Solar Panel Battery Charger 6-12V - Research Paper Example

?CHAPTER 2 METHODOLOGY 5.0 INTRODUCTION The circuit design for the solar panel battery charger will be explained in detail with all relevant calculations for voltage, current, load, power, etc. 5.1 CIRCUIT DIAGRAM Figure 8: Block diagram for solar panel battery charger. The block diagram represents a 6-12V, 300mA circuit that charges batteries using solar cells that draw power directly from sunlight. The circuit can be divided into 3 parts: the voltage regulator, the comparator and the battery voltage checker. The voltage regulator controls the voltage and current from the solar panels to the battery. The comparator compares the voltage from the battery and acts as a switch for the voltage regulator circuit. Finally, the battery voltage checker checks the voltage of the battery (as it received from the panel) to determine if the battery needs to charge more. The circuit is designed to be simple, efficient and reliable by using easily available field replaceable parts. It uses a 12V, 5W solar panel rated from 100 milliamps to 1A and a lead acid or other rechargeable battery that is rated from 500 milliamp hours to 40 amp hours of capacity. 5.2 CIRCUIT CHARGING The 6-12V photovoltaic charger circuit consists of three different circuits as shown in Figure 9 below: Figure 9: Charging circuit. Voltage regulator circuit: This circuit regulates the voltage flow from the photovoltaic panel to the lead acid battery. It can produce currents up to 150mA. When external pass transistors are added to this circuit, output currents can reach up to 10A. The maximum input voltage to this circuit is 40V (LM723, 2004) with an output voltage adjustable between 2V and 37V. This circuit consists of a series regulator, LM723. LM723 (voltage regulator): Description: LM723 is a monolithic, integrated and programmable voltage regulator mainly used as a series regulator (Voltage regulator, 2012). Although, it can supply output currents of 150mA through its internal circuit, additional currents can be obtained by adding external transistors. This chip has low standby current drain and is particularly used for linear or fold-back limiting of current (LM723 Variable Power Supply with Over-Current Protection, 2012). Figure 10 represents the voltage regulator circuit as below: Figure 10: Voltage regulator circuit. Figure 11 and Figure 12 are the connection diagram and the datasheet circuit (Voltage regulator, 2012) as below: Figure 11: Connection Diagram Figure 12: Datasheet Circuit The basic building blocks of LM723 are: 1 The Reference Voltage Amplifier 2 The Error Amplifier 3 The Series Pass Transistor. The equivalent circuit of LM723 (LM723/LM723C Voltage Regulator, 1994) is shown in Figure 13 as below: Figure 13: Equivalent circuit of LM723. The main components used in the voltage regulator circuit are (Table 1): Table 1: Main components of the voltage regulator circuit Quantity Component Value 1 LM723 - 1 R1 4.87k ? 1 R2 7.15k ? 2 Transistor 2N3055 1 Diode 1N4007 1 VR 10k ? 1 C1 0.1?F 1 C2 500PF Calculations: Following are the calculations for design and operation of the circuit: Output voltage- Vout = Vref x ((R1+R2)/R2) Where R1= 4.87K, R2= 7.15K and Vref= 7.35V from the datasheet. Vout= 7.35 x ((4.87 + 7.15)/7.15) = 12.36V. Figure 14: Output voltage. Current- The current is established from the Darlington transistor pair in the regulator circuit (Q1 and Q2 in Figure 10). Q1 and Q2 (2N3055) are silicon, Epitaxial-Base Planar NPN transistor mounted in a Jedec TO-3 metal case (Charger Circuit for 6V or 12V Car Battery, 2012) and are recommended for use in power switching circuits, series and shunt regulators, output stages and high fidelity amplifiers. Figure 15 shows the Darlington transistor pair: Figure 15: Darlington transistor pair used in voltage regulator