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This report presents the process of design and implementation of a battery charger for. a Li ion battery The development of this battery charger includes the component from. Linear Technology LTC4015 This component integrates the functions of a battery charger. configured as a buck step down converter This device must be integrated in a Printed. Circuit Board with a specific design Also it must be configured using a microcontroller. named Raspberry Pi which also performs the measurements. The method of design is divided in two parts One is focused on developing the printed. circuit board which includes the simulation of the device and the development of the PCB. and the second one is focused on developing the program used in the microcontroller to. manage the operation of the LTC4015, The result is a charge controller device that can be used with different configurations. with a buck converter topology The different parts of the design process are the simulation. the design and the implementation Each of these parts have a section of results in this report. The simulation section includes results obtained with LTSpice and the device LTC4020. which is a similar device to the LTC4015 but without the Maximum Power Point Tracking. mode which is not modelled in LTSpice, PV is the main power source considered to charge the battery and is carefully studied. in this project The PV input is studied with LTSpice first simulating the I V curve of the. schematic of the solar cell Second integrating a solar cell in the simulation of the LTC4015. Third operating the device LTC4015 with a solar panel that is also characterized. The design section includes the electronic components used for the development of the. board that integrates the charge controller the LTC4015 in this case based on the. calculations performed for the requirements of the LTC4015 Finally the implementation. section includes the description of the board implemented but also the description of the. configuration and measurement code, The conclusions presented in this report show that the LTC4015 is a battery buck. charger with different functions that make it suitable to be used in different solar. applications Also this report opens new future work lines such as the full characterization. of the board the implementation of a test bench and the integration of the board in different. applications for solar energy systems,Acknowledgment. I would like to thank professor Thomas Walter for his help in this project and for giving. me the possibility of using the laboratory of Hochschule Ulm in which I have acquired very. interesting skills especially in the fabrication of PCBs which I find most interesting Also I. would like to thank Peter Adelmann for lending me his Li ion battery Finally I would like. to thank Volker Schilling for his help with the design of the board. From H gskolan Dalarna I would like to thank D sir e Kroner for supervising my. work and Frank Fiedler for helping me in the beginning of the master program I would also. like to thank the rest of the professors of the master for their time and their willingness to. give their students their knowledge and also their patience. I will like to thank all the people who has been with me during this year sharing my path. and helping me while needed and among these especially my family and my friends from. Dalarna University Thank you very much to all,1 Introduction 1. 1 1 Aim and objective of the work 1,1 2 Method of development 2. 1 3 Previous work 3,2 Development of the Li ion battery charger 4. 2 1 Introduction to solar cells 4, 2 2 Description of a buck and buck boost converter 5. 2 3 Introduction to Li ion batteries 6, 2 4 Hardware design Simulation of the LTC4020 with LTSpice 7. 2 5 Hardware design Selection of the components 12. 2 6 Hardware design Design and implementation of the board 14. 2 7 Software design Description of the code 18,2 8 Software design Auxiliary functions 22. 2 9 Software design Main functions 24,2 Results obtained from the experiments 28. 2 1 Results obtained from the simulation with LTSpice 28. 2 2 Results obtained from the experiments performed with the LTC4015 battery. charge controller 33,3 Discussion of results and conclusions 37. 3 1 Discussion of results 37,3 2 Conclusions 38,3 3 Future work lines 38. 4 Bibliography 39,Appendix A Bill of materials A,Appendix B Schematic of the circuit B. Appendix C Gerber files of the circuit C,Index of Figures. Figure 1 Parts of the development process 2,Figure 2 Band diagram of the p n junction 5. Figure 3 Buck converter circuit 5, Figure 4 Description of a Li ion battery operation Liu et al 2016 7. Figure 5 Schematic of the equivalent circuit of the solar cell for LTSpice 8. Figure 6 Schematic of the device LTC4020 used for the simulation in LTSpice 10. Figure 7 LTC4015 battery charger schematic application for Li ion battery 12. Figure 8 Layout of the board described in EAGLE representing the routing 15. Figure 9 Top layer of the PCB 16,Figure 10 Bottom layer of the PCB 17. Figure 11 Flow diagram of the configuration and telemetry program 20. Figure 12 Charge algorithm for Li ion batteries Linear Technology 2016 21. Figure 