Design and testing of a high-performance solar LED streetlight

This paper presents the development and validation of a high-performance solarpowered charging streetlight. Our controller incorporates various charging modes, including MPPT charging, constant current charging, and constant voltage charging, and the controller can automatically change the charging mode based on the battery level. The streetlight can achieve brightness adjustment through PWM adjustment. The controller turns off the streetlights once the battery voltage reaches the over-discharge protection voltage. However, turning off the streetlights inevitably results in a voltage rebound above the over-discharge protection voltage. Consequently, the streetlights enter a cycling pattern of illumination and extinguishment. We have addressed this issue by incorporating a voltage rise threshold in the controller program, effectively preventing the streetlights from cycling between illumination and extinguishment. The MCU (Micro Controller Unit) can output multiple sets of PWM waves, where each set can control the on/off state of an LED light group. Combining multiple LED groups enables the driving and control of high-power LED street lights. We designed and constructed two solar-powered LED lighting systems to demonstrate the practicality of our approach. Extensive long-term outdoor field tests validate the robust performance of our control system.


Introduction
When charging batteries using a constant AC power supply, staged charging is commonly employed [1].This approach typically involves constant current and voltage charging [2].For instance, in 2021, Khalid et al. [3] studied staged charging methods using AC power as the energy source.In his research, he employed constant current charging when the battery capacity was low and switched to constant voltage charging when the charging voltage reached the maximum battery voltage.This staged charging method ensures both charging efficiency and safety.In solar charging, the focus lies in maximizing the efficiency of charging photovoltaic batteries under specific conditions, known as Maximum Power Point Tracking (MPPT) charging, due to the significant environmental impact on photovoltaic sources [4].Various algorithms have been developed to achieve MPPT charging.For example, Shang et al. [5] explored MPPT charging methods based on Incremental Conductance Algorithms.At the same time, Shi and Li [6] studied MPPT charging technology using the Perturbation and Observation Control method.Despite extensive research on MPPT charging in solar applications [7], there remains a scarcity of designs that integrate MPPT with other charging modes.Therefore, this study aims to develop a technology combining MPPT charging with constant current charging and voltage charging to enhance efficiency while ensuring battery safety.
LED driver chips can be classified into AC-powered and DC-powered [8].AC-powered LED driver chips typically offer higher driving power and can effectively drive high-power LED lamps [9].Conversely, DC-powered LED driver chips often have limited driving capabilities.For instance, Nguyen-Van conducted a study on a GaN-based hybrid DC-DC converter, which achieved a maximum output power of approximately 30 W [10]. Commonly used DC constant current chips, such as the constant current source chip SY7203DBC (Silergy Corp.San Francisco, USA), typically have a maximum driving power of around 60 W. The restricted driving power of DC-driven constant current chips contributes to the relatively low power output of current solar streetlights [11].Additionally, existing street light controllers cannot often control multiple groups of LED light arrays [12].To address this limitation, developing a controller capable of simultaneously controlling multiple groups of LED light arrays would enhance the controller's utilization efficiency and reduce the cost of solarpowered street lighting systems.Moreover, combining several groups of LED light arrays makes it possible to drive high-power LED solar streetlights.Therefore, another objective of this study is to design a controller capable of controlling the on/off state of multiple groups of LED light arrays, enabling the driving of high-power LED solar streetlights by combining several groups of LED light arrays.
Most existing solar street light controllers primarily rely on light sensors to control the on/off state and brightness of street lights [13].However, the brightness adjustment lacks a connection to specific dates and times.Furthermore, these controllers do not consider the battery voltage when adjusting the brightness [14].For example, Huang et al. [15] developed a solar street light controller that determines the on/off status of the street light solely based on detecting daytime and nighttime conditions.This leads to inefficient battery usage and rapid depletion, even when the battery level is low.Therefore, another objective of this study is to develop street lights with adjustable LED brightness that adapt based on time and battery voltage values, ensuring rational brightness adjustments and optimized battery utilization.
Addressing another challenge, LED discharge should cease to preserve power for the controller's operation when the battery voltage falls below a certain threshold.However, turning off the streetlights inevitably results in a voltage rebound above the over-discharge protection voltage.The increase in battery voltage may cause the LED to light up again, leading to undesired cycling effects [16].The commonly employed solution in current practice is integrating Schmitt triggers (threshold switch circuits) into the system [17].This incorporation effectively mitigates the issue of street lights cycling on and off due to voltage fluctuations.However, such hardware-based approaches increase costs due to additional electronic components.This study aims to address this issue through software programming by employing a set voltage threshold and monitoring the battery voltage to prevent cyclic LED switching.
To address the issues above, this study presents the design of a solar street light controller with the following functionalities: (1) Integration of MPPT charging, constant voltage charging, and constant current charging, with automatic switching between different charging modes based on battery capacity.(2) Driving multiple groups of LED light arrays to enable the operation of high-power LED solar street lights.(3) Implementation of automatic adjustment of LED light intensity based on clock time and battery voltage.(4) Incorporation of software-configurable settings to prevent cycling on and off of LED street lights caused by voltage fluctuations when the battery reaches the threshold for light extinction.Additionally, experimental tests will be conducted on the LED street lighting system to validate the proposed solutions.

