Microcontroller Based Solar Charger

As the sources of conventional energy deplete day by day, resorting to alternative sources of energy like solar and wind energy has become need of the hour.

Solar-powered lighting systems are already available in rural as well as urban areas. These include solar lanterns, solar home lighting systems, solar streetlights, solar garden lights and solar power packs. All of them consist of four components: solar photovoltaic module, rechargeable battery, solar charge controller and load.

In the solar-powered lighting system, the solar charge controller plays an important role as the system’s overall success depends mainly on it. It is considered as an indispensable link between the solar panel, battery and load.

The microcontroller-based solar charge controller described here has the following features:

1. Automatic dusk-to-dawn operation of the load
2. Built-in digital voltmeter (0V-20V range)
3. Parallel- or shunt-type regulation
4. Overcharge protection
5. System status display on LCD
6. Deep-discharge protection
7. Low battery lock
8. Charging current changes to ‘pulsed’ at full charge
9. Low current consumption
10. Highly efficient design based on microcontroller
11. Suitable for 10-40W solar panels for 10A load

The circuit of the solar charge controller is shown in Fig. 1. It comprises microcontroller AT89C2051, serial analogue-to-digital converter ADC0831, optocoupler MCT2E, regulator 7805, MOSFETs BS170 and IRF540N, transistor BC547, LCD and a few discrete components. Component description is given below.

Microcontroller. Microcontroller AT89C2051 is the heart of the circuit. It is a low-voltage, high-performance, 8-bit microcontroller that features 2 kB of Flash, 128 bytes of RAM, 15 input/output (I/O) lines, two 16-bit timers/counters, a five-vector two-level interrupt architecture, a full-duplex serial port, a precision analogue comparator, on-chip oscillator and clock circuitry. A 12MHz crystal is used for providing the basic clock frequency. All I/O pins are reset to ‘1’ as soon as RST pin goes high. Holding RST pin high for two machine cycles, while the oscillator is running, resets the device. Power-on reset is derived from resistor R1 and capacitor C4. Switch S2 is used for manual reset.
   
Serial ADC.  The microcontroller monitors the battery voltage with the help of an analogue-to-digital converter. The ADC0831 is an 8-bit successive approximation analogue-to-digital converter with a serial I/O and very low conversion time of typically 32 μs. The differential analogue voltage input allows increase of thecommon-mode rejection and offsetting of the analogue zero input voltage. Inaddition, the voltage reference input can be adjusted to allow encoding of anysmaller analogue voltage span to the full eight bits of resolution. It is available in an 8-pin PDIP package and can be interfaced to the microcontroller with only three wires.
   
LCD module. The system status and battery voltage are displayed on an LCD based on HD44780 controller. The backlight feature of the LCD makes it readable even in low light conditions. The LCD is used here in 4-bit mode to save the microcontroller’s port pins. Usually the 8-bit mode of interfacing with a microcontroller requires eleven pins, but in 4-bit mode the LCD can be interfaced to the microcontroller using only seven pins.
   
Solar panel. The solar panel used here is meant to charge a 12V battery and the wattage can range from 10 to 40 watts. The peak unloaded voltage output of the solar panel will be around 19 volts. Higher-wattage panels can be used with some modifications to the controller unit.
   
Rechargeable battery. The solar energy is converted into electrical energy and stored in a 12V lead-acid battery. The ampere-hour capacity ranges from 5 Ah to 100 Ah.
   
Dusk-to-dawn sensor. Normally, in a solar-photovoltaic-based installation—for example, solar home lighting system, solar lantern or solar streetlight—the load (the light) is switched on at dusk (evening) and switched off at dawn (morning). During daytime, the load is disconnected from the battery and the battery is recharged with current from the solar panel. The microcontroller needs to know the presence of the solar panel voltage to decide whether the load is to be connected to or disconnected from the battery, or whether the battery should be in charging mode or discharging mode. A simple sensor circuit is built using a potential divider formed around resistors R8 and R9, zener diode ZD1 and transistor T1 for the presence of panel voltage.
   
