Energy harvesting with “tiny” solar cells for IoT projects – part 2

This article is the continued investigation of the Texas Instruments BQ25570, the MPPT solar charging chip. In the following article, I will present my field tests as well as their results. I tested it with two types of solar panels, of various sizes. The tests were performed indoors as well as outdoors. The results are striking and much better than I anticipated.

If your starting hypothesis was right and your lab test results were valid, then your field outcomes should be similar. Maybe that is not always the case, but at least it was for me this time 😌. I must admit, we had surprisingly good weather these days in Sarajevo. The temperatures were considerably high, hence most of the outdoor tests I performed on the nearby mountain, Trebevic. I understand, not everyone lives in the south, where it is bright and sunny, therefore I will provide you with tests with solar panels that can “compensate” for cloudy days.

For the test, I used two distinct battery capacities, 110mA and 800mA. Two different kinds of solar panels were used in the tests, monocrystalline and polycrystalline solar panels. The reason why I used these two types, is simply that I had both lying around. I used them earlier for some experiments. The first used in the experiment were polycrystalline solar cells. You can see the first solar cell in figure 2.

Indoor and outdoor tests with polycrystalline solar cells

The rated power for this solar cell is P=0.18W. Their open circuit voltage is 2V and short circuit current 90 mA, according to their specifications. I think I ordered them from AliExpress, just search for: “2V 0.18W 90mA” and you will find them. Each cost around 1$ USD. I connected two of them in parallel just to make sure there is enough current supplying the circuit. The first performed test was indoors. I put the solar panel close to the window. The window is located on the east side.

I didn’t use any data logger but rather a cheap Chinese voltage meter, which can be seen in figure 3. My basic idea was to capture the states, i.e. the voltage levels with my camera. That would record the time when I took the picture. In every picture, I am measuring the voltage of the battery. After some time I took the second picture, figure 4.

It was obvious, the circuit works flawlessly. The BQ25570 boosts the voltage, of ~1.8-1.9V, to the required voltage to charge the battery. I was surprised how quickly the battery was charged by these two small cells. It was getting warmer inside. I decided to go outside, to the mountains because it would be cooler. Consequently, the experiment continued on the mountain. The MPPT algorithm operation can be seen in the video below.

Live MPPT operation of the BQ25570

In the video, I will be measuring the voltage at the solar cells, to see the MPPT algorithm in action. As you can see, the IC finds the maximum power point (MPP) at around ~1.5V since the solar panel is in the shadow. In other words, that means the open-circuit voltage was at ~1.875V because of 1.875V*80%=1.5V. Consequently, the MPP voltage should grow when I place the solar panel in the sun. Once I move the solar cells into the sun, the solar cells generate higher voltage and the maximum power point is at ~1.8V. As a result, the open-circuit voltage was at around ~2,25V. This proves the MPPT operation as described in the previous article.

When I measured the short circuit current of the solar cells outdoors and indoors, I noted a difference. The solar cells outdoors produced almost the double amount of current when the solar rays hit them compared to when indoors. I did not do any research on it why it behaves that way. Perhaps glass reflects some light, refraction may be the reason as well or glass filters out certain wavelengths, I am doubtful about what causes this. Possibly, the angle of the solar cell was better-directed towards the sun while outdoors.

Next time I will measure the temperature of the solar panels. The temperature may affect the amount of produced current. I observed the effect while I was driving my car. The solar cell was placed at the front of the front windshield where the air-conditioned air came out. During the 30-40 minutes drive in the sun, the battery charged surprisingly fast.

Monocrystalline cells and what to do when it is cloudy

The above mentioned, polycrystalline, solar cells have a small size. When it is not sunny, the circuit works however, the generated current is insignificant. As a result, of small amounts of generated current, it would take an “extensive” amount of time to charge your battery. Is there a solution to that problem? Yes, there is. This may not be the optimal solution for our circuit but it will do the job. Whenever you have to deal with any limits or boundaries of something, always consider the worst-case analysis. In this case, simply make an excessive current generating system. Simply said, find solar cells that produce more current in cloudy weather, even if in the sunnier days you can not optimally use the excessively generated current (at least not with the BQ25570.)

Monocrystalline solar cells are more efficient as opposed to the polycrystalline cells. Luckily, I had a couple left from a previous project. They were expensive and of a high-quality American brand, SUNPOWER. As the saying goes, you get what you pay for! It is true in the solar market as well.

One cell produces around ~0.6V in open-circuit and a current of ~2.5A when short-circuited. Thus, to get a similar voltage of ~1.8V, I connected three of these cells in series. Obviously, they are bigger and it is sound to say with a greater surface more power will be generated. However, this excessive power can not be used by our chip! This would probably operate well in the winter months or in locations where you get smaller quantities of solar irradiance. Nevertheless, this circuit has also worked great but it is far from being the optimal solution or well-used resources.

Comparison of results

Conclusion

The BQ25570 works as specified in the datasheet. I was surprised by how well it works indoors. Even tiny solar cells can be used to extract their energy and charge lithium-ion batteries. It stops charging once it enters the saturation charge zone, to be exact at 4.16V. It can be used both indoors and outdoors to charge your battery for your IoT system. The tests have been performed in July and August, summer month. A solution was given for winter months but it is not optimal.