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Evaluation of Balancing Circuits Used in Battery System

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CAREER EPISODE 2

2.1 INTRODUCTION

My second career episode describes my project that is called 'Evaluation of balancing circuits used in battery system'. This project was completed as a part of my minor thesis in the fulfilment of my master's degree program from Swinburne University of Technology, Australia in December 2016. I completed my master's degree in electrical and electronics engineering science. It was completed under the supervision of Prof. Weixing Shen, senior lecturer at the university. And was also supported by Prof. George Banky.

2.2 BACKGROUND

Electric vehicle which operates on battery doesn't operate on a single battery as sufficient energy is not supplied through it. Thus, many batteries that are either connected in series or parallel are required for the generation of required voltage and current for the system to function. The difference in the cell voltage of the stack reduces the string capacity and hence a system is required that will be able to achieve and maintain the voltage and current that is required by the system along with considering the issue of reliability and performance. For this purpose, a BMS is used. Battery Management System (BMS), which can oversee the voltage and the current level and can also charge the battery.

2.3

The main aim and goal behind implementing this project was to design a 48V battery stack power system that consists of 12 lithium ion battery cells. The monitoring and supervision of each cell operation and voltage level and its charging status can be done with the help of LTC3300-1/LTC6803-2 bidirectional cell balancer along with the BMS system. It also considers the safety concern of the batteries that are used. And the other objective behind implementing this project was to understand LTC330-1/LTC6803-2 bidirectional cell balance kit's performance along with checking its results based on QuikEval software and DC590B USB serial controller when connected to the computer.

2.4

The idea is to balance the voltage supply of the cells so that it provides constant battery voltage to the shelter of the electric vehicle. The overcharging of the battery results in voltage battery degradation and can have consequences like it may catch fire or explosion. Whereas undercharging of the cell or having voltage under the required level may affect the life of the cells. So, I have made use of lithium ion batteries which are then further connected to LTC3300-1/LTC6803-2 bidirectional cell balancer that would assist in achieving the required voltage level of the cell and it can also help in increasing the life of the cell or battery long with providing the supply voltage of EV.

2.5

The project was completed in a group of two. And I took the opportunity to become the team leader. I first involved with my supervisor in planning the topic and how to start working on the project. Based on that I prepared a flow chart in which all the major task was divided and assigned among team member along with a deadline so that the final project gets completed in time and there will be time for troubleshooting. After deciding the topic and assigning task, I made myself busy with researching and with literature review of the topic by referring various journal, books, and internet material. The later part was designing and implementing the hardware structure along with its software part for its functioning. I had to weekly report to the Professor about my project update and took feedback on my project which helped me improve a lot. Weekly meetings with the supervisor was either done through emails or small presentations.

2.6 PERSONAL ENGINEERING ACTIVITY

I started my project with literature review on cell balancing where cell balancing is a process that involves adjusting the cell voltages to same level i.e. where the battery cells are not over charged or over discharged. Also, its internal parameters are the reason for its unbalance like its time of manufacturing, internal resistance etc. It also depends on some of the external resources like thermal difference between battery cells. The process of battery balancing in lithium ion batteries is an easy process due to its property of charge-discharge capability. It also provides increased life span and the right voltage for its application. After understanding the concepts that were going to be used in my project I started performing research on the components that could be used for my system. It took lot of survey and suggestions were taken from seniors and lecturer about it. and later I finalized the following hardware components that suited best for the project.

2.7

1. Lithium ion (Li-ion) batteries - this battery was used due to its high-energy efficiency and has relatively longer battery cycle life which is an important feature for my system as the electric vehicle requires battery than can store more as well as can frequently recharge. Basically, there are two parameters that affect the overall performance of the battery. They are power and the battery diving range.

2. LTC3300-1/LTC6803-2 bidirectional cell balancer -  for cell balancing and monitoring purpose I made use of two ICs that are functioning as bidirectional cell balancer. Active balancing principle is used by BMS. The two ICs are LTC3300-1, which can balance 6 cells whereas the LTC6803-2 microcontroller IC can support 12 cells when connected to each other in series connection. The function of LTC3300-1 is to equalize the SoC i.e. state of charge when the battery modules are connected in series manner. It can charge the battery stack faster and extent the runtime and enhancing the overall performance by cell balancing and charging. Moreover, this IC can balance 6 cells connected to each other in series when it functions as transformer based on bidirectional active balancing system that has synchronous fly back topology. Looking at the other IC i.e. LTC6803-2 is also a microcontroller IC that can support up-to 12 batteries connected in series connection. It uses different chemical material than the other IC for monitoring the batteries. It gives more accurate results and low error rate, as low as 0.25% of total. It generally operates in three modes: measure, shutdown and standby.