13 Simulation of the solar panel I V curve with a sweep of the series resistance 28. Figure 14 Simulation of the solar panel P V curve with a sweep of the series resistor 29. Figure 15 Simulation of the solar panel I V curve with a sweep of the parallel resistor 29. Figure 16 Simulation of the solar panel P V curve with a sweep of the parallel resistor 30. Figure 17 Charging curves of a Li ion Battery for different current values 31. Figure 18 Switching frequency of the LTC4020 32, Figure 19 Charge current and voltage applied by the LTC4020 to the battery 32. Figure 20 Battery voltage during the charging process 33. Figure 21 Charge current applied by the LTC4015 charger 34. Figure 22 Measurement of the Coulomb Counter 34, Figure 23 Battery voltage against State of Charge SoC 35. Figure 24 I V characterization of the solar panel used for the test 35. Figure 25 P V characterization of the solar panel used for the test 36. Figure 26 Schematic of the LTC4015 for the circuit developed B. Figure 27 Gerber files for the top a and bottom b copper layer C. Figure 28 Gerber file for the drill position and size of the board D. Index of Tables, Table 1 LTSpice simulation modes Linear Technology 2011 7. Table 2 MPP values of the solar panel for different series resistance values 28. Table 3 MPP values of the solar panel for different parallel resistance values 30. Table 4 Comparison of the results obtained from the characterization and the test with the. LTC4015 36,Table 5 Bill of materials A,Nomenclature. ACK Acknowledgement message, ARM Advanced RISC Machine Microcontroller architecture. Buck converter Step down converter Voltage reducer. Buck boost converter Step down Step up,CC Constant current. CC CV Constant current Constant voltage,CV Constant voltage. EAGLE Easily Applicable Graphical Layout Editor PCB editor. Float voltage Maximum battery voltage, Gerber File format for PCB layout standard representation. I2C Inter Integrated Circuits communication protocol. IC Integrated Circuit, JEITA Japan Electronics and Information Technology Industries. Li ion Lithium Ion battery chemistry,LTC4015 Buck battery charge controller. LTC4020 Buck boost battery charge controller, MOSFET Metal Oxid Semiconductor Field Effect Transistor. MPP Maximum Power Point,MPPT Maximum Power Point Tracking. P P Pick and Place,PCB Printed Circuit Board,Power Path High current driving path. Raspberry Pi ARM based Single Board Computer microcontroller. RISC Reduced Instruction Set,SCL Serial Clock Line. SDA Serial Data Line,SMB System Management Bus I2C subset. SMBAlert System Management Bus Alert line,SoC State of charge of the battery. Trickle voltage Recharge voltage of the battery,Via Vertical interconnected access. 1 Introduction, Charge controllers are one of the main components of PV stand alone systems as they. protect batteries from degradation due to operation conditions outside of the acceptable. range Batteries are needed for storing the energy generated with the sun panels as for PV. stand alone systems the demand of electricity covers all the day while the supply of energy. covers only part of the day due to the nature of solar energy While some batteries can be. charged directly with low exposure to degradation for PV stand alone systems a charge. controller is recommended as the output of the PV panels is not constant during the day. Li ion batteries are used more widespread nowadays and they cover a wide range of. applications such as electric vehicles hybrid electric vehicles mobile devices and PV systems. Li ion batteries are selected due to their high energy density their low self discharge rates. and their low maintenance required On the other hand Li ion batteries have strict operation. limits and require complex protection circuits they are expensive and they are subject to. degradation due to aging even if they are not used More research is required in this field to. achieve optimum performance of Li ion batteries,1 1 Aim and objective of the work. The aim of this project is to develop a solar charge controller for a Li ion battery that. will be used in a solar trike There is not a device specifically suited for this application and. the design of this device will provide a solution The LTC4015 Linear Technology 2016 is. the component considered for the development of the project and this device is a recent. implementation of Linear Technology The LTC4015 performs the control of the battery. according to the instructions given by the user which are transmitted digitally by another. controlling device such as a microcontroller The microcontroller Raspberry Pi is a useful. tool to develop a control system due to its flexibility. The LTC4015 gives more flexibility to the design as it already includes many operation. modes that will be useful for the implementation of the charge controller These options. include the monitoring of the temperature voltage and current of the system the control of. the MPP tracking module the control of the charging current limiter the monitoring module. of the State of Charge SoC and the communication interface between the microcontroller. and the LTC4015 I2C NXP Semiconductors 2003 With the Raspberry Pi the control. instructions for the