circuit. In this circuit, a voltage of 0.7V is applied to the base (B) of the first transistor (TR1) to switch it on. A current of 300 milliamps passes through the first transistor from the collector (C) to the Emitter (E). The emitter of TR1 is connected to the second transistor (TR2) which is then switched on due to current flowing from the emitter of TR1. Also, heat sinks are used for the transistors as the current flowing through them produces heat that needs to be dissipated to the surroundings through the heat sink. Calculations for the transistor current- Current at the collector (IC) of the transistor can be measured by the following equation: IC = hFE x IB Where IC is the Collector Current, IB Base Current and hFE is the DC Current Gain. With hFE is taken as the minimum, IC = 5 x 300mA = 1500mA = 1.5mA With hFE is taken as the maximum, IC = 70 x 300mA = 21000mA = 21mA Calculations for the heat sink- Rth j-a = Rth j-c +Rth c-h + Rth h-a Where Tj= 200, Tair= 25 and Rth j-c= 1.5 Co/W T j-a= 200 Co-25 Co =175 Co P = IC x VCE = 1.5A x 14.5V = 21.75 W Rth j-a = (T j-a)/P = 175 Co /21.75 W = 8 Co /W Assuming Rth c-h = 0, Rth h-a = Rth j-a – Rth j-c = 8 Co /W - 1.5 Co/W = 6.5 Co/W The heat sink applied to the transistors is less than 6.5 Co/W. Operation: The voltage regulator circuit is designed to supply 14.5 V and 1.5A. A 300mA current from the solar panel input passes through the 1N4007 Darlington transistor pair to increase up to 1.5A. A supply voltage of 14.5V from the solar panel input (+ve), switches ON IC1 (LM723) which in turn produces a 6V or 12V regulated output by comparing the inverting input voltage that is set at pin4 of IC1. The inverting input voltage is derived from internal reference voltage at pin6 (7.15V) which is adjustable through a 10K variable resistor R3 and the non-inverting voltage at pin5. The non-inverting voltage is the sampled battery voltage for 12V operation produced through R1 and R2. Further, a 6V battery can be charged by using a variable resistor connected between the reference voltage and the non-inverting voltage to split the reference voltage to half (Vref= 3.5v) (see Figure 10). The comparator circuit: LM324 is used to acts as a switch for the Darlington transistor. Figure 16 shows the comparator circuit of the charging circuit: Figure 16: Comparator circuit. Figure 17 shows the connection diagram of low power quad operational amplifier chip, LM324C: Figure 17: Connection diagram of LM324C. Table 2 shows the component list in the comparator circuit: Quantity Component Value 1 LM324 - 2 R4-R5 10k ? 1 R5 1k ? 1 R5 500k ? 2 Switch - Description: The comparator circuit of the solar battery charger circuit consist of four independent, high gain, internal frequency compensated operational amplifiers which operate over a wide range of voltages from a single power supply (LM324    Low Power Quad Operational Amplifier, 2012). The circuit can also operate with split power supplies with the low power supply being independent of the value of voltage of the power supply. Calculations: Calculations for the 12V battery: Vout = (R4/(R4+R5)) x Vin When Vin ? 10V, Vout = (10/(10+10) x 10 = 5V This implies that the inverter input is smaller than the non-inverting input making the comparator output 14.5V and the Darlington transistor in the voltage regulator circuit remains ON. When Vin = 13.5V, Vout = (10/(10+10) x 13.5 = 6.75V Implying that the inverter output is greater than the non-inverting output with the zero comparator and the Darlington transistor in the voltage regulator circuit remains OFF. Calculations for the 6V battery: Vout = (R6/(R6+R7)) x Vin When Vin ? 