Solar controller based on PICl6F676

Abstract: A simple solar controller is designed to conveniently control the charging and discharging of the battery. The controller uses a low-power PICl6F676 microcontroller as the core of the entire control circuit, monitors and controls the voltage at both ends of the battery in real time, and uses a liquid crystal screen to intuitively display its voltage and power. A temperature compensation diode is used to overcome the change in battery terminal voltage caused by changes in ambient temperature, and an "adaptive three-stage" charging mode is adopted. By calling the corresponding charging and discharging electronic program under different conditions, the battery is charged and discharged in the best way, which better protects the battery.
Keywords: solar controller; temperature compensation; bridge rectifier; hysteresis effect; PlCl6F676

In recent years, as the limited resources such as oil and coal on the earth have been developed and utilized in large quantities, resulting in energy shortages, people have paid more and more attention to the use of renewable energy, namely solar energy. This paper designs a small solar controller that can directly couple solar panels with 16 V batteries. It uses a low-power microcontroller PICl6F676 as the core of the control loop to monitor the terminal voltage of the battery in real time. Under different conditions, different methods are used to intelligently control the charging and discharging of the battery, improve the utilization efficiency of solar cells, and extend the service life of the battery.

1 Solar Panel Volt-ampere Characteristics
The volt-ampere characteristic curve of solar panels is the basis for analyzing indicators such as circuit design, system optimization operation reliability, service life and operation cost in photovoltaic systems, and is the main parameter of solar cells. Figure 1 shows 8 sets of solar panel data UI curves and corresponding PU curves measured under different environments. Series 1 to Series 5 are curves obtained by solar panels under different sunlight intensities, Series 6 is the curve obtained when the solar panel faces away from the sunlight, and Series 7 and Series 8 are curves obtained under energy-saving lighting and ordinary lighting, respectively.

To obtain the maximum power in a solar power generation system, it is necessary to track the sunlight intensity and ambient temperature conditions, and continuously change the load impedance to achieve the best match between the array and the load, thereby improving the system efficiency. Commonly used control methods include CVT (constant voltage tracking) and MPPT (maximum power point tracking).

2 Solar Control System
The solar control system is mainly composed of solar cells, batteries, single-chip control modules, display devices, temperature compensation modules, loads and other external components. The control system structure diagram is shown in Figure 2.

As shown in Figure 2, the solar cell is controlled by the microcontroller control module to charge the battery and supply power to the external circuit load. At the same time, considering the changes in the battery's environment, the temperature compensation control circuit is used to protect the battery.

3 Hardware circuit of solar controller
3.1 Voltage acquisition module
The voltage is divided by the voltage-dividing resistors at both ends of the battery, and the collected signal is input into the single-chip microcomputer. In order to reduce the nonlinear error of conventional resistors, 4 2 kΩ precision resistors are used to divide the 1/4 battery voltage as the analysis voltage of the single-chip microcomputer A/D conversion, thereby reducing the error of the analysis voltage. When the voltage is collected, the sudden change of the external ambient light will cause the single-chip microcomputer to misjudge and affect the error control of the charging circuit. Therefore, it is necessary to add an appropriate delay function to the software to distinguish the "authenticity" of the ambient light.
3.2 Battery voltage and power display module
The collected and processed voltage is output to the LCD LCDl602 for display. The power consumption per square centimeter of the LCDl602 liquid crystal screen is μA level, and the display is stable. When considering the power display, since the capacity of the battery is related to its terminal voltage, the voltage difference between the over-discharge UMin and over-charge UMax of the battery can be indirectly and approximately linearly divided to obtain the power S:

In the formula, U is the actual voltage across the battery.
3.3 Controller input and output control module
3.3.1 Input module
The input module is the solar cell charging module. Since the solar cell panel is affected by light and the properties of its own material, its charging current has certain fluctuations. If the generated current is directly charged into the battery or directly supplied to the load, it is easy to cause damage to the battery and the load, seriously reducing its service life. Therefore, it is necessary to control its charging part. Here, the "adaptive three-stage charging mode" is adopted: 1) In the charging stage, the battery voltage U is relatively low, less than 12.5 V, and constant current charging is used; 2) When the battery voltage U reaches 12.5 V, it enters the constant voltage floating charge state (achieved by controlling the duty cycle); when the current drops to the set value (judged by comparing the voltage difference across the high-precision resistor with the set value), U reaches 14.5 V at this time, and constant current charging is performed: 3) When U reaches the set overcharge voltage of 16 V, constant voltage trickle charging is performed. When the trickle current is small to a certain extent (judged by comparing the voltage difference across the high-precision resistor with the set value), the charging circuit is cut off. Figure 3 is a circuit for charging a battery from a solar panel. The filter unit is composed of a single-phase bridge rectifier circuit VD and an inductor L filter circuit. VD allows the solar panel to always charge the battery, while the battery will not supply power to the solar cell. When the solar panel generates AC power when the light changes suddenly or is reversely connected to the battery, it can only charge the battery. The filter circuit L filters out the ripple of the output voltage after rectification, making the charging current more stable.