Block diagram of the street lighting system
Figure 1 illustrates the power supply and control circuitry of the street lighting system.The photovoltaic (PV) panel charges the battery through the charging circuit for battery charging.The MCU controls the charging circuit, determining the connection and disconnection of the circuit.In particular, it begins by measuring the output voltage and current of the PV panel through Sensor 1 and the charging voltage and current of the battery via Sensor 2. Subsequently, the MCU employs the data from these two sensors to effectively regulate the on/off state of the charging circuit, thus ensuring precise control over the battery charging process.Battery discharge involves two components.Firstly, it entails powering the LEDs via the driver circuit, which can be configured to accommodate multiple groups for driving various LED clusters.The MCU regulates the power supply to the LEDs based on battery voltage and clock signals.Secondly, power is supplied to the MCU, voltage current Sensor 1, voltage current Sensor 2, and real-time clock module through the DC/DC module.A switch module controlled by the MCU is implemented to prevent battery over-discharge.When the battery is in a state where the power supply is stopped due to over-discharge protection, the PV panel can power the controller through DC/DC conversion.

Battery charging circuit and control process
2.2.1.Battery charging circuit.The charging process employs a BUCK circuit, as depicted in Figure 2(a).The BUCK circuit comprises components such as a MOS transistor (Q1), a Schottky diode (D1), a freewheeling diode (D2), an energy storage inductor (L1), a filtering capacitor (C1), two sampling resistors (R11 and R13) and a battery.The functionality of the BUCK circuit hinges on effective control of the conduction and cutoff states of the MOS transistor (Q1).Upon turning on the MOSFET (Q1), the freewheeling diode (D2) enters a reverse bias state, rendering it cut off.Consequently, the power source PV can provide power to the inductor (L1), capacitor (C1), and battery through the switch transistor (Q1).As the current in the inductor increases, energy is stored in the magnetic field.When the switch transistor (Q1) is turned off, the current in the inductor (L1) cannot change abruptly, in line with the characteristics of the inductor.The inductor (L1) induces an electromotive force with positive polarity on the left and negative polarity on the right, thereby forward biasing the freewheeling diode (D2).The stored energy in the inductor (L1) and capacitor (C1) is subsequently utilized to supply power to the battery, ensuring a stable voltage at the battery.Throughout this process, the current in the inductor decreases linearly.
The MOS transistor (Q1) serves as the control core of the entire BUCK circuit, achieving highfrequency switching between the on and off states under the control of PWM pulses.The relationship between the battery input voltage (V0), PV output voltage (Vs), and PWM duty cycle (D1) is determined by Equation ( 1): = × The MCU controls the conduction and cutoff time of the MOS transistor (Q1) by generating PWM signals with varying duty cycles.This enables the realization of different charging states.