Charge control. Relay RL1 connects the solar panel to the battery through diode D1. Under normal conditions, it allows the charging current from the panel to flow into the battery. When the battery is at full charge (14.0V), the charging current becomes ‘pulsed.’ To keep the overall current consumption of the solar controller low, normally-closed (N/C) contacts of the relay are used and the relay is normally in de-energised state.
   
Load control. One terminal of the load is connected to the battery through fuse F1 and another terminal of the load to an n-channel power MOSFET T3. MOFETs are voltage-driven devices that require virtually no drive current. The load current should be limited to 10A. One additional MOSFET is connected in parallel for more than 10A load current.

Circuit description

Basically, there are two methods of controlling the charging current: series regulation and parallel (shunt) regulation. A series regulator is inserted between the solar panel and the battery. The series type of regulation ‘wastes’ a lot of energy while charging the battery as the control circuitry is always active and series regulator requires the input voltage to be 3-4 volts higher than the output voltage. The current and voltage output of a solar panel is governed by the angle of incidence of light, which keeps varying.
   
Parallel regulation is preferred in solar field. In parallel regulation, the control circuitry allows the charging current (even in mA) to flow into the battery and stop charging once the battery is fully charged. At this stage, the charging current is wasted by converting into heat (current is passed through low-value, high-wattage resistor); this part of the regulation dissipates a lot of heat.
   
In this project, we have used parallel regulation technique but instead of wasting the charging current as heat, we have made it pulsed and applied to the battery to keep the battery topped-up.
   
After power-on, the microcontroller reads the battery voltage with the help of the ADC and displays the values on the LCD. It monitors the input signal from the dusk-to-dawn sensor and activates the load or charging relay RL1 accordingly. The digital voltmeter works up to 20V. As V_ref of the ADC is connected to V_CC (5V), the input voltage to the ADC cannot exceed +5V. A potential divider is used at pin 2 of the ADC (IC2) using resistors R5, R6 and R7 to scale down the voltage from 0V-20V to 0V-05V. The ADC output is multiplied four times and displayed on the LCD as battery voltage.
   
When the solar panel voltage is present, the dusk-to-dawn sensor provides a signal to the microcontroller, which then displays ‘charging’ message on the LCD. During charging, the battery voltage is continuously monitored. When the voltage reaches 14.0V, the microcontroller interrupts the charging current by energising the relay, which is connected to MOSFET BS170 (T2), and starts a 5-minute timer. During this stage, the LCD shows “battery full.”
   
After five minutes, the relay reconnects the panel to the battery. This way, the charging current is pulsed at the intervals of five minutes and the cycle repeats until the panel voltage is present.
   
When the panel voltage falls below the zener diode (ZD1) voltage of the dusk-to-dawn sensor, the microcontroller senses this and activates the load by switching on MOSFET T3 via optocoupler IC3 and “load on” message is displayed.
   
In this mode, the microcontroller monitors for low battery. When the battery voltage drops below 10 volts, the microcontroller turns off the load by switching off MOSFET T3 and “battery low—load off” message is displayed.

Normally, when the load is switched off, the battery voltage tends to rise back and the load oscillates between ‘on’ and ‘off ’ states. To avoid this, the microcontroller employs a hysteresis control by entering into a ‘lock’ mode during low-battery state and comes out of the lock mode when the dusk-to-dawn sensor receives the panel voltage (the next morning). During lock mode, the microcontroller keeps converting the ADC value and displays the battery voltage on the LCD.

Code:

