09-07-2011, 09:32 AM
Presented By
Jeevan Reddy N
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ABSTRACT
Battery is the most widely used energy storage device. Since its invention, it has become a common power source for various household, commercial and industrial applications. Despite its ever increasing importance, many challenges remain unsolved to characterize and manage the battery. Among them, one fundamental issue is the estimation of state of charge (SoC). SoC, expressed in percentage, refers to the amount of capacity available in a battery. SoC is critical for modeling and managing batteries. If SoC is 100%, reflects a full battery and if SoC is 0%, reflects an empty battery.
Information on SoC can be used to control charging and discharging process of the batteries. A good SoC estimation offers many advantages such as longer battery life, better battery performance and increased reliability of battery pack. There are several methods for determining SoC. Some of the popular methods are Coulomb counting, Voltage estimation and Impedance measurement method. There have been many attempts in literature to estimate SoC by synthesizing circuit models based on measured voltage and current at battery terminals. The final goal of any SoC algorithm is to predict the remaining capacity accurately. Developing efficient yet accurate SoC estimation algorithms remains a challenging task.
This project aims at developing a novel method to estimate the SoC and remaining runtime of a rechargeable battery which overcomes the drawbacks of existing methods. The proposed method is based on renowned Coulomb Counting technique. The proposed method predicts the SoC by Coulomb Counting method and corrects it using PI controller by employing a closed loop to estimate actual SoC. The proposed method is simple and easy to implement. The SoC as well as remaining runtime are estimated accurately.
Based on the new method, a model is developed using MATLAB/SIMULINK. The code corresponding to develop model is dumped in a target PC and is run in real time for online estimation of SoC. The required parameters such as voltage and current at the battery terminals are acquired by target PC and SoC is estimated. Estimated SoC and remaining runtime are used for control the charging and discharging process of the battery. A hardware test bench is developed for acquiring voltage and current at the battery terminals for online estimation of SoC.
1. BATTERIES
The modern society relies completely on the fossil fuels such as natural gas, coal and oil for its energetic needs. Their reserves are however limited and the environmental concerns are nowadays haunting the society. The utilization of energy in a sustainable way is the only pursuable solution to cope with these problems. Thus, the conversion and storage of energy is becoming necessary step for global efficiency of the energy generation and utilization process. Battery is the most widely used energy storage device. An electric battery is a device that converts chemical energy directly into electrical energy. Battery powered applications have become ubiquitous in the modern society. The recent rapid expansion in the use of these applications creates a strong demand for fast deployment of battery technologies at an unacceptable rate.
1.1 Construction
An electric battery is one or more electrochemical cells connected in series (or) parallel (or) series-parallel combination.
1.1.1 Electrochemical Cell
An electrochemical cell is a device capable of delivering electrical energy from chemical reactions (or) facilitating chemical reactions through the introduction of electrical energy. The main parts of an electrochemical cell include:
(a) Two half cells
(b) Electrolyte and
© Electrodes
(a) Half Cells
Each half cell consists of a conductive electrode surrounded by a conductive electrolyte. The two half cells may use same electrolyte, (or) they may use different electrolytes. A salt bridge is often employed to provide ionic contact between two half cells with different electrolytes to prevent the solutions from mixing and causing unwanted side reactions. The construction of an electrochemical cell is shown in Figure 1.
Figure 1: Electrochemical Cell Construction
(b) Electrolyte
An electrolyte is a substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible. Commonly, electrolytes are solutions of acids, bases (or) salts. Electrolyte solutions are normally formed when a salt is placed into a solvent such as water and the individual components dissociate due to the thermodynamic interactions between solvent and solute molecules, in a process called “salvation”. For example, when table salt (i.e.) “NaCl”, is placed in water, the salt dissolves into its component ions, according to the dissociation reaction given below.
NaCl(S) Na+(aq) + Cl-(aq) (1.1)
Furthermore, some gases may act as electrolytes under conditions of high temperature (or) low pressure. Electrolyte solutions can also result from the dissolution of some biological and synthetic polymers, termed polyelectrolyte, which contain charged functional groups. Molten salts can also act as electrolytes as well. For instance, when Sodium Chloride is molten, it conducts electricity.
© Electrodes
An electrode is an electrical conductor. An electrochemical cell contains two electrodes with one in each half cell. An electrode in an electrochemical cell is referred to as either an “Anode” (or) a “Cathode”. The anode is defined as electrode at which electrons leave the cell and oxidation occurs. In contrast, the cathode is defined as the electrode at which electrons enter the cell and reduction occurs. An electrode may become either anode (or) the cathode depending on the direction of current through the cell.
1.2 Helmholtz Layer
The Helmholtz layer also called as “Double Layer” (DL) (or) “Electric Double Layer” (EDL) is a structure that appears when an electrode is placed into an electrolyte. Figure 2 shows an example of Helmholtz layer.
Figure 2: Helmholtz Layer
The EDL refers to two parallel layers of charge surrounding the electrode. The first layer (i.e.) the surface charge (either positive or negative), comprises ions absorbed directly onto the electrode due to a host of chemical reactions. The second layer comprises ions attracted to the surface charge due to Coulomb force, electrically screening the first layer. The second layer is loosely associated with the electrode, because it is made up of free ions which move in the electrolyte under the influence of electric attraction and thermal motion rather than being firmly anchored. The EDL plays an important role in the operation of batteries. The entire chemical reaction that occurs in an electrochemical cell occurs in this EDL.