5V Vout = (500K/(500K+1K)) x 5 = 4.99V When Vin = 7V Vout = (500K/(500K+1K)) x 7 = 6.98V Operation: The LM324C is a low power quad operational amplifier that is used to switch the Darlington transistor pair, ON and OFF. It also functions as a voltage comparator based on the inverter input signal. When the input signal on the inverter is greater than the input on the non-inverter, comparator output signal will be almost 0V and when the input to non-inverter is greater than the input to inverter, comparator outputs 12V resulting in turning ON of the transistor and allowing flow of current from collector to emitter. Voltage checker circuit: This circuit also uses the low power quad operational amplifier, LM324. Figure 18 shows the schematic of voltage checker circuit which is the third part of the battery charger circuit: Figure 18: Voltage checker circuit. Figure 19 shows the connection diagram of LM324 (LM124/LM224/LM324/LM2902 Low Power Quad Operational Amplifiers, 2004) used in the voltage checker: Figure 19: Connection diagram of voltage checker circuit. Table 3 shows the main components in the voltage checker circuit: Quantity Component Value 1 LM723 - 1 R1 2 ? 1 R2 10.2k 2 R3 1.5k 1 R4 575 ? 1 R5 165 ? 1 R6 7.5k 4 R7-R10 1k 1 C 100?F 1 Zener Diode 5.4V 1 Switch - 4 LED 0.5mA Description: The voltage checker circuit uses LM324 low power op-amp which has four op-amps that are used in this particular circuit as comparators. This circuit is switched ON through a switch connected to the battery. Calculations: Calculating non-inverting input value: From Figure 18, the voltage applied to any inverting input is the ration of the resistance between that inverting terminal and ground to the total resistance (R2+R3+R4+R5+R6) where R2=10.2K ? R3= 1.5K ? R4= 575 ? R5= 165 ? and R6= 7.5K ? Further, the status (ON/OFF) of the first comparator (IC2A) is represented by a Green LED when voltage reaches 11V and Rtotal (RT) =R3+R4+R5+R6 =1.5+0.165+0.575+7.5 = 9.74 ? and Vinverter = (RT/(RT+R2)) x Vin = (9.74/19.74) x 11 = 5.43V The second comparator (IC2B) uses an Orange LED to show the status of the inverter voltage when voltage reaches 12.8V and Rtotal (RT) = R4+R5+R6 =0.165+0.575+7.5 = 8.42? Vinverter = (RT/(RT+R3+R2)) x Vin = (8.42/ (8.42+10+1.5)) x 12.8 = 0.423 x 12.8 = 5.41V The third comparator (IC2C) uses a Yellow LED to show the status when the voltage reaches 13.3V and Rtotal (RT) = R5+R6 = 0.575 + 7.5 = 8.075 ? and Vinverter = (RT/(RT+R3+R2+R4)) x Vin = 8.075/(8.075+0.65+10) x 13.3 = 0.409 x 13.3 = 5.43V The fourth comparator (IC2D) uses a Red LED to show the status when voltage reaches 14.5V and Rtotal (RT) = R6 = 7.5 K? and Vinverter = (RT/(RT+R3+R2+R4+R5)) x Vin = (7.5/(7.5+0.575+0.165+1.5+10)) x 14.5 = 0.3799 x 14.5 = 5.5V Operation: The inverting terminals (A to D) of the op-amp (LM324) are connected to the positive supply rail through a potential divider chain consisting of R2 through R6 resistors. The voltage applied at any of the inverting input of the comparators becomes the ratio of the resistance between the inverting terminal and the ground to the total resistance (R2+R3+R4+R5+R6) when a reference voltage of 5.4V is applied to the non-inverting input terminals of the op-amp through a Zener diode (ZD2) at the same voltage of 5.4V. The switch (S1) in ON and the circuit is connected to the battery, the analyzing voltage checker circuit samples the battery voltage. The different levels of the battery voltage are indicated using LED1 through LED4 but when the voltage level goes below 11V, all the LEDs are OFF indicating that the battery needs to be charged. When the supply voltage that is sampled at the non-inverting input (reference voltage) of the op-amp exceeds the reference voltage of the inverting inputs (R2 through R6), the output of the op-amp of the analyzer circuit goes high and the relevant LED (based on voltage level) is ON. Table 4 shows the status of LEDs for the different battery voltages: LED Voltage (v) State Of LEDs Levels Red 11 On-off-off-off Low Level Orange 12.8 On-on-off-off Normal Level Yellow 13.3 On-on-on-off High Level Green 14.5 On-on-on-on Full Charge Level 5.3 TESTING OF THE CIRCUIT A laboratory experiment is conducted to test the charging efficiency of the battery. 