Introduction
Solar panel generate electricity when it is placed in bright sunlight. The time of energy production is dependent on certain parameters like ambient temperature, the incident ray angle and the material of the solar cell. This energy is not produced at night. To use this energy at night we have to store this energy in the battery. The battery is charged when there is enough energy during the day. And then the stored energy can be used anytime to power electronic equipment.
The system will store solar energy in the lead-acid battery by charging it and also keeping the control system active, when the solar energy is not there the control system will take its energy from the battery which will discharge the battery. After the battery is discharged it will charge when solar energy will be available.
During the daytime or when solar energy is available the control system along with peripheral equipment are powered directly by the solar. And when solar energy is not available the battery is used. So, in a way, the system has a redundant power supply. Which reduces the overall downtime of the system.
The peripherals connected are current-limited and short-circuit protected. Which increases the durability of the peripheral by minimizing the damage. When the system is overloaded or a short-circuit event occurs the system safely cuts off power and generates a fault signal which can be reset only by manual triggering.
Battery Information
A lead-acid battery is selected as the energy storage device because of its fault-tolerant nature and overall reliability.
Manufacturer: EXIDE
Nominal Voltage without load: 13V (approx.)
Number of cells: 6
After quite a bit of reading, I have found a safety tip about all the batteries from a safety standpoint it is better to undercharge them rather than overcharge them; overcharging deteriorates at a faster rate and also overcharging causes security concerns such as heating.
So, according to cycle use, we can use 2.38V to 2.43V per cell for charging them.
So for a 6-cell battery that will be 14.28V to 14.58V.
I selected 14.2V as the charging voltage.
Generally Charging of the battery is done in three steps
- Constant Current mode
- Constant voltage mode
- constant current mode (Lower current limit mode 1 )
To charge the battery it is important to know the charge rate parameter.
Charging rate calculations
battery capacity = 7000mAh
charging current = 290mA
To charge a fully discharged battery to its full capacity it will take
battery capacity 7000mAh
———————- = —————– = 24 hour
charging current 290mA
C rate = 0.0414C
Most sealed lead acid battery manufacturers specify that the best method of charging the battery is the current limited constant voltage method.
I have chosen to employ only current limited constant voltage mode.
I have a constant load of 20mA attached to the system at all times whether it is day or night.
Since the system automatically detects day and night. There are different loads that are attached according to the mode.
During the daytime, I have a LED panel of 30 3V white LEDs which are fed by 9V and consume around 30mA.
So in Day time, the total load is around 60mA. There are some other components which also consume power.
And at night time the total load is 30mA + 15mA(modified LED light bulb). So, 45mA
Making a general assumption that the 290mA total current is available 8 hours during the day from the solar panel.
The 290mA will be split into two components charging current and the load current.
Since I have only one sensing resistor employed so the load current will also be added with the charging current in the calculation. That is why I am splitting the 290mA.
So, during the daytime, I have around 290mA – 60 mA= 230mA available for charging the battery.
The battery mostly gets discharged during the early morning and in the evening and in night.
So a total of 16 hours.
in 16 hours x 45 mA = 720mAh will be discharged from the battery.
To recharge this amount at 230mA.
time to recharge 720mAh/230ma = 3 hours (approximately)
SOLAR PANEL
I have a 180 Watt solar panel on the roof. Which provides me with
Open circuit voltage of 21V
Maximum short circuit current of around 10A.
Try to think of solar panels as a constant power device.
So I have 180Watt at my disposal. This is not entirely true it will be in test conditions which are not always present.
Sometimes we will have 6A and sometimes 2A time even 1A. the only constant thing is the voltage.
The point to remember when dealing with the solar panel is that it only produces enough energy to power your system. That means if you have a solar panel capable of producing 180Watts and the load only needs 1Watt. It is only producing 1W at that time.
To understand solar PV characteristics.
Voc(open circuit voltage ): This is the voltage which is produced when there is no load attached to the panel.