Figure 4 is a temperature compensation circuit. It is known that in areas with large temperature changes, the battery capacity will change accordingly, and the previously set charging conditions are no longer suitable. The above points need to be corrected accordingly (software programming settings), otherwise the battery life will also be reduced. Therefore, it is necessary to add this temperature compensation module. The temperature compensation circuit is mainly composed of a temperature compensation diode, and its PN junction voltage is directly added to the port. The diode is used for temperature compensation by using the characteristic that the PN junction forward voltage drop will decrease by 2 to 2.5 mV for every 1°C increase in temperature near room temperature.

Of course, household batteries cannot always be protected from overcharge by controllers. For general batteries, it is also necessary to consider the battery acid stratification caused by long-term charging and discharging, gassing and water loss, which causes the battery capacity to decay and produce memory effect. Therefore, it is necessary to regularly shield the battery overcharge protection to overcharge the voltage to reduce the above adverse effects.
3.3.2 Output module
The output module is the module that supplies power to the load from the battery. When the battery meets the discharge conditions, the controller will open the relay on the same path of discharge to supply power to the load.

Figure 5 shows a two-stage electrical protection device to prevent external short circuits or other situations from causing a sudden increase in current and damaging the battery and other electronic components. The first-stage protection (software and hardware method) collects the voltage across the high-precision resistor. When the product of the set maximum discharge current and the resistance value of the high-precision resistor (i.e., the voltage across the two ends) is met and lasts for 20 seconds, the short circuit is confirmed. At this time, the microcontroller controls to cut off the discharge circuit. The second-stage protection (hardware protection) is to prevent the first-stage current from being too large and not lasting for 20 seconds, which will cause damage to the circuit. At this time, the SR30 series resettable fuse is added. When the current flowing through reaches the rated value. The temperature of the resettable fuse rises, the resistance increases rapidly, and the current decreases rapidly. When the overcurrent disappears, the electronic fuse automatically returns to the initial state, without the need for manual replacement, which simplifies the maintenance of the controller and greatly improves the safety performance of the system.

3.4 Voltage stabilization circuit
The voltage stabilization circuit is composed of high-precision resistors, voltage regulators L7809CV, L7805CV, filter capacitors and other components, as shown in Figure 6. In Figure 6, four 2 kΩ precision resistors are used to divide 1/4 of the battery voltage as the A/D conversion analysis voltage of the microcontroller, thereby reducing the error of the analysis voltage. Considering that the battery voltage is actually controlled between 11.5 and 16 V, and the optimal voltage difference of the three-stage integrated voltage regulator is 2.5~4 V. First, use L7809CV to stabilize the battery voltage at 9 V, and then use L7805CV to stabilize the 9 V voltage at 5 V. In this way, the maximum voltage difference of L7809CV is 6 V, and the voltage difference of L7805-CV is 4 V. Attaching a heat sink to the voltage regulator can ensure that the voltage regulator will not be damaged due to overheating. In order to improve the stability of the microcontroller, two 5 V power supplies are used to supply power to the microcontroller and the relay respectively to avoid mutual interference.

4 Software design of solar controller
Figure 7 shows the control flow of the solar controller software design.

During trickle charging, the charging circuit is closed only when the detected trickle current is smaller than the set value; during temperature compensation, △μ (positive or negative) is the change value of the corresponding charge and discharge point voltage value in the program. It is also necessary to consider the impact of sudden changes in ambient light intensity and sudden changes in instantaneous current of electricity consumption on system control, so it is necessary to add appropriate delay functions to the software.
In addition, during the test, the "hysteresis effect" of the battery is encountered, that is, when the battery is at the overcharge point and overdischarge point, due to the existence of the power load, the power supply system continues to oscillate at the protection value, which will cause damage to the electronic components. Therefore, when designing the program, it is necessary to "determine whether it is the first power-on", call subroutines with different critical values, and make it work normally when the voltage at both ends of the battery drops or rises to the specified value.

5 Conclusion
A solar controller based on PICl6F676 microcontroller is designed, which has low power consumption and stable performance. It uses the "adaptive three-stage charging mode" to automatically control the battery charging and discharging in the best way. The temperature compensation function is considered to ensure that various "charging and discharging points" can be automatically changed under the change of external ambient temperature, and it can be used in areas with large temperature differences. The memory effect and hysteresis effect of the battery are considered, and a secondary protection device is used in the load circuit to better protect the battery.
The solar controller has also achieved initial success in field tests based on the successful simulation test. If the manual control circuit and the subsequent inverter circuit are further improved on the basis of this controller, the circuit stability and overall function can be improved, which will give it a good market prospect.