Solar Panel Array With Chinese MPPT Module

About: Fishin', buildin', dreamin'...

A brief description of my take on making solar panels work well, and rather cheaply at that…

I absolutely do not guarantee any of the contents, they might just be the ramblings of a mad man, in fact I strongly suspect they are...

Some pictures have been found on-line and are believed to be free to use, if you find a copyrighted picture drop me a note.

Solar panel ratings are not to be taken as anything but a very rough guide, the published specs are what is achievable under lab conditions with specific light sources etc. In practice it’s not possible to get this performance under realistic conditions. However, they do give a starting point when deciding what to get. As far as I’ve found specifications are only comparable within a manufacturer’s portfolio, comparing between different manufacturers is a chance at best.

Cheap solar panel regulator modules can be found on eBay, AliExpress or similar sites. Despite being quite different they all claim to work perfectly for optimizing solar panel performance. Unfortunately they do not all tell the truth.

When I built my first solar panel arrays a couple of years ago I had to sift through quite a lot of information before finally getting a grasp of what I now believe to be the truth, of course continuous development may lead to something completely being true tomorrow.

Basically there is the choice between PWM and MPPT regulators, and for solar panels MPPT is the way to go.

A MPPT regulator tries to use the solar panel where the panel delivers the most power, MPPT = Max Power Point Tracking. Any other type of regulator will give you lower efficiency since the panel may not be be living up to it’s full potential, this is true whether you’re using cheap Chinese panels or something else.

The cheap Chinese MPPT regulator I use is very basic, you set the MPP Voltage and the regulator tries to keep it there. More advanced regulators will regularly do a ”sweep” to find the MPP (where the curve is flat). The cheap ones are fine for simple projects, but if you want to squeeze every bit of juice from your panels you can’t skimp on this – and as an added bonus you can just ho up everything without reading this ”how-to”…

Step 1: Gather Your Resources

Further down I do a brief walk-through of the steps I did, sort of turorial-ish.

You will need the following.

- Solar panel(s) (I use a bunch of cheap Chinese panels from Ali, connected in arrays)

- MPPT module (I use cheap Chinese modules from Ali)

- Schottky diodes (1 per solar panel)

- Power resistors, fixed values (I use a mix of 10, 33, 47, 120, 330 Ohms, rated 3/4/5/9/10W)

- Variable power resistor (I use a 100 Ohms/2A slide resistor)

- DMMs, I recommend 2, one for measuring DC Voltage and one for measuring DC Current

- Adjustable DC voltage source

- Cables

- Beer, might need quite a few if the weather is good

Step 2: Determine Intended Use

What’s the intended use for your solar panel(s)?

What’s the desired output voltage from the MPPT mpdule?

In my case I have a couple of pretty similar uses.

Array 1 - Portable mobile phone charger used when hiking and scouting (Target voltage 12.3V)

Array 2 - Charging the trolling motor battery for a tiny (12ft) rowing boat (Target voltage 13.6V)

Array 3 - Charging the starter battery for a small (15ft) motor boat (Target voltage 13.6V)

Step 3: Connect Solar Panels in Arrays

Depending on your use it might be necessary to connect the panel in series or parallell, maybe even in combinations thereof, to achieve the necessary Volts/Amps.

I start with soldering the Schottky diodes in place, and then the connecting cables between the panels to form arrays. The Schottky diodes are necessary since the panels differ slightly, and I don’t want to waste power by reverse feeding the panels across.

Array 1: CNC145x145-6, Star Solar. 4 panels connected in series.

Array 2: CNC170x170-18, Star Solar. 6 panels connected in parallell.

Array 3: CNC170x170-18, Star Solar. 4 panels connected in parallell.

Step 4: Prepare the Load

I soldered the fixed power resistors in series leaving the tag ends long, this is to allow quick adjustment of the fixed load by moving the alligator clips.

The variable power resistor is connected in series with the fixed resistors.

Step 5: Preparations

Wait for a day with clear skies, even the tiniest cloud will affect your beer consumption.

Place the array in the sun, make sure no parts are shadowed.

Of course, if you only have a single panel the same measurements are done for this.

Note: Cloudy conditions greatly affect the achievable output, my guess is that better panels are probably affected less than the cheap ones I have.

Pop open a beer and enjoy life for a moment, have a second beer ready if needed.

Step 6: Measure Panel Parameters

These steps are rather important, unless you're using some fancy self calibrating MPPT controller, which I'm not...

Get on with it!

For each solar panel array I measure the parameters as per below.

1. Connect a DMM (set to DC voltage) across the array connections, measure and write down the voltage (Voc). Voc = _____V

2. Next connect a DMM (set to DC current 10A) between the array connections, measure and write down the current (Isc). Isc = _____A

3. Do some quick measurements to determine approximate MPP (Max Power Point).

3a. Connect a DMM (DC voltage) across the array connections and another DMM (DC current) in series with the load.

3b. Write down the measured Voltage and Current whilst varying the load.

3c. By calculating the power for each registered measuring point (P = V x I) we can quickly determine the approximate Max Power Point. Approx MPP: _____V

3d. An alternative (quick and dirty) way to get approximate MPP is to calculate;

Vmpp = Voc x 0.8, Impp = Isc x 0.9

4. Select suitable connection points for the fixed resistors, allowing to focus measuring around the MPP (from 3c). Slowly adjust the variable resistor while writing down the voltages and currents.

I try to aim for 0.1V jumps between measurements.