Battery charging control process.
The charging control process consists of three charging modes: constant voltage charging stage, constant current charging stage, and MPPT charging.As shown in Figure 2, the MCU (U1) receives battery voltage data and determines the charging mode based on the battery voltage.MPPT charging is executed when the charging voltage (battery two-stage voltage) is below 12.6 V, and the current is less than 4 A. Constant current charging is applied when the charging current exceeds 4 A. Constant voltage charging is implemented when the charging voltage equals 12.6 V.Each charging mode corresponds to a subprogram, and the flowcharts for each subprogram are depicted in Figure 3.
In the constant voltage charging stage, as shown in Figure 3(c), it was charging proceeds when the charging voltage exceeds the set voltage threshold (Vthreshold).Initially, it is determined if the charging voltage surpasses the maximum battery voltage (Vmax).If so, the PWM duty cycle (D1) is increased to limit the charging voltage from exceeding the Vmax.If the charging voltage falls between the Vthreshold and Vmax, current judgment is performed.If the charging current exceeds the set minimum current value (Imin), the PWM duty cycle remains unchanged, maintaining constant voltage charging.Conversely, if the charging current is below Imin, indicating a fully charged battery, the PWM duty cycle is set to 0 to stop charging.In the constant current charging stage, as shown in Figure 3(b), charging occurs when the charging voltage is below the Vthreshold and the charging current surpasses the set current threshold (Ithreshold).During constant current charging, if the charging current exceeds the set maximum current (Imax), the PWM duty cycle is reduced to decrease the charging current.When the charging current falls between the Ithreshold and Imax, the PWM duty cycle remains unchanged, maintaining constant current charging.
For cases where the charging voltage is below the Vthreshold and the current is below the Ithreshold, the MPPT charging method based on the conductance increment algorithm is employed, as shown in Figure 3(a).The PV is a semiconductor that produces electricity by converting energy from sun irradiance to electricity [4,7].Figure 4(a) illustrates the relationship between the output power and output voltage of the PV panel.The black dots represent the maximum power points for charging, and the three curves represent the power-voltage variation under different irradiance levels.The figure shows that increased irradiance results in an increase in both generated power and voltage.Figure 4(b) depicts the relationship between the output current and output voltage of the PV panel.Similarly, the black dots represent the maximum power points for charging, and the curves represent the voltagecurrent variation under different irradiance levels.It can be observed that increased irradiance affects both the generated current and the maximum voltage.According to the photovoltaic characteristics curve of the PV panel, MPPT charging technology can be employed to maximize the power output of the PV panel.The output power of the PV panel is given by the Equation ( 2 In Equation ( 2), U represents the output voltage of the PV panel, I represents the output current of the PV panel, and P represents the output power of the PV panel.Taking the derivative concerning U, we obtain Equation (3): Based on Figure 4(a) and Equation ( 3), it can be seen that when dP/dU = 0, indicating dI/dU = -I/U, it corresponds to the maximum power point.At this point, it is only necessary to keep the voltage constant, meaning the PWM duty cycle remains unchanged.When dI/dU > -I/U, the charging point is on the left side of the peak, and to reach the MPPT point, the voltage needs to be increased, resulting in an increased PWM duty cycle.Conversely, when dI/dU < -I/U, the charging point is on the right side of the peak, and to reach the MPPT point, the voltage needs to be decreased, resulting in a decreased PWM duty cycle.Figure 4(b) illustrates that when dU = 0 and dI > 0, the MPPT point shifts to the right, requiring an increase in the PWM duty cycle.Similarly, when dU = 0 and dI < 0, the MPPT point shifts to the left, necessitating a decreased PWM duty cycle.Figure 3(a) provides a route map for the five PWM duty cycle adjustments mentioned above.The filtering capacitor (C2), high-power storage inductor (L2), freewheeling diode of the switching power supply (D2), and storage capacitor (C3) work in conjunction with the internal switch of SY7203DBC to accomplish the boost function.Pin 9 of the SY7203DBC serves as the PWM input pin, allowing LED brightness control by adjusting the PWM duty cycle.Pin 9 of the four SY7203DBC chips in Figure 5 are connected to Pins 10, 18, 19, and 40 of the MCU shown in Figure 2 to receive the PWM signals generated by the MCU.The specific calculation equation is: Regarding PWM control, it is adjusted by the MCU based on the battery voltage, date, and realtime clock.The specific adjustment process is illustrated in the upper part of Figure 6: The MCU acquires the battery voltage, date, and real-time clock data.Firstly, the MCU determines whether the current time corresponds to day or night based on predefined periods.If it is daytime, the voltage threshold parameter (Fthreshold) is set to 0, and the PWM is set to 0 to turn the street light off.Throughout the daytime, the battery voltage (V1) is continuously recorded.During the night, the brightness of the street light is determined by comparing V1 with specific thresholds.If the current state is nighttime, the current battery voltage is subtracted by the voltage threshold parameter Fthreshold (0 during the daytime).The lamp brightness control program is executed if the result is greater than or equal to the set minimum battery voltage (Vmin).The lamp brightness control program proceeds to the initial voltage (daytime charging voltage V1) and period determination process.The program defines two voltage ranges and identifies the range in which V1 falls.Each voltage range comprises multiple periods, and the PWM duty cycle is determined based on different periods to achieve varying brightness levels for the street light.When the battery voltage during the night is less than or equal to Vmin, the over-discharge protection program is executed.In this scenario, the voltage threshold parameter Fthreshold is set to 0.2, and the PWM is set to 0 to turn off the street light.Although the extinguishing of the street light causes a slight increase in battery voltage, the increase in voltage is still smaller than the increase in the new Fthreshold.Therefore, when the voltage minus the new Fthreshold is still less than Vmin, the system will continue to execute the battery over-discharge protection program, keeping the street light in the off state.This process repeats until the next day when Fthreshold is reassigned as 0 during the daytime, commencing a new cycle.