;************************************************************************************** ; SOLAR CHARGE CONTROLLER WITH DIGITAL VOLTMETER-ADC0831+LCD+8051 ;************************************************************************************** $MOD51 ; LCD 4-BIT MODE CONNECTIONS RS EQU P1.7 ;LCD REGISTER SELECT LINE EN EQU P1.6 ;LCD ENABLE LINE DB4 EQU P1.5 ; DB5 EQU P1.4 ; DB6 EQU P1.3 ; DB7 EQU P1.2 ; ; ADC0831 CONNECTIONS CS EQU P3.0 CLK EQU P3.1 DO EQU P3.2 ;INPUT & OUTPUT DYI EQU P3.4 ; SOLAR PANEL VOLTAGE SENSOR CHG_RL EQU P3.5 ; CHARGING CONTROL RELAY LD_RL EQU P3.7 ; LOAD CONTROL RELAY DSEG ORG 0020H VAL1: DS 1 VAL2: DS 1 VAL3: DS 1 ADC_VAL: DS 1 BUF: DS 1 CNT1: DS 1 CNT2: DS 1 IMG: DS 1 FLAGS: DS 1 OCF BIT FLAGS.0 ; OVER CHARGE FLAG LBF BIT FLAGS.1 ; LOW BATT FLAG CSEG ORG 0000H JMP MAIN ORG 000BH ;Timer Interrupt0 JMP COUNTDOWN MAIN: MOV SP,#50H MOV P3,#0FFH MOV P1,#0FFH CLR CHG_RL CLR LD_RL LCALL PWR_DELAY LCALL INIT SETB CLK SETB DO SETB CS SETB DYI MOV VAL1,#00H MOV VAL2,#00H MOV VAL3,#00H MOV FLAGS,#00H LOADCHAR: MOV BUF,#40H LCALL CMD MOV DPTR,#RCHAR REP: CLR A MOVC A,@A+DPTR JZ SCREEN1 MOV BUF,A LCALL DAT INC DPTR SJMP REP SCREEN1: MOV BUF,#80H LCALL CMD MOV DPTR,#MSG1 HERE: CLR A MOVC A,@A+DPTR JZ NEXT MOV BUF,A LCALL DAT INC DPTR SJMP HERE NEXT: MOV BUF,#0C0H LCALL CMD MOV DPTR,#MSG2 HERE1: CLR A MOVC A,@A+DPTR JZ OVER MOV BUF,A LCALL DAT INC DPTR SJMP HERE1 OVER: LCALL ONE_SEC_DELAY LCALL ONE_SEC_DELAY LCALL CLEAR MOV BUF,#0C0H LCALL CMD MOV DPTR,#MSG7 HERE2:CLR A MOVC A,@A+DPTR JZ CONVERT MOV BUF,A LCALL DAT INC DPTR SJMP HERE2 CONVERT: LCALL DDELAY CLR CS ; INITIATE CONVERSION SETB CLK CLR CLK ; FIRST CLOCK SETB CLK CLR CLK ; SECOND CLOCK MOV A,#00H ; CLEAR A MOV R5,#08H ; 8 CLOCK PULSES AGAIN: MOV C,DO RLC A SETB CLK CLR CLK DJNZ R5,AGAIN SETB CS MOV ADC_VAL,A MOV B,#79D MUL AB ; PRODUCT IN AB MOV R1,B ; HIGH BYTE IN B MOV R2,A ; LOW BYTE IN A LCALL HEX2BCD MOV VAL1,R7 MOV VAL2,R6 MOV VAL3,R5 LCALL SENDVAL2LCD CHECK: JNB LBF,CHECK2 ; SEE IF ANY FLAGS ARE SET ,i,e LOW BATT FLAG OR BATT FULL FLAG JB DYI,CONVERT CHECK2:JNB OCF,PROCEED JB DYI,NIGHT SJMP CONVERT PROCEED:JB DYI,NIGHT CLR LD_RL ; OFF LOAD CLR LBF ; CLEAR LOW BATT FLAG MOV A,VAL2 ; SEE IF BATT.IS FULL XRL A,#04H JZ FULLCHG CLR CHG_RL ; CONNECT BATT. TO PANEL MOV DPTR,#MSG4 ; DISPLAY CHARGING MSG MOV IMG,#00H LCALL SENDSTAT2LCD LJMP CONVERT FULLCHG: SETB OCF ;SET OVERCHARGE FLAG SETB CHG_RL ;DISCONNECT BATT.FROM PANEL MOV TH0,#03CH ;START 5 MIN TIMER HERE MOV TL0,#0B0H ;DISCONNECT BATT FROM PANEL MOV CNT1,#200D MOV CNT2,#30D SETB ET0 SETB TR0 SETB EA MOV DPTR,#MSG5 ; DISPLAY BATT.FULL MSG MOV IMG,#01H LCALL