LTC4015 will be executed using the communication interface provided. so that the charge controller operates in real time. Both hardware and software are part of this design as the LTC4015 operates with the. instructions given by the user The hardware design must accommodate the software that is. implemented This software works through the communication interface of the LTC4015. and the Raspberry Pi and the hardware that includes the LTC4015 needs an I2C input to be. able to connect it to the Raspberry Pi,1 2 Method of development. The development process is divided in two parts which correspond to the software and. the hardware design These two parts are also divided in different subsections that are. explained here The different parts of the hardware development are the simulation of a. charge controller the selection of the components the design of the board and the. implementation of the board On the other hand the different parts of the software. development are the study of the communication interface I2C the implementation of the. auxiliary functions and the development of the main functions of the program The. Raspberry Pi stores and executes the software implemented for the project The different. parts of the development process can be seen in Figure 1. Design of a buck converter,battery charger,Hardware design Software design. Simulation Communication interface,Selection of the components Auxiliary functions. Design and implementation of,Main functions,Figure 1 Parts of the development process. The first part of the hardware development is the simulation of a charge controller. which will be performed using LTSpice This simulation tool has different models of charge. controller Unfortunately the LTC4015 is not one of them as the MPPT is not included in. the model The simulation will be performed using the model of the LTC4020 which is a. buck boost converter without the MPPT function, The second part of the hardware design is the selection of the components of the charge. controller The datasheet of the LTC4015 includes the information needed to select the. components of the board according to the design specifications The third part of the. hardware design is the design and implementation of the board The software tool used to. design the board is EAGLE and the result of this design can be sent to a manufacturer to. produce the board that integrates all the components. The first part of the software design is the study of the communication interface I2C. This study is required to understand the communication process of the LTC4015 and the. Raspberry Pi and implement the software accordingly The second part of the software. design is the development of the auxiliary functions that perform the conversion of the data. from the Rapsberry Pi and the LTC4015 The third part of the software design is the. implementation of the main functions of the program that perform the configuration of the. LTC4015 and the measurement of the operation parameters of this device which will be. stored on a database in the Raspberry Pi,1 3 Previous work. Different research projects on charge controllers for Lithium ion batteries have been. done given the complexity of these batteries to operate efficiently and their sensitivity to. extreme operation conditions which lead to the degradation of the system The projects. presented in this section are aimed at systems present in different fields like automotive. systems portable devices and PV off grid installations and they describe charge controllers. In Zhihao et al 2010 a charge controller for a Li ion battery is presented which. includes the development of a state machine to control the operation of the charge controller. This state machine controls the rest of the modules of the system which includes an Under. Voltage Lockout UVLO to monitor the input voltage A Battery Thermal Control BTC. module is also included to monitor the temperature and control the other modules integrated. in the device through the state machine Finally an oscillator and a counter are included to. control the charging time of the battery, Also in Armando et al 2009 a charge controller for a Li ion battery integrated in an. electric scooter is presented This device has a higher level of complexity due to the AC. motor integrated in the scooter which requires also a 3 phase inverter to operate The charge. controller also includes a power factor correction system to adjust perfectly to the AC motor. The system has a 3 phase boost converter to optimize the operation of the AC motor and. the battery The system is developed with a FPGA, In Tien Ha et al 2014 a high efficiency buck converter is implemented which allows. controlling the battery for a wide range of load values This buck converter device uses a. PWM PFM model for charging the battery The system switches between the Pulse Width. Modulation PWM and the Pulse Frequency Modulation PFM for high and low load. currents respectively The PFM mode reduces the switching losses so that the overall. efficiency is increased, Finally in Amin et al 2009 a microcontroller based charge controller is introduced. based on the PlC16F877A RISC microcontroller This system is also based on a finite state. machine