3. DC590B USB serial controller - it is a kind of controller that is specially designed using QuikEval software for display and demonstration of the results and for technologies involving linear family circuits. It is a controller that is based on USB. It is generally isolated or separate from the host computer. So therefore, no external or additional power supply is needed to drive the controller.

4. Power supply - GPS18500 instrument is one of a kind device which is used in my system as for this project a DC power supply of +5V is required externally to drive the unit. This power supply has many different configurations that include variable voltage and fixed voltage. It has the feature of supplying 2 different power supply which can work and be used independently. In this device, a slot that gives out fixed supply of +5V DC is provided to the LTC3300-1 bidirectional PCB.

2.8

After having finalized the hardware components, they were implemented on the evaluation board. The diagram attached shows the circuit and hardware connection for this project. It can be observed that a DC2064A GUI evaluation board, 14 conductor ribbon cables, computer that has QuikEval software installed and USB/serial port cable along with DB590B USB series controller is been used and a power supply of +5V is supplied externally using the GPS18500 device discussed earlier.

Various precaution and safety measure was considered while connecting all the different components to the evaluation board. This was done precisely and carefully as any wrong connection made, it can damage the whole circuitry and the cost of application will increase indirectly. The two ICs LTC3300-1 and LTC6803-2 are soldered or wired to the evaluation board DC2064A GUI along with the capacitors, MOSFET and transformers. The voltage level and the temperate can be monitored with this circuit board and has the capability to balance the cell voltage. This board also has a special feature of advance fault detector that helps to secure the performance of the board in case of any spark or short circuit. 3.7V and 2600mAH configured lithium ion battery is used in my system. Thus 6 batteries giving a output voltage of 22.8V when connected in series are used on the board as the board can withstand a maximum of 12 batteries for balancing.

2.9

After successful implementation of the hardware, testing and trouble shooting was performed. For that a QuikEval software was used to monitor the battery stack. To use the software, it had to be configured accordingly for both the microcontroller ICs that were used in the system. After proper implementation, the screen looked as below.

From that it can be depicted that the when we press the button called START CONTINUES READ CELL, voltage level can be retained and measured from each battery connected in the battery stack. From performing the test run, I found the cell voltage for each cell was almost 3.7V. But if there is any minor change in the voltage level of any cell it can be equalized by transferring cell voltage manually for 5sec and charge the required cell by discharging other cells with high voltage to get an equilibrium stage. All the charging/discharging or any error situation status can be seen on the screen as that status will be highlighted.

2.10

The struggle in implementing this project was getting the expected outcome. It different the actual outcome. It took many efforts, there were issues like the hardware didn't receive any signals from the software. The problem was solved by reconnecting few hardware wires. Also, it required changing the voltage level of the components by choosing higher level components. After the final configuration, the hardware received the signal and the voltage level of each cells were determined. The later challenge was the important task and the main idea of our project i.e. cell balancing by voltage levels. The output was taken and written down at each time for charging/discharging function and the experiment was carried out until the satisfied result was attained. There were times when I didn't get the desired result even after performing the experiment several times. During this phase, my faculty members and supervisors helped me a lot and gave me steps or hints on how I can solve the error.

2.11 SUMMARY

To conclude, my project used 3.7V lithium ion battery that has 2600mAH of capacity for active cell balancing technique while making use of two microcontroller ICs configured on evaluation board. The transfer of charging from one cell to another is done using the six lithium ion batteries that are connected in series on the evaluation board. Along with that the software QuikEval was used to trace the output. The project has an open window in terms for expansion for future project work based on this system. This project can be used for self-controlling function to detect faulty or imbalance situation in the system along with capability for temperature measurement unit.

Though I could only achieve partial results from my experiment as it varied from expected outcome. But this experiment helped me in many other ways. It taught me to handle complex circuits and outcomes, though I didn't partially achieve what I expected from this but it gave me the strength and knowledge on how to overcome such situations. It was a new learning experience by working in groups and team mates. It involved managing different views and coming to one conclusion before implementing the idea. Also, I had the opportunity to apply my previous engineering knowledge into this project.


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