12V battery charging and discharging: Charging- Figure 20: Lab test setup for 12V battery charging. Instead of the lead acid battery, a DC source is used to set the voltage to test the battery charging. The battery’s voltage (set at 12.31V or less) is the non-inverting input at the comparator which works as a switch. The inverting input is the reference voltage at the comparator and it is seen that the non-inverting (12.31V battery voltage) voltage is less than the reference voltage. This turns ON the comparator to supply to the Darlington transistors present in the voltage regulator circuit. This also switches ON the charger to supply the battery with 12.24V until the battery reaches its peak voltage level. Discharging: Figure 21: Lab test setup for 12V battery discharging. When the battery reaches its full charging capacity at 12.84V (as in Figure 21 above), the comparator compares the inverting input or the reference voltage with the non-inverting input or the battery voltage to turn ON or OFF the supply to the Darlington transistor in the voltage regulator circuit. When the non-inverting input (battery voltage) is greater than the inverting input (reference voltage), the comparator turns OFF to stops supplying current to the battery. 6V battery charging and discharging: Charging: Figure 22: Lab test setup for 6V battery charging. Performing the same operation for a 6V battery as for testing a 12V battery by connecting a DC source instead of the lead acid battery, the comparator circuit switches ON after comparing the inverter input and the non-inverter input when the battery voltage is set to 6.5V. At this voltage, the charger supplies 6.47V to the battery until it reaches its full charging capacity. Discharging: The comparator circuit switches OFF when the battery voltage reaches the full charge at 7.15V. This is when the charger stops supplying voltage to the lead acid battery. Solar panel lab test: A 12V, 5W solar panel is used in the design with the the panel consists of 36 solar cells that are connected in a series with open circuit voltage of 19.17V when no current is passing through the cells and with a peak power (PMax) of 5 Watts and max power current (Imp) of 0.29Amps and a max power Voltage (Vmp) of 17.5 Volts. The luminous power per area of the solar panel is 2630 lux. Figure 23 shows the solar panel and Figure 24 shows the open circuit voltage schematic: Figure 23: Solar panel Figure 24: Schematic for open circuit voltage The open circuit voltage (Voc) is the voltage when I=0 measured by directly connecting the panel to a voltmeter, and is represented by V (at I =0) = Voc The short circuit current (Isc) of the panel is calculated when voltage is zero which corresponds to a short circuit condition when impedance is low and can be measured with by directly connecting an ammeter. I (at V=0) = Isc Figure 25 shows the short circuit schematic. It is measured that the luminous power per area is 2630 lux with a short circuit current of 266.33 mA. Figure 25: Short circuit current schematic. In the lab test conducted to measure the voltage and current with load, many different loads were connected. One example test conducted is explained below. Figure 26 shows the lab test setup for measuring the voltage and current with different loads: Figure 26: Measuring voltage and current in the lab with different loads. Table 5 represents the measured voltage and currents for different loads: Power (W) Voltage (v) Current (A) Resistor (?) 