Isc(Short Circuit Current): This is the current which is produced when there is a short circuit.
Now, look at the blue curve which is the power curve. It just multiplies the current with the voltage.
Look at the power at point Isc, 0.
Look at the power at point Voc, 0.
The power is maximum when the Voltage is less than Voc. That will happen when there is a load attached to the system. But the current produced by the system is dependent upon the various parameters such as temp, angle of incidence and position of solar panel etc.
Let’s say we have a panel which is capable of producing 1A and the load needs only 0.5A. Where does the other 0.5A go? Well, the answer to this question is that it is not produced so, it will not damage anything. This answer is very basic and there is a lot of analytical physics behind it. Which is outside the scope of my project.
Here is what my panel specification looks like
Circuit Diagram
R2 sets the current limit.
It is based on the forward bias voltage of the base-emitter p-n junction of transistor Q1 which is 0.7V(approx) at 25 degrees Celsius.
R2 = 0.7/Current limit
For 290mA
R2 = 0.7/0.29 = 2.4(approx)
LM317T needs a proper heatsink operating at 290mA. It can handle current up to 1A safely but you have to put on a decent chunk of aluminium on it. And also the system needs to be well ventilated.
As the battery will charge the charging current will drop. At the beginning charging current will be a maximum of 290mA.
I have used two types of diode one is 1N4007 and the other is 1N4004. So this circuit is based on them.
Diode D2 and D3 make an OR Gate. This configuration is also known as diode ORing.
You can make this using the Schottky diode which lowers the forward voltage.
B1 is a 12 V Sealed lead-acid battery. You can increase the system capacity by connecting the battery in a parallel configuration.
RV1 is a potentiometer in a variable resistance configuration to set the output voltage of the LM317T.
D5 is used to protect the system reverse current when the solar panel voltage is below the battery voltage.
D7 is used for flyback protection from voltage spikes.
D9 is not connected to anything it is just there.
All the transistors are BC546B.
All the capacitors despite their capacitance are rated for 50Vdc.
Here is the photograph of the night led bulb.
solar automatic day and night light with a manual alarm circuit
I have added a manual alarm to this project and an SPDT switch from which you can control the light in day or night mode. The alarm will be turned on at the Sunrise. It will remain ON till you manually switch it OFF.
Since it is continuous it will be hard on your ears and will force you to wake up and switch it OFF.
The alarm has to be turned back on at night so that it can sound at sunrise.
Battery Readings
These are the reading that I took from the battery using a multimeter.
date time event
23/7 7:35PM night lights on
11:05PM battery voltage check 12.9V
24/7 2:01AM battery voltage check 12.85V
24/7 5:35AM sunrise night light OFF
7:35PM Night Light On
7:35PM battery voltage check 13.12V
7:44PM battery voltage check 13.11V
9:17PM battery check 13.07V
25/7 12:44AM battery check 13V
12:53 PM 7/25/2022 battery check 13.7V
2:23 PM 7/25/2022 battery check 13.7V
7:40 PM 7/25/2022 battery check 13.14V
8:36 PM 7/25/2022 bettery check 13.11V
26/7 12:42 AM 7/26/2022 battery check 13.06V
3:45 PM 7/26/2022 battery check 13.72V
27/7 1:33 AM 7/27/2022 battery check 13.04V
5:03 AM 7/27/2022 12.99V
6:48 AM 7/27/2022 13.26V
7:17 PM 7/27/2022 13.07
7:27 PM 7/27/2022 13.11
11:55 PM 7/27/2022 13.02
28/7 1:00 AM 7/28/2022 13
8:03 PM 7/28/2022 13.07
29/7 11:40 AM 7/29/2022 13.61
7:30 PM 7/29/2022 13.07
8:22 PM 7/29/2022 13.04
31/7 9:00 PM 7/31/2022 13.06
10:33 PM 7/31/2022 13.02
From these observations, I can observe that the system is working. It is charging and maintaining the charge. The charge is useable at nighttime.