Voltage and time parameter settings of LED lighting discharge system
The testing location is in Heze City, situated near 35 degrees north latitude, experiencing four distinct seasons.Consequently, the year is divided into four quarters, and brightness adjustment is performed based on date, time, and battery voltage.Table 1 illustrates the brightness percentage during different periods when the initial battery voltage exceeds 11.85 V, while Table 2 displays the brightness percentage when the initial battery voltage exceeds 11.85 V. Taking the period from January to March as an example, the brightness control of the street light is as follows: When the battery voltage exceeds 11.85 V, the brightness levels are adjusted according to the time of day.From 17:30 to 22:00, the brightness is set to 100%; from 22:00 to 00:00, the brightness is set to 70%; and from 00:00 to 7:00, the brightness is set to 40%.During other times, the street light is turned off.Additionally, if the battery voltage falls below 11.85 V during the illumination process, the street light will adjust its brightness according to the corresponding values specified in Table 2 at the respective time points.

Test of solar streetlight systems
The solar street light controller circuit board is depicted in Figure 7.It primarily comprises the charging circuit module, voltage measurement module, clock circuit module, microcontroller module, 4 street light drivers, 4 street light interfaces, battery interface, and photovoltaic panel interface.Each street light interface can drive an 18 W LED street light, with eighteen 1 W LED chips connected in a configuration of 3 in parallel and 6 in series.The design incorporates a 12 V lithium battery.To validate the functionality and reliability of the system, this study conducted three experiments.Firstly, the controller charging experiment was conducted.An 80 W solar photovoltaic panel with a maximum charging voltage of 18 V and a maximum charging current of 5 A was used.A 12 V lithium battery with a capacity of 38AH was employed in this experiment.The objective was to verify the controller's ability to switch between various charging modes automatically.
Secondly, the controller's ability to regulate street light brightness was tested.An 18 W street light and a 12 V lithium battery with a capacity of 25AH were used.In this experiment, the automatic adjustment of the street light brightness was measured from sufficient to low battery capacity.Therefore, only battery discharge was conducted in this experiment without charging the battery.
Thirdly, long-term field tests of solar LED lighting systems are needed.Regarding the selection of battery capacity, this design necessitates that when fully charged, the battery can sustain the illumination of the street light for three days.The first two days require high-power lighting, while the last day requires low-power lighting.For the configuration of 18 W street lights, a 12 V lithium battery with a capacity of 38AH (operating within the voltage range of 11.1-12.6V) and an 80 Wp photovoltaic panel are employed.For the configuration of 36 W street lights, a 12 V lithium battery with a capacity of 76AH and a 150 Wp photovoltaic panel are utilized.Following the system assembly, the solar panel faces south at an inclination angle of 46°.The lighting status of the street lights is determined by monitoring the current, and the lighting conditions are recorded.The duration of the experiment spans one year.

Charging test result
To validate the charging process of the controller, a test was conducted on the charging process of the 18 W solar street light battery (12 V 38AH).Figure 8 presents the outdoor charging test results of the street light controller during daylight hours, with a sampling interval of 10 minutes for each data point.The results indicate that from approximately 6:00 in the morning until around 10:30, the charging current and voltage gradually increase.This corresponds to the MPPT charging stage, where the controller optimizes the power output from the solar panel.From around 10:30 to approximately 13:00, the current reaches the threshold for constant current charging at 4 A, and the charging process continues at a constant current of 4 A. During this stage, the voltage shows an upward trend.At around 13:00, when the voltage reaches 12.6 V, the controller switches to constant voltage charging mode.Throughout the constant voltage charging process, the current gradually decreases until it reaches 0 at approximately 16:30, indicating the completion of the charging process.
Based on the voltage and current variation curves observed during the charging process, it can be concluded that this street light controller can effectively switch between different charging modes based on the battery voltage and charging current values.The controller provides battery overcurrent and overvoltage charging protection, ensuring safe and efficient charging operations.