SENDSTAT2LCD LJMP CONVERT NIGHT: CLR CHG_RL ; RECONNECT BATT. TO PANEL CLR TR0 ; STOP TIMER0 INCASE ITS RUNNING CLR OCF ; CLEAR OVER CHARGE FLAG SETB LD_RL ; CONNECT LOAD TO BATT. MOV A,VAL1 XRL A,#00H JZ LOWBAT MOV DPTR,#MSG3 ; DISPLAY LOAD ON MSG MOV IMG,#02H LCALL SENDSTAT2LCD LJMP CONVERT LOWBAT: SETB LBF CLR LD_RL ; DISCONNECT LOAD FROM BATT. MOV DPTR,#MSG6 ; DISPLAY BAT.OW AND LOAD OFF MSG MOV IMG,#03H LCALL SENDSTAT2LCD LJMP CONVERT ;****************************************************************************** SENDVAL2LCD: MOV BUF,#0C7H LCALL CMD MOV A,VAL1 ORL A,#30H MOV BUF,A LCALL DAT MOV A,VAL2 ORL A,#30H MOV BUF,A LCALL DAT MOV BUF,#'.' LCALL DAT MOV A,VAL3 ORL A,#30H MOV BUF,A LCALL DAT RET ;****************************************************************************** SENDSTAT2LCD: MOV BUF,#080H LCALL CMD HERE3: CLR A MOVC A,@A+DPTR JZ PICT MOV BUF,A LCALL DAT INC DPTR SJMP HERE3 PICT: MOV BUF,#0CEH LCALL CMD MOV BUF,IMG LCALL DAT BACK: RET ;****************************************************************************** ; TIMER0 ISR (5 MINUTES TIMER) ;****************************************************************************** COUNTDOWN: CLR TR0 MOV TH0,#03CH MOV TL0,#0B0H SETB TR0 DJNZ CNT1,BACK2 MOV CNT1,#200D DJNZ CNT2,BACK2 CLR TR0 ; OFF 5 MIN TIMER CLR ET0 CLR OCF ; CLEAR OVER CHARGE FLAG CLR CHG_RL ; RE-CONNECT BATT TO PANEL BACK2: RETI ;****************************************************************************** Hex2BCD: MOV R3,#00D MOV R4,#00D MOV R5,#00D MOV R6,#00D MOV R7,#00D ACALL H2B RET H2B: MOV B,#10D MOV A,R2 DIV AB MOV R3,B ; MOV B,#10 ; R7,R6,R5,R4,R3 DIV AB MOV R4,B MOV R5,A CJNE R1,#00H,HIGH_BYTE ; CHECK FOR HIGH BYTE SJMP ENDD HIGH_BYTE: MOV A,#6 ADD A,R3 MOV B,#10 DIV AB MOV R3,B ADD A,#5 ADD A,R4 MOV B,#10 DIV AB MOV R4,B ADD A,#2 ADD A,R5 MOV B,#10 DIV AB MOV R5,B CJNE R6,#00D,ADD_IT SJMP CONTINUE ADD_IT: ADD A,R6 CONTINUE: MOV R6,A DJNZ R1,HIGH_BYTE MOV B, #10D MOV A,R6 DIV AB MOV R6,B MOV R7,A ENDD: RET ;************************************************************************************ ONE_SEC_DELAY: MOV R0,#10D ; One second delay routine RZ3: MOV R1,#100D RZ1: MOV R2,#250D RZ2: NOP NOP DJNZ R2,RZ2 DJNZ R1,RZ1 DJNZ R0,RZ3 RET ;************************************************************************************ PWR_DELAY: ; 15 mSec DELAY FOR LCD TO INTIALIZE AFTER POWER-ON MOV R4,#100D H2: MOV R3,#250D H1: DJNZ R3,H1 DJNZ R4,H2 RET ;********************LCD SUBROUTINES************************************** CMD: PUSH ACC ; SAVE ACCUMULATOR SETB EN CLR RS ; SELECT SEND COMMAND MOV A,BUF ; PUT DATA BYTE IN ACC MOV C, ACC.4 ; LOAD HIGH NIBBLE ON DATA