but in this case it is implemented in the microcontroller The system also integrates. a temperature sensor a voltage transducer and a current transducer to monitor the main. parameters of the system The charge controller also uses a Pulse Width Modulation PWM. charging model with a variable duty cycle,2 Development of the Li ion battery charger. This chapter includes the description of the design and implementation of the software. and hardware components of the device The chapter is divided in nine sections that include. an introduction to solar cells an introduction to Li ion batteries and an introduction to. battery chargers and its topologies buck and buck boost converters Then the remaining. sections are identified with the corresponding design part The hardware part consists of the. simulation of a charger with LTSpice the selection of the components for the device and. the design and implementation of the board The software part consists of the study of the. communication interface I2C the implementation of the auxiliary functions and the. implementation of the main functions of the program. 2 1 Introduction to solar cells, Solar cells are devices that convert the energy coming from the sun into electricity These. devices are based on a semiconductor material such as silicon Once a photon coming from. the sun falls upon the surface of the semiconductor the energy of the photon is supplied to. an electron If the energy supplied by the photon is high enough it will move the electron to. a higher energy level In this case two levels of energy are considered which are the valence. band and the conduction band This means that once the photon falls upon the surface of. the semiconductor an electron is moved from the valence band which is then charged. positively to the conduction band which is now charged negatively This generates a pair of. electron negative charge hole positive charge, The solar cell is also composed of two semiconductor materials named p. semiconductor which has an excess of holes and n semiconductor which has an excess of. electrons The resulting material is named p n junction and both materials of the junction. have a valence band and a conduction band Also the region between the p semiconductor. and the n semiconductor is called depletion region The electrons and the holes of their. respective materials will move towards the opposite material and then will be placed in the. depletion region according to the type of carriers electrons will be placed near the p. semiconductor and holes will be placed near the n semiconductor generating an electric. field between the p semiconductor and the n semiconductor inside the depletion region. Once photons impact on the surface of the junction pairs of electron holes are. generated which will move according to this electric field producing again an excess of. electrons in the n semiconductor side and in the p semiconductor side Then this flow of. electrons that move following the direction of the electric field is called photocurrent The. incidence of light generates carriers pairs electron hole that move inside the junction. There are two types of carrier flows inside the device The current from the n. semiconductor to the p semiconductor named as drift current and the current from the p. semiconductor and the n semiconductor named as diffusion current The drift current is. the result of the electric field generated inside the depletion region of the p n junction while. the diffusion current is produced by thermal effects in the junction. Figure 2 Band diagram of the p n junction, 2 2 Description of a buck and buck boost converter. A charge controller regulates the operation of the battery to avoid its degradation This. degradation can be caused due to exceeding the operation limits of the battery This is more. critical in the case of Li ion batteries as they have stricter operation ranges and conditions. The charge controller must ensure that the charging voltage temperature and SoC are within. the safety limits There are also different types of input sources that may be able to charge a. battery These source inputs can supply higher equal or lower voltages compared to the. voltage of the battery to the system and the charge controller must be able to adapt this. input voltage to the voltage of the battery, If the input voltage is higher than the voltage of the battery the converter has a step. down or buck topology On the other hand if the input voltage is lower than the voltage of. the battery the converter has a step up or boost topology There are charge controllers that. can be adapted to both situations and they are called buck boost converters or step. down step up converter These converters use a passive device in this case a coil that is. able to store electromagnetic energy in order to release it adequately according to the voltage. needs of the system In this report two charge controllers are presented which are the. LTC4015 and the LTC4020 The first one has a buck converter topology while the second. one has a buck boost converter topology Figure 3 presents the schematic of a buck. converter topology charger The main components are a coil a switch and a diode. Figure 3 Buck converter