Solar Battery Charger With MPPT Using A Microcontroller

C.Suchetha

Photo Voltaic Cells

Photo Voltaic cells are devices that generate electromotive force as a result of absorption of ionizing radiation.

Advantages of PV cells

• Direct conversion of solar radiation to electricity

• Reliable, Maintenance Free

• Quite, compatible

• Durable

Disadvantages of PV cells

• Low conversion efficiency

• High fabrication cost

• Continues supply is not possible

Three essential approaches to overcome the drawbacks

• Improving manufacturing process of solar arrays

• Controlling insolation input to PV arrays

• Utilization of output electric power of solar arrays

Conventional battery charging system

Maximum Power Point Tracking

• MPPT is an electronic tracking in which the controller compares the panel output with the battery input and pushes the operating point to the maximum power point.

• MPPT is used in PV systems to continuously tune the system so that it draws maximum power from the solar array irrespective of weather or load conditions.

Types of MPPT Techniques

• Look –up table method

• Perturbation & observation (P & O) method

• Computational method

Perturbation & Observation Method

Types of P & O Method

• Comparison with a constant reference value

• Incremental Conductance Method

• Feed Forward MPPT Controller

Present System

An MPPT control system has been developed ,consisting a Buck converter which is controlled by a microcontroller based unit

The main difference between this and existing methods is that the power output from the panel is directly used to control the converter output.

MPPT is more effective under these conditions

• Winter or cloudy days

• Cold weather

• Low battery charge

Block diagram

The block diagram consists of four blocks.

•PV array•Buck converter •Battery•Microcontroller

The PV array specification used in this project is as follows

• Maximum voltage: 35 volts

• Open circuit voltage: 44 volts

• Maximum current: 15 amps

A sealed lead acid battery is used in this project. The specification are

• Battery voltage : 24 volts

• Battery capacity : 150 Ah

Buck converter

ATMEL’s ATmega 128L low-power consumption, CMOS microcontroller with built in EEPROM, FLASH memory, ADC, PWM generator is used.

• Advanced RISC Architecture

• 128K Bytes of In-System reprogrammable Flash

• In-System Programming by On-chip Boot Program

Schematic Diagram

Software Development Tools

The tools used in developing this software WinAVR GCC compiler and AVR Studio 4 IDE for AVR series of microcontroller.

The C program written is complied using the WinAVR GCC compiler. This compiler converts the C file to the file with extension .cof. This .coff file is used to build, simulate and debug the program using the AVR Studio 4. The AVR Studio 4 generates .hex file which is downloaded to microcontroller.

Main Program

MPPT Charge Function

Trickle Charge Function

The testing was done for three different conditions

1. Connection of the panel directly to the

battery.

2. Connection of the panel to the battery

through an ordinary charger.

3. Connection of the panel to the battery

with MPPT.

Results

From the results obtained the percentage of the power wasted the three different conditions is

• 23.8% when directly connected.

• 18.63% when connected through a ordinary charger.

• 11.38% when connected to a MPPT charger.

Conclusion

• The MPPT technique assures maximization of energy transfer to the battery bank.

• The battery lifetime is increased because the battery is operating at higher state of charge.

Future Extensions

• This design can be further be worked upon to include GSM for remote controlling of the load.

• This control system can be used for a grid connected system also.