Chapter 1: Battery management: State of charge

Transcription

1 Chapter 1: Battery management: State of charge Since the mobility need of the people, portable energy is one of the most important development fields nowadays. There are many types of portable energy device and the most of them are around of a state of charge measurement system. a. Definition State of charge (SoC) is the available capacity of a battery and it could be compared to the fuel gauge in a fuel vehicle. This parameter is used to be represented as a percentage of a fully charged reference, which should be reset and update periodically; although, rarely, it is shown as absolute measure in Coulombs (C), Ampere-hour (Ah) or kilowatts-hour (kwh). Basing in the amount of available energy which is stored in a battery referred to the complete range of energy that this battery could store, it is possible to find out how much longer the battery will be working properly and when it needs be recharged. State of charge measurement is essential to reach an optimum behavior in a system which manages electric or hybrid cars in order to maximizing the function of the electric motor versus the engine. The capacity of a battery is not always the same value because of the ageing. It means that the capacity of an old cell is lower than the capacity of a new one due to the charge-discharge cycles in its life. The rated capacity must be calculated always in reference to the current value to the battery, not in reference to the value that it had at the beginning, for that reason the reference has to be update periodically. Otherwise, SOC reaches never the maximum value 100% when a battery is not new. But ageing is not the only factor which is involved in the SOC estimation, there are several ones: 11

2 Charge Discharge Rate. The electrochemical process in the cells takes always a finite time and it may be longer than the continuously electrical stimulus which is charging the cell. The effect of a charging pulse may not be completed if a discharging pulse is given very fast and it could be a source of inaccuracy in measurement of SOC. On the other way, the contrary process wastes energy in the electrochemical reaction and the discharge energy is less than the stored one. The ratio between discharge energy and charge energy is called Coulombic efficiency and can reach up to 3 % of the available capacity. Graph 3 Hysteresis The open circuit voltage of a battery in a charge is higher than this voltage in a discharge. 12

3 Graph 4 Temperature and discharge rate The available capacity of a battery depends directly on the temperature in which is involved. The look up shows an example of how it depends on the temperature and the discharge rate Graph 5 13

4 Self discharge Aside of the charging-discharging cycles, the self discharge effect is consuming energy from the available energy of the battery. This amount is typically less than 3 % per month and if the time between charges is too long, the effect can be important. b. State of art: Methods Many different methods have been developed to measuring the SOC of a battery stack. The research work to improve the SOC determination has been increased in recent years and it depends on the type of battery, its chemistry and its parameters. Direct measurement This method is hypothetical and theoretical since a constant discharging current rate is needed, which would be multiplied by the total discharging time. This integration method is not available because the discharging current rate is variable for all practical batteries. Furthermore, another important drawback of this theoretical estimation is that, in the most of the practical cases (not evaluation or testing), users should be able to measure the SOC without discharging the battery or, at least, before it becomes to be completely discharged. Specific gravity (SG) measurement Also known as Relative density measurement, this method is only available for cells which can offer access to their internal liquid electrolyte. This parameter is referred to the ratio of density of a substance to the density of a reference substance, which is used to be water. The specific gravity is decreasing linearly with the ampere-hour discharge of the battery cell. Hence, measuring the density of the electrolyte provides an approximate indication of the state of charge of the cell. Even though this method is rather accurate to estimate the SOC of 14

5 one cell, it is not able to determine the battery capacity. Graph 6 shows one example of specific gravity and voltage measurement depending on the charge discharge time. Graph 6 Internal impedance The composition of the chemical components of the cell is changing in charge-discharge cycles and it derives internal impedance variation. This parameter could also be an indicative of the estimation of SOC. However, the measurement of the impedance is difficult during running as well as it is depending on the temperature. Voltage based estimation Cell voltage measurement is another method of SOC estimation. This estimation is based on the current battery voltage as a direct relationship of the remaining capacity. Depending on the type of battery this relationship can be linear or non linear. When it is not linear, like on Lithium-ion cell; this estimation may not be accurate enough at intermediate values. Graph 7 shows the characteristic curve of a discharging cycle of a Lithium-ion battery. 15

6 Graph 7 At the time close to total discharge, an abrupt fall is marked showing imminent critical discharge. In many applications, this situation is forbidden and it can be an important drawback. Moreover, at intermediate range of the voltage, the SOC estimation is not accuracy at all due to the dependant factors of the battery: temperature, discharging rate, hysteresis Thus features make Voltage estimation a good method in order to estimate the total charge and the imminent discharge point, but some method else is needed to make this measurement reliable at the whole range. Current based estimation This method is also called as Coulomb Counting and it consists on the integration of the current entering (charging) and leaving (discharging) the battery. Basically, this estimation takes into account the current flowing to and from the cell and integrates it over the time. Hence, since the integration of current is charge, Coulomb counting is currently the most accurate method to estimate SOC due to the directly measure of the charge flowing over the battery. However, this estimation needs to be combined with one more measurement else to 16