circuit, The circuit presented in Figure 3 operates in the following way When the switch is. closed the inductor is charged The input voltage is Vi while the output voltage is Vo Then. the voltage of the inductor will be 0 If the time equation of the inductor. presented in 1 is integrated during the on time to obtain the current increase in the inductor. the result will be 0 in which ton is the amount of time that the. switch is on In this case it can be expressed as which means that the circuit is. open a fraction of the cycle T which corresponds to the duty cycle of the signal D The. duty cycle is expressed as a fraction of the total cycle and takes values in the range 0 1. The process for the calculation of the current increase when the switch is open is similar. but in this case the voltage of the inductor is 0 Also the amount of time that the. switch is off is 1 which means that the switch is closed a fraction 1 D. of the cycle which is the opposite of the duty cycle The resulting of integrating equation 1. over the off time is 0 Given that the sum of both current increases. must be 0 the resulting equation is presented in 2 and the result is the ratio between the. input voltage and the output voltage which is the duty cycle 0. 2 3 Introduction to Li ion batteries, Li ion batteries have one of the highest ratios of energy and power density which makes. them suitable for many applications especially portable devices Li ion batteries provide. energy by a Redox chemical reaction which means that one of the components of the. reaction suffers oxidation loses electrons and the other suffers reduction gains electrons. There are two parts in the battery The first one is the anode that provides the electron to. the circuit when the battery is being discharged oxidation The second one is the cathode. that receives the electrons from the circuit when the battery is being discharged When the. electric circuit is closed the battery starts to discharge and the Redox reaction starts so that. the electrons move from the anode to the cathode There is also an electrolyte that allows. the movement of ions between the anode and the cathode while separating both electrodes. In the case of a Li ion battery the cathode is the Lithium based component which. depends on the type of battery one of the most used elements is the LiCoO2 Lithium has. the lowest reduction potential so that the cell voltage will be much higher Nitta et al 2015. Also it is possible to find in Nitta et al 2015 a discussion on new electrode and electrolyte. materials During the discharging process the anode supplies electrons to the circuit while. the cathode lithium based component supplies Lithium ions Li It is possible to see the. operation of the device in Figure 4 which is obtained from Liu et al 2016. Figure 4 Description of a Li ion battery operation Liu et al 2016 Reprinted with permission. 2 4 Hardware design Simulation of the LTC4020 with LTSpice. This section includes the simulation of three devices which are a solar cell a Li ion. battery and the LTC4020 which as mentioned before it is a charge controller with buck. boost converter topology This means that it can be configured as a buck or as a boost. converter The aim is to simulate a device similar to the LTC4015 and the LTC4020 will be. configured as a buck converter according to the datasheet of the device. LTSpice features six simulation schemas which are described in Table 1 The transient. analysis and the DC sweep will be used depending on the device under simulation and it will. be indicated for each device in the corresponding section. Table 1 LTSpice simulation modes Linear Technology 2011. Evaluation mode Brief description, Transient analysis For time evaluation mode Analog or digital. AC sweep For analogic systems frequency,DC sweep For DC systems. Noise analysis For analog systems with noise,DC transfer For DC input output analysis. Self defined For general purpose,2 4 1 Simulation of a solar panel. The equivalent circuit of the solar panel is presented in Figure 5 There are four. components in this schematic The current source represents the photocurrent of the panel. The diode represents the behavior of the solar panel as a semiconductor The series and. parallel resistors represent the energy losses of the solar panel The datasheet of a commercial. solar panel Solarworld 2012 provides the values of the photocurrent and the number of. cells The simulation of the solar panel also includes a sweep of the values of the resistors. to evaluate the impact of the losses on the performance of the device. Figure 5 Schematic of the equivalent circuit of the solar cell for LTSpice. The electric analysis of this circuit gives equation 3 as a result which includes the. effects of the diode the photocurrent and the two resistors This expression is similar to the. Shockley equation of the diode with the resistors Cubas et al 2014. The diode D is represented by three parameters in equation 3 which are VD VT and. N The first one represents the breakthrough voltage while N is the ideal factor of the diode. The ideal factor represents the deviation of the I V curve with respect to the diode.
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