5. Repeat the power calculation above and determine Vmpp and Impp (where Max Power is).

6. Might be interesting to see how the measured MPP compares to calculated MPP;

Measured MPP; Vmpp = _____V, Impp = _____A

Calculated MPP; Vmpp = Voc x 0.8 = _____V, Impp = Isc x 0.9 = _____A

7. One may, just for fun, calculate the Fill Factor at this point, FF = (Vmpp x Impp) / (Voc x Isc)

Step 7: Adjust the MPPT Module to Suit Your Needs

Above we selected our desired output voltage, this along with the parameters derived in 2.1 will be vital for correctly adjusting the MPPT module. We also need to know the max charging current (Ichg) and at which current charging is considered done (Idone).

Vmpp: _____V / Vout: _____V / Ichg: _____A / Idone: _____A

Procedure:

1. Connect a DMM to the MPPT output (set to DC voltage)

2. Turn the CC and CV trimpots fully clockwise, turn the MPPT trimpot fully anti-clockwise

3. Connect an adjustable DC voltage source to the MPPT input, set voltage to zero before turning on.

4. Set the adjustable DC voltage source to Vmpp, slowly turn the MPPT trimpot clockwise until the output voltage just stops increasing.

5. Turn the CV trimpot anti-clockwise until desired Vout is set.

6. Short-circuit the output through a DMM (set to DC current 10A). Turn the CC trimpot anti-clockwise until desired Ichg is set.

7. LED trimpot adjusts at which current the LED will change color, default is 0.1 x Ichg. To adjust, connect a load that gives Idone, turn the LED trimpot until the LED changes color.

Note: Nothing will actually happen apart from the LED changing color.

8. The MPPT module is now adjusted and ready for use.

Step 8: Walk-through, My Array 1

Specifications:

Panel: CNC145x145-6, 4 panels in series.

Dimensions: 145x145x3mm

Ratings: 6V / 3W per panel. 4 panels: 24V / 12W

1. Gather the stuff needed.

2. Schottky diodes and panel connections are already in place.

3. Measuring setup as shown.

4. I start off measuring Voc and Isc.

5. Next I mess around a bit with the load to get an approximate MPP.

6. I reconfigure my fixed resistors so I can focus my measurements around MPP, I did two series to try to pinpoint the exact MPP.

Results:

Voc: 25.9V / Isc: 325mA

Vmpp: 20.0V / Impp: 290mA

Calculated Pmpp: Vmpp x Impp = 5.8W

Just for fun and comparison:
Calculated MPP; Vmpp = Voc x 0.8 = 20.7V, Impp = Isc x 0.9 = 292mA

Fill Factor: FF = (Vmpp x Impp) / (Voc x Isc) = 0.69

Unfortunately I seem to have misplaced the excel work sheet I was using, so no graphs or recorded series for this panel array.

MPPT module adjustment:

Next is the MPPT module adjustments.

When selecting Vout I decided that I can either sharge a 12V Li-Ion battery, or connect the output to a 5V/2A USB charging module (input 7.5-28VDC).

The MPPT module was adjusted using the following parameters:

Vin = 20.0V / Vout = 12.3V / Ichg = 600mA / Idone = 100mA

1. I ”reset” the trimpots as described, connect my DMMs, and set my adjustable DC voltage source to Vin = 20.0V

2. I adjust MPPT trimpot until the output voltage just stops rising, next using the CV trimpot to set the output voltage to Vchg = 12.3V

3. Short-circuiting the output through a DMM (set to DC current 10A) I adjust the CC trimpot to Ichg = 600mA

4. Connecting my resistor load I adjust the load until I get output current = Idone = 100mA, next adjusting the LED trimpot so that the LED just changes color.

5. Varying the load confirms that the LED changes color as intended. DONE!

Step 9: Results - My Array 2

Specifications:

Panel: CNC170x170-18, 6 panels in parallell.

Dimensions: 170x170x3mm

Ratings: 18V / 4.5W per panel. 6 panels: 18V / 27W

Results:

Voc: 20.2V / Isc: 838mA

Vmpp: 15.6V / Impp: 821mA

Calculated Pmpp: Vmpp x Impp = 12.8W

The panel array delivers lightly less than half of the rated power.

MPPT adjustments:

The MPPT module was adjusted using the following parameters:

Vin = 15.6V / Vout = 13.6V / Ichg = 850mA / Idone = 100mA

Step 10: Results - My Array 3

Specifications:

Panel: CNC170x170-18, 4 panels in parallell.

Dimensions: 170x170x3mm

Ratings: 18V / 4.5W per panel. 4 panels: 18V / 18W

Results:

Voc: 20.5V / Isc: 540mA

Vmpp: 15.8V / Impp: 510mA

Calculated Pmpp: Vmpp x Impp = 8.1W

The panel array delivers lightly less than half of the rated power.

MPPT adjustments:

The MPPT module was adjusted using the following parameters:

Vin = 15.8V / Vout = 13.6V / Ichg = 550mA / Idone = 100mA

Step 11: Results - My Array 3 (cloudy Day)

Specifications:

Panel: CNC170x170-18, 4 panels in parallell.

Dimensions: 170x170x3mm

Ratings: 18V / 4.5W per panel. 4 panels: 18V / 18W

Results:

Voc: 18.3V / Isc: 29mA

Vmpp: 14.2V / Impp: 26mA

Calculated Pmpp: Vmpp x Impp = 0.37W

Same array and setup as used in previous step, but with clearly different results.

Compared to achieved output on a sunny day it’s quite clear these panels will not be of much use under cloudy conditions.

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