7 know the conditions of the measurement, for example the starting point for the current integration. The reference point is the fully charged point and the charging losses or self-discharge are not taken into account in this method. An additional method has to be added to get more accuracy by this estimation. There are several sensors to measure the current: o Current shunt: measuring the voltage drop a low ohmic value, high precision, sense resistor which does not cause large power loss. This method is not accurate for low current. o Hall Effect: it consists on the user of transducers to measure the current but they are susceptible to noise and not acceptable to high current even more expensive. o GMR: Magnetoresistive sensors are more expensive but they obtain higher signal level and are more stable at high temperature. Its sensitivity is higher. Studying the state of the art of SOC measurement and regarding on the influence factors to estimate this important parameter, Coulomb counting with the support of the battery voltage measurement has been chosen to develop this project. The current integration obtains the available charge capacity into the battery and the voltage measurement is used to acquire the reference the extreme of the function battery range. The voltage is also used to refresh and update the available capacity of the fully charged battery. 17

8 c. Objective and organization Designing a complete system to estimate the battery SOC of Toyota Prius is the scope of this project. The project development has been performed by this documentation s author. This process has been faced knowing the difficulties that the design over one car can reach. Prius battery stack is sited on the backside of the car and the screen to be observed by the driver is on the front side of the car inside of the drive cabin. Since this situation, the communication between battery measurement and screen displaying turns out complex. To solve this drawback, the use of wireless communication has been integrated to communicate both sides. The work was divided into different stages depending on the advance: Documentation and research System block diagram and components choice Schematics Building Layout design Soldering Programming Debugging and mistakes correction Evaluation 18

9 These stages has always been accompanied by documentation writing as well as each stage was preceded by one period of learning and practicing of the respective software or handling of tools to be used. Anyway, every stage was performed by the documentation author. d. Tools and materials To develop the whole project in order to provide it the maximum precision and efficiency, several of the most famous and accurate software have been used. It has been considered to be named in this documentation by the fact of the understanding of some schematics, layouts and pictures in general which are part of the project document. Every software is licensed by austriamicrosystems AG. This list of software and programs is attached with a series of physical materials and hardware needed to connect this software between them. As software, this hardware is provided by austriamicrosystems AG and its suppliers. Following, the different programs and tools which have been used are described as well as its respective uses in the envelopment of the various stages of the design. PSpice Student Version It is a limited version for several types of simulation of simple schematics with a limited number of nodes and devices. This software is supplied by Orcad and this version is free distribution. This version provides some types of simulation of circuits, being able to calculate AC simulation, DC simulation and transient analysis for example. The behavior of one circuit can 19

10 be showed in multiple outputs with the stimulation of some inputs regarding on either the time either the frequency. PSpice has been used to specify the values of the passive components of the anti-aliasing filter sited on the input of the current measurement. This behavior will be explained in details later. Cadence SPB 16.2 It is one of the most famous tools for complete PCB design. This is also Orcad supplied software but, in this case, it is not a free distribution program. From top to bottom, a whole PCB can be designed with the help of Cadence software. In purpose of this project, two important tools of Cadence have been utilized: o Allegro Design Entry CIS This tool is used to create schematics and produce connectivity and simulation information for printed circuit board and programmable logic designs. Allegro PCB Design CIS is a complete printed circuit board design solution that integrates PCB tools for creating design projects, managing libraries, capturing schematics, packaging, physical placement and routing. The complete schematics of transceiver and receiver board has been designed with this tool as well as the netlists of the layouts were built to be routed in the next step. 20

11 o OrCAD Layout Plus After assigning the respective footprint to each component of a schematics circuit, Layout Plus is used to route the PCB layout. This tool is also part of Cadence software. It is provided a library manager which is able to design the footprint of the component to be used on each PCB and, furthermore, some libraries are added with the most of the most useful component footprint on the current market. This tool has been used to route the layout of the transceiver and receiver boards and some concrete footprints has been designed with this specific software. AVR Studio 4 This programming tool is powered by ATMEL to program ATMEL microcontroller from a visual and intuitive interface. It can be found on the official ATMEL web page and is freeware. Any ATMEL microcontroller can be programmed with AVR Studio because it provides libraries for all types of ATMEL microcontroller. This tool gives the option of programming with C programming language or Assembly programming language. This tool is also a debugger program to run the code step by step and debug the programming. The programming of both microcontroller (transceiver and receiver) has been done and debugged with this tool using C language. Compiling, debugging and object building are some of the mainly functions of it. 21

12 Extreme Burner It is software to program AVR microcontrollers using USBasp programmer. This tool is used to program EEPROM, Flash and fuses of ATMEL microcontrollers with a graphical, simple interface. It also is able to read the values already programmed on the device memory. This software was used to load the programming on the microcontroller of both PCBs with the help of USBasp. USBasp USBasp is an USB programmer for AVR microcontroller. This hardware is connected by serial interface to program the target and debug it. This unit was ordered to the webpage Figure 5 22

13 The schematics of the USB programmer is shown in the next picture Schematic 1 As it is shown in the schematic 1, this device is easily built with one microcontroller and some passive component and it is very useful working with ATMEL AVR controllers. Standard Laboratory tools and materials During the development of this project, some standard tools have been used on the laboratory, like soldering stations, soldering materials, wires, computer, computer complements To sum up, the standards utilities sited into a standard lab environment. 23