Street light brightness adjustment and power failure protection experiment result
Starting from February 15, 2022, the initial voltage was 12.6 V, and the variation curves of the battery voltage and discharge current are depicted in Figure 9. Before 17:30 on February 15, the current was at 0, indicating that the street light was switched off.At 17:30, the current increased to approximately 1 A, and the street light started operating at 100% brightness for 4.5 hours.Subsequently, it operated at 70% brightness (current approximately 0.7 A) for 2 hours, followed by 40% brightness (current approximately 0.4 A) for 7 hours.Eventually, the street light turned off.By February 16, 2022, at 17:30, the initial battery voltage had dropped to 11.64 V, which was lower than 11.85 V.The street light initially operated at 80% brightness for 4.5 hours, followed by 50% brightness for 1.5 hours.When the voltage fell below 11.1 V, the current reached 0, indicating that the street light was turned off.Although the voltage slightly increased to 11.14 V after the street light turned off, the current remained at 0, and the street light remained off.The results demonstrate that when the initial voltage was 12.5 V, the activation of the street light aligned with the brightness adjustment requirements in the first quarter of Table 1, confirming the program's feasibility.On the subsequent day, with an initial battery voltage of 11.64 V, which was lower than 11.85 V, the brightness of the street light consistently followed the values outlined in the first quarter of Table 2. On the second day of the experiment, when the battery voltage dropped below 11.1 V, the control circuit board activated the battery over-discharge protection program, resulting in a current of 0 and the street light being turned off.Even though there was a slight increase in the battery voltage to 11.14 V after the street light was turned off, the LED remained inactive.This can be attributed to the adjustment of the voltage threshold Fthreshold, which shifted from 0 to 0.2 when the battery voltage fell below 11.1 V.In the subsequent program iteration, the street light remained off when the battery voltage minus Fthreshold remained below the set minimum battery voltage (Vmin) for the street light to activate.

Long-term field test of solar LED lighting systems results
Figure 10 and Figure 11, respectively, document the on-off status of the 18 W and 36 W street lights throughout the year.The testing results over a year indicate that in the second and third quarters, both the 18 W and 36 W street lights were successfully illuminated within the designated time frame without any instances of turning off.However, during the first and fourth quarters, the 18 W street lights failed to meet the required illumination time on only 12 days, resulting in 58.7 hours of reduced illumination.Similarly, the 36 W street lights had 18 days when they did not achieve the required illumination time, resulting in 87.5 hours of reduced illumination.

Conclusion
The PV panel is linked to a BUCK charging circuit, facilitating the battery charging.The charging power is adjustable through PWM signals that control the MOSFET's on/off state within the BUCK circuit.This technique enables tracking of the maximum power point on the PV curve, thereby optimizing the PV panel's operation at its peak power output.The STM32 microcontroller can function as a PWM signal generator.By employing PWM control, it effectively combines MPPT charging technology with constant voltage and current charging methods, ensuring rapid charging and battery safety.The STM32 microcontroller can generate multiple PWM signals, which can be employed to control constant current chips and regulate the brightness of street lights.Utilizing these multiple PWM signals emitted by the STM32 makes it feasible to govern multiple sets of LED street light driver circuits, consequently facilitating the operation of multiple groups of LED light arrays.This configuration enables the efficient functioning of high-power LED street lights powered by a DC power source.Furthermore, the controller leverages PWM to adjust the LED brightness based on the battery voltage and time conditions, imbuing the streetlights with enhanced intelligence and promoting more judicious battery utilization.

Figure 1 .
Figure 1.Block diagram of the street lighting system.

Figure 2 .
Figure 2. Battery charging circuit and charging control diagram: (a) BUCK circuit for battery charging, (b) Drive circuit for MOSFET in BUCK circuit, (c) Real-time clock circuit, (d) MCU minimum system circuit, and (e) Voltage and current detection circuit.

Figure 3 .
Figure 3.The three charging modes of the controller, the triggering conditions for each mode, and the PWM regulation of each charging mode.(a) The MPPT charging, (b) the constant current charging, and (c) the constant voltage charging.

Figure 4 .
Figure 4. Photovoltaic characteristics curve of the PV panel.(a) illustrates the relationship between the output power and output voltage of the PV panel, and (b) depicts the relationship between the output current and output voltage of the PV panel.

Figure 5 .
Figure 5. Driving circuit for 4 groups of LED light arrays.

Figure 6 .
Figure 6.Discharge control system for LED lighting.(1) Daytime light intensity control program, (2) light intensity control program for battery over-discharge, and (3) nighttime light intensity control program.

Figure 7 .
Figure 7.The solar street light controller circuit board.

Figure 8 .
Figure 8.The outdoor charging test results of the street light controller during daylight hours.

Figure 9 .
Figure 9.The variation curves of the battery voltage and discharge current.

Figure 10 .
Figure 10.The daily duration of illumination for 18 W solar street lights throughout the year.

Figure 11 .
Figure 11.The daily duration of illumination for 36 W solar street lights throughout the year.

Table 1 .
The brightness percentage of illumination during different periods when the battery voltage exceeds 11.85 V.

Table 2 .
The brightness percentage of illumination during different periods when the battery voltage is less than 11.85 V.