The entire voltage range needs to be swept to avoid local power maxima caused by shade, otherwise it could get stuck in a local peak without realizing the true maximum is somewhere else.

[Sunking]

It isn't any different than at any other point in the bulk charging stage..

You are either blind or do not know what you are talking about. Which is it? You are proving you do not understand how a battery charges.

[sensij]

You are either blind or do not know what you are talking about. Which is it? You are proving you do not understand how a battery charges.

You can see that all the charts posted on this thread show the *input* side of the CC, yes? The only thing causing differences in the charts shown are differences in *inputs*... Irradiance, temperature, and shade. Every chart illustrated here shows the controller operating at the maximum power point it finds for those specific conditions, which means they were taken during the bulk stage. (Except maybe the chart showing 3W of production).

The OP has talked about observing behavior of the CC when it is in a constant voltage stage, but hasn't shown any pictures yet of how it controls itself there.

[BackwoodsEE]

You can see that all the charts posted on this thread show the *input* side of the CC, yes? The only thing causing differences in the charts shown are differences in *inputs*... Irradiance, temperature, and shade. Every chart illustrated here shows the controller operating at the maximum power point it finds for those specific conditions, which means they were taken during the bulk stage. (Except maybe the chart showing 3W of production).

This is true, and even in the 3W plot, the CC was still trying to get as much power as possible from the PV array. I haven't yet posted any plots where the CC is limiting its output, but I'll try to remember to do that tomorrow.

The point you are making, which I agree with, is that the CC is loading the PV panels as much as possible, and optimally using its MPPT capabilities. It wouldn't matter if I had some wonderfully perfect load that Sunking keeps talking about. The plots posted would be exactly the same, with the charge controller pulling everything it can from the PV array.

This really is not that complicated.

As an aside, I've had my capacitor-loading box inserted in the circuit (after this monitoring device and before the charge controller), and when it momentarily interrupts current flow to the CC and switches in the capacitors to do its own sweep (independent of what the CC does), I see the exact same curves. That's with a pretty much ideal dynamic load for sweeping the IV curve of a photovoltaic device, using a technique that's been employed for decades for that very purpose. And that result is the same as what you've seen, except that my capacitor load starts at about 5V as soon as the capacitors get switched in, instead of the 13-14 V battery voltage. (The ESR of the caps plus MOSFET on-resistance switching them in plus wiring, multiplied by Isc, limits the lower end of the voltage range. Doesn't really matter.) I don't use the capacitor box because what I'm really interested in is the behavior of the overall system, which often does not use all the power the panels can provide. It also consumes a little bit of power due to the MOSFET on-resistance (28 mOhm) and diode drop (around 1.2 V) necessary to switch between capacitor and CC current paths.

[BackwoodsEE]

Complicated morning shading. The TS-MPPT-60 is trying its best to get everything it can with a half-empty battery and some significant inverter loads. (I'll post a plot of the CC running in absorb mode this afternoon.) A pic of the array is below the plot.

Note how the steps in the IV (current vs. voltage) curve occur at around 10 V increments. That's because there are three substrings of PV cells inside each panel, and each string produces a third of the panel's total voltage of around 30 Vmp and 36 Voc. As the total current through the overall string of panels decreases, each panel substring produces more voltage, and the transitions shown in the stair-step plot show the different points when substrings overtake their bypass diodes.

In this plot, the CC has found the maximum power point at 52 V and 2.32 A. If it pulled any more current, the voltage would decrease without enough of a gain in current to make up for it. To get the voltage to increase, it would have to reduce current draw significantly. There is a transition right next to the max power point (that's what "MPPT" stands for, of course) where a bypass diode would have to turn off that is allowing quite a lot of the string's total current to flow.

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Here's what this shading looks like on the array:

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[Sunking]

Now take off the Inverter loads and see what happens. My point to you is to test any Switch Mode Power Supply, that is exactly what a MPPT Controller is, is you want to clamp the voltage under test. Say 10.5 to 12 volts which is an over load.

Here is where battery Ri and surface charge comes into play. You have a 12 volt 100 AH battery correct? What is the Panel Wattage and Charge Rate? A battery will limit itself. You want to remove that self limiting property and Surface Charge. What you really want to see is the I/O efficiency at maximum stress. Just using a battery gives you a Ball Park number like using a fixed load to measure Amp Hours. You get errors. Wave forms may not change, but conversion efficiency changes.

Measure the battery Ri. It is easy to do using Delta Voltage / Delta Current. Apply a load of say 1 amp. Measure and record the actual current and voltage accurately. Call it E1 and I1. Repeat at say 10 or 20 amps can call them E2 and I2. The actual currents are not important. What is important is how accurately you can measure the voltage and current, and the difference between high and low current. You want as much difference as possible. Perform test on a rested battery with a SOC of 50 to 90% SOC.

Dv = E1 - E2
Da = I2 - I1
Ri = Dv/Da

Or use a deeply discharged Lithium Ion Battery as the load. Ri is much lower and almost no Surface Charge. A LiFePO4 will clamp at 3.2 volts x 4 = 12.8 for a long time. Stay at or just below maximum charge rate to minimise Ri effects.

[BackwoodsEE]

Now going full tilt with the battery still about half discharged plus about 150 W on the inverter. The last two times it updated MPPT, the CC didn't bother with a full sweep, just a quick look above the input voltage it's currently operating at, up to Voc.

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