BUS MOV DB4,C ; ONE BIT AT A TIME USING... MOV C, ACC.5 ; BIT MOVE OPERATOINS MOV DB5,C MOV C, ACC.6 MOV DB6,C MOV C, ACC.7 MOV DB7,C CLR EN NOP SETB EN ; PULSE THE ENABLE LINE MOV C, ACC.0 ; SIMILARLY, LOAD LOW NIBBLE MOV DB4,C MOV C, ACC.1 MOV DB5,C MOV C, ACC.2 MOV DB6,C MOV C, ACC.3 MOV DB7,C CLR EN NOP SETB EN ; PULSE THE ENABLE LINE LCALL MSDELAY POP ACC RET ; ********************LCD SUBROUTINES************************************** DAT: PUSH ACC ; SAVE ACCUMULATOR SETB EN SETB RS ; SELECT SEND DATA MOV A, BUF ; PUT DATA BYTE IN ACC MOV C, ACC.4 ; LOAD HIGH NIBBLE ON DATA BUS MOV DB4,C ; ONE BIT AT A TIME USING... MOV C, ACC.5 ; BIT MOVE OPERATOINS MOV DB5,C MOV C, ACC.6 MOV DB6,C MOV C, ACC.7 MOV DB7,C CLR EN NOP SETB EN ; PULSE THE ENABLE LINE MOV C, ACC.0 ; SIMILARLY, LOAD LOW NIBBLE MOV DB4,C MOV C, ACC.1 MOV DB5,C MOV C, ACC.2 MOV DB6,C MOV C, ACC.3 MOV DB7,C CLR EN NOP SETB EN ; PULSE THE ENABLE LINENOP LCALL MSDELAY POP ACC RET ;********************LCD SUBROUTINES************************************** CLEAR: MOV BUF,#01H LCALL CMD RET ;****************************LCD SUBROUTINES**************************************** DDELAY: MOV R5,#41D ; 4.1 mS DELAY QT2: MOV R6,#50D QT1: DJNZ R6,QT1 DJNZ R5,QT2 RET ;****************************LCD SUBROUTINES**************************************** MSDELAY: MOV R5,#26D QT22: MOV R6,#50D QT11: DJNZ R6,QT11 DJNZ R5,QT22 RET ;****************************LCD SUBROUTINES**************************************** INIT: MOV BUF,#30H ; FUNCTION SET - DATA BITS, LINES, FONTS LCALL CMD ACALL DDELAY ;INITIAL DELAY 4.1MSEC MOV BUF,#30H ; FUNCTION SET - DATA BITS, LINES, FONTS LCALL CMD ACALL DDELAY ;INITIAL DELAY 4.1MSEC MOV BUF,#30H ; FUNCTION SET - DATA BITS, LINES, FONTS LCALL CMD ACALL DDELAY ;INITIAL DELAY 4.1MSEC MOV BUF,#28H ;2 LINES 5X7, 4-BIT MODE LCALL CMD MOV BUF,#0CH ; DISPLAY ON LCALL CMD MOV BUF,#01H ; CLEAR DISPLAY, HOME CURSOR LCALL CMD MOV BUF,#06H ; SET ENTRY MODE LCALL CMD ; INCREMENT CURSOR RIGHT, NO SHIFT RET ;************************************************************************************ ORG 0320H MSG1: DB ' SOLAR CHARGE ',00H MSG2: DB ' CONTROLLER-EFY ',00H MSG3: DB ' LOAD ON ',00H MSG4: DB ' CHARGING ',00H MSG5: DB ' BATTERY FULL ',00H MSG6: DB 'BAT.LOW-LOAD OFF',00H MSG7: DB 'Volts: ',00H ORG 0400H RCHAR: DB 04D,31D,17D,31D,17D,31D,17D,31D ; CHARGING DB 04D,31D,31D,31D,31D,31D,31D,31D ; FULL DB 31D,31D,14D,04D,04D,14D,31D,31D ; LOAD DB 04D,31D,17D,17D,17D,17D,17D,31D ; LOW BATT DB 31D,31D,31D,31D,31D,31D,31D,31D DB 31D,31D,31D,31D,31D,31D,31D,31D DB 31D,31D,31D,31D,31D,31D,31D,31D DB 031D,31D,31D,31D,31D,31D,31D,31D,00H END