# Maximum Power Point Tracking for Solar Cell

1. May 6, 2014

### Nabeel

have two solar cells which I have connected in Parallel to increase the current provided to the system. I want to calculate the maximum Power point tracking for my solar cells. There was no information provided by the seller except the one below.

I have attached the open circuit reading of the cells under a lamp. If you can assist me in this regard it will be very much appreciated.

Also, if these cells can be used to charge a 3.7V battery?

Each Solar Cell specifications provided by the seller are:
Each one is a 5V
2.5W Power
500mah Mini Encapsulated Solar Cell Epoxy
Size is 130×150mm
92g each
Conversion rate of 17%

2. May 6, 2014

### Staff: Mentor

There is no attachment.
If you have enough light and make sure the battery does not get overcharged, probably.

What do you mean with "calculate [...] tracking"? Do you want to calculate something, or track something?

3. May 6, 2014

### Staff: Mentor

You can consider determining the MPP experimentally for various insolation rates. You will need to be able to vary the load resistance and measure the output voltage and current. Graph about 10 points for each level of sun intensity (you can use several semi-opaque sheets to simulate lower insolation conditions, or just wait for less-bright days or times of day). From the graphs you can calculate the MPP for each insolation condition, and decide where you want your DC-DC converter circuit to set its input conditions.

BTW, keep in mind that your battery charger circuit must be intelligent -- it cannot blindly charge the battery or it will over-charge it. Maybe do some reading about optimum battery charger circuits for the type of battery that you want to charge.

4. May 6, 2014

### mjhilger

I would suggest that you look into creating a system that determines the MPP dynamically at run time. I say this because many variables factor into the point that change your cells output characteristics, dirt on the glass, outside temp, angle, haze, etc. Since you want to charge 3.7 volt cell (I will put myself at risk at assume Li-xx), and this really needs some intelligence as was suggested above, use a microcontroller to both charge and determine the MPP at the same time. This will involve using the A/D to measure voltage and current and manipulating the load to move the point to the max output while charging. I believe it might take a little more work, but the end result will be a much more capable system.

5. May 6, 2014

### Staff: Mentor

Good suggestion!

6. May 6, 2014

7. May 6, 2014

### Baluncore

The maximum power point is really only useful if you have a switching converter and can move the load point.

With a linear controller you may operate the cells in a short circuit mode for maximum current.

Use the +5V solar cells in parallel, to supply a 3.7V low drop out linear regulator, which will prevent overcharging of the cell.

8. May 6, 2014

### Staff: Mentor

Interesting. When the OP asked about the MPP, I assumed he was asking about DC-DC converters. It never occurred to me that he might be thinking about a linear regulator. But based on the nature of his question (Nabeel seems new to electronics), you might be right...

9. May 8, 2014

### Nabeel

I do have non-inverting buck/boost converter to which I will input the voltage from the solar cells(connected in parallel). I will buck down the voltage to 3.7V and if the voltage at the input falls below 3.7V the converter will boost the voltage to 3.7V. I just needed to find a way so that I can input maximum power from the solar cells into the converter so that I can achieve maximum current.

10. May 9, 2014

### mjhilger

Let me first state that these next few words are my opinion on a design philosophy to maximize power from a solar cell.

The design request is to operate a solar cell using MPP to charge a battery pack. I view this a an advanced circuit. The reason I state this is that in order to actually maximize the power output using a switcher, you will need to operate with two (or more) inductors and two (or more) switches so ALL the solar power is used. A "buck" switcher does not pull energy from the source when the switch is off. So to actually maximize energy use and keep the voltage output constant, you need to grab the energy available while one switch is off with another inductor and switch on. (Actually you might need more than 2, but two should achieve a high efficiency transfer.) Using a microcontroller to keep track of the switching and charging will allow this, but you will need to watch the coding to move the switching times (to find MPP) and operate more than one switch AND monitor charge at the same time. You could use more than one micro, but honestly one should be enough, you will just need to write good code that has bounded latency for interrupt servicing and carefully monitor the voltage and current to achieve the MPP point on the power curve of the SYSTEM (this point will change dynamically as the pack gets charged). So again I will say that while this is possible and will be great fun and learning experience for the OP, it is by no means trivial in my opinion.

11. May 9, 2014

### Baluncore

As mjhilger suggests, with a switching regulator, it will be necessary to use a capacitor in parallel with the cell and regulator input. An inductor between the cell and capacitor could reduce ripple further but only if the inductor had a very low series resistance.

The problem presented in this thread is not typical since the cell voltage is close to the regulated output voltage. It is my opinion that in this special case an MPP sensor and a switcher are unnecessary.

I will try to explain why a boost/buck converter with MPP tracking is not needed. Under low light conditions, when the MPP and open cell voltage are below about 4V, there will be insufficient power from the cells to run a switching converter. A 5V solar cell in good light should always have MPP above 3.7V. That more or less eliminates any need for a voltage boost component.

A linear regulator is low cost, simple and EM quiet. In this case the minimum efficiency of a low-dropout-voltage linear-regulator will be 3.7/5.0 = 74%. Under lower light conditions, (for a 100 mV drop-out), efficiency will approach 3.7/(3.7 + 0.1) = 97%.

For relatively high input voltages, a 3.7V output switching regulator, (using an idler diode with a 1V forward voltage drop), will be about 3.7/(3.7+1.0) = 79%. This is not much better than a linear regulator with low voltage input.

But for a 5V input, with 3.7V output, a buck converter employing an idler diode will have a switched-on duty cycle of 3.7 / 5.0 = 0.74, 0.26 idle. That gives an overall efficiency near 79%*0.26 + 95%*0.74 = 91%.
By replacing the output idler diode with a mosfet as a synchronous rectifier, the efficiency of a switching buck converter will be closer to 95%.

If by wiring the two cells in series, rather than in parallel, the regulator input voltage was increased to 10 volts, then the game would change. A linear regulator could no longer be considered, a buck switching converter, (possibly with MPP optimisation), would then be necessary to maintain efficiency. With a 3.7V output, a switching regulator should employ a synchronous rectifier as the idler diode to maximise efficiency.