My First Solar Cell Project (Prototype)

This is a small-scale prototype PV (PhotoVoltaics) project before making a decent and practically useful home solar panel system later.

Therefore, this is the testing purpose of basic circuits made for understanding the operational scheme of a solar cell.

The overall circuit configuration is very basic and power is rated less than 2W (using 5V and 400mA solar cell).

As this is the first time for me to utilize solar cells in a DIY project, I'm wondering how this component is working and really any electricity is produced from it.

I'm using only one small solar panel and produced electricity from it is stored in a single 18650 battery through the TP4056 charger module.

With this circuit, I can understand the basic operational characteristics of a solar cell as well as the method of storing energy in a lithium-ion battery.

Through experiences and knowledge with this circuit, I can make a more useful solar energy generating system someday.

As shown in the schematics above, the solar power generating circuit itself is simple.

As I'm interested in the operational scheme of the solar cell, several measuring circuits are included such as a voltage meter, ampere meter, and battery capacity level meter.

I'll explain each measurement circuit one by one in the later step or at the other instructable as each device is a little bit complex.

The operational scheme of this solar circuit is simple like below.

-     Solar cell produces about 50mA current when it receives sunlight (Although the solar cell specification claims a maximum of 400mA, the actual current I got under the sunlight through windows is 50mA)

-     TP4056 lithium-ion battery charger circuit storing electric energy from the solar cell to 18650 battery (This module prevents overcharging and over-discharging of 18650 battery while operating)

-     The 18650 battery storing energy from TP4056

-     Battery level indicator circuit showing the capacity level of 18650 battery (This circuit was explained in other Instructables)

-     1Watt LED circuit is a load of this solar circuit and consumes energy stored in the 18650 battery

As this kind of solar circuit that charging battery is very common, maybe no further explanation will be necessary.

The most important part is of course solar panels.

For making this experimental solar circuit, I chose a small size solar panel that's dimension is 125mm x 125mm.

The following are used parts.

-     5V and 400mA solar panel

-     TP4056 single 18650 battery charger break-out

-     20L15T Schottky diode as blocking current from battery to panel (this is a 20A device but you don't need this much high capacity diode. I only have this device in my inventory)

-     2900mAh Samsung 18650 battery

-     1W LED as the testing load

-     Others (Battery level indicator, voltage meter, and current meter circuits)

According to the TP4056 charger IC specification sheet, an additional blocking diode (such as the 20L15T Schottky diode listed above) is not necessary.

Internally power MOSFET is included in the TP4056 and the MOSFET includes a parasite diode that can be acting as a blocking diode role.

But for the reliable solar circuit operation, this additional Schottky diode is used as a safety measure.

As the circuit is simple, no additional wiring diagram will be necessary for explaining the part connection details.

The solar panel is connected with the TP4056 charger via the Schottky diode.

The diode prevents current flow from the battery to the solar panel during night hours.

The 18650 battery is connected to the Battery(+/-) pin and the TP4056 charge current to the 18650 with a regulated 4.1V level. (So output power from the solar panel is less than 4.1V, actually charging seems not occurring)

For a load of battery (Stored energy consumer), 18650 battery level circuit and 1W power LED are used as details explained in the following instructable.

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https://www.instructables.com/1W-Power-LED-Flasher/

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As I was curious about how much current can be produced from the solar panel, voltage and current measuring meters are produced and connected. (I'll explain details about these meters later)

Firstly I'm supposing 5V is produced by the solar panel I bought.

As the specification of the solar panel is rated as 5V and claims to produce 400mA, I naively assume that a constant 5V can be observed when the panel is exposed to sunlight.

But to my surprise, during the daybreak and sunset, the output voltage is less than 3V as shown in the graph above.

Even under the sunlight, the voltage produced continually changes from 3.3V to 4.9V.

The voltage measurement is conducted from 3:30 pm ~ 6:40 pm under the sunlight on a very bright day.

I don't know why the voltage is changing continually under the clear and bright sunlight. (Really that day is very clear and without any cloud)

The output voltage is measured every 5 seconds with an Arduino voltage meter which will be explained in the next step.

Anyway, I can see the actual solar panel operational scheme with the voltage meter.

As this is my first project with the solar circuit, I want to know detail about the solar panel operational scheme.

Therefore, I want to see more detail about the electrical characteristics of solar panels.

That's why I made this voltage meter circuit.

As the result of voltage measuring, the solar panel is highly dependent on the strength of sunlight. (Of course, this is a very simple truth even a fool can understand)

But after I measured the actual voltage output, I can understand solar panel is not constant energy generator.

Without this Arduino voltmeter, you can measure voltage with a simple tester that can measure DC voltage.

But to know more detail about voltage change trend according to the sunlight condition changes, RTC (Real Time Clock) and SD card data storage is essential.

By collecting timestamps and measured voltage results, you can understand the actual working details of the solar panel.

That's why I'm making this device as part of this solar circuit project.

This is typical data logging Arduino circuit using DS3231 RTC and SD card break-out modules.

As a result, I can understand that the solar panel produces less than 4.1V output voltage during quite significant times.

As the TP4056 is charging the battery only at the regulated 4.1V level, the charging circuit is idly stand-by about 1/3 times of solar panel operation.

Therefore, the 18650 battery level increased by less than 100mAh for all day long. (Increasing battery voltage from 3.5V to 3.6V)

This is a quite disappointing result with the 128mm x 128mm size solar panel.

Maybe I need a bigger and more efficient solar panel for fully charging the 18650 battery within 8~5 daylight hours.

As measuring voltage is not complex by using a voltage divider circuit, the sketch program logic is also straightforward.

Per 5 seconds, Arduino is measuring the changed resistance value from the A0 analog port by the variance of input voltage.    

Then data is stored in the SD card with a timestamp created by RTC.

This voltage meter can be a useful device when you are necessary to measure any voltage change on a long-term base.

As mentioned at the head of this story, this is an experimental project and makes a testing solar circuit.

My future plan is to make a 20~30 watt garden lamp to lighten my home in the country.

As shown in the picture above, a higher power of parts will be necessary for making a practical street or garden lamp.

Before making some useful appliances, I need to know more about Photovoltaic devices and components.

That's why I'm making these small-scale solar circuits by myself.

Anyway, I learn a little bit of helpful knowledge for making electric energy from the solar cell in this project.

Thank you for reading.