0 19.76 0 open circuit 2.11 17.6 0.12 80 2.15 16.6 0.13 70 2.38 15.96 0.15 60 2.54 15.85 0.16 50 2.84 15.8 0.18 40 3.15 15.75 0.2 30 3.92 15.7 0.25 20 4.07 15.67 0.26 10 0 0 0.27 short circuit It is observed that as the value of the load increases, the output voltage increased significantly as shown in Figure 27: Figure 27: Voltage vs load Also, when the load increased, the current value decreased significantly as shown in Figure 28: Figure 28: Current vs load The output of the solar panel is found to exhibit a characteristic behaviour (as shown Figure 29) that is common to all solar panels in that the maximum power is generated at the operating point, also called the ‘knee’ of the curve (in this case it is 4.07W) corresponding to a load of 10 ?. At this point (knee), the maximum power point occurs, which is the area of the rectangle beneath the curve and is given by the product of maximum current and voltage. 5.4 PCB DESIGN Figure 30 shows the PCB board designed using Eagle software program (website 1): Figure 30: PCB for the battery charger circuit. Also, a circuit board cover has been designed to prevent any damage that may occur due to heat from sun or water. The circuit is mounted with four screws to keep it in place and two switches are placed to show the 6V and 12V marked outside the cover. Further, the LEDs in the voltage checker circuit are covered with four coloured glass pieces which makes it easier to know the voltage level of the charging battery. Figure 31 shows the protection cover that is designed: Figure 31: Circuit protection cover 5.5 PROJECT ECONOMICS Table 6 represents the budget for the project for one year: Quantity Component Value Cost 1 R1 4.87k ? 0.69? 1 R2 7.15k ? 0.69? 2 R4-R5 10k ? 1.38? 1 R5 1k ? 0.69? 1 R5 500k ? 0.69? 1 R1 2 ? 0.69? 1 R2 10.2k 0.69? 2 R3 1.5k 0.69? 1 R4 575 ? 0.69? 1 R5 165 ? 0.69? 1 R6 7.5k 0.69? 4 R7-R10 1k 0.69? 1 VR 10k ? 0.95? 3 Switch - 10.47? 1 C1 0.1?F 0.24? 1 C2 500PF 0.25? 1 C3 100?F 0.24? 2 Zener Diode 5.4V 1.58? 4 LED 0.5mA 2.56? 2 Transistor 2N3055 5.18? 1 Heat Sink 6.5/W 3? 3 Diode 1N4007 2.37? 2 LM723 - 1.9? 1 LM723 - 0.95? 1 Solar Panel 12V (5watts) 65? Total Cost 103.68? 5.6 CONCLUSION AND RECOMMENDATIONS Conclusion- A 6-12V lead acid battery charger was designed and tested successfully. A DC source was used to test the solar charger in the laboratory. Adding a pair of Darlington resistors to the voltage regulator circuit increased charge protection with a heat sink improving the charger operation by protecting against heat from the transistors. Recommendations- A digital LED system can be used to improve the appearance of the voltage checker circuit and Solar panel with more than 5W can be designed to speed up charging of the battery. REFERENCES- Charger Circuit for 6V or 12V Car Battery. 2012. EEWeb. Available online: http://www.eeweb.com/blog/circuit_projects/charger-circuit-for-6v-or-12v-car-battery Accessed on: 06th March 2012. LM124/LM224/LM324/LM2902 Low Power Quad Operational Amplifiers. 2004. National Semiconductor. Available online: http://www.ti.com/lit/ds/symlink/lm124-n.pdf Accessed on: 06th March 2012. LM324 Low Power Quad Operational Amplifier. 2012. National Semiconductor. Available online: http://www.national.com/mpf/LM/LM324.html#Overview Accessed on: 06th March 2012. LM723 Variable Power Supply with Over-Current Protection. 2012. EEWeb. Available online: http://www.eeweb.com/blog/circuit_projects/lm723-variable-power-supply-with-over-current-protection Accessed on: 06th March 2012. LM723. 2004. Texas Instruments. Available online: http://www.ti.com/product/LM723#parametrics Accessed on: 06th March 2012. LM723/LM723C Voltage Regulator. 1994. National Semiconductor. Available online: http://wiki.sj.ifsc.edu.br/wiki/images/e/e9/LM732.pdf Accessed on: 06th March 2012. Voltage regulator. 2012. National Semiconductor. Available online: http://www.national.com/mpf/LM/LM723.html#Overview Accessed on: 06th March 2012. Website 1: http://www.pcb-design-directory.com/designpcb/ Read More