Technical University of Cluj-Napoca

Eng. Dorin Vasile Cadar ~abstract of Phd thesis~

CONTRIBUTIONS TO BATTERY MANAGEMENT SYSTEMS

Scientific Coordinator Prof. Dorin Petreuş Phd

2011

Table of Contents

List of figures

List of tables

List of symbols

List of abreviations

Chapter 1 Introduction

1.1 Thesis motivation 1.2 Scope of the thesis 1.3 Thesis organisation REFERENCES

Chapter 2 General information regarding batteries

2.1 Description of battery parameters 2.2 Comparative analysis of battery cells

2.2.1 Lead acid batteries 2.2.2 Nickel Cadmium batteries 2.2.3 Metal hydrid batteries 2.2.4 Lithium batteries 2.2.5 Flow batteries 2.2.6 Vanadium-redox batteries

REFERENCES

Chapter 3 Battery modelling

3.1 Analytical models 3.2 Stochastic models 3.3 Electrochemical models

3.3.1 Peukert Equation 3.3.2 Shepherd model 3.3.3 Unnewehr model

3.4 Step discharge battery models 3.5 Equivalent circuit models

3.5.1 Thevenin electric model 3.5.2 Linear electric model 3.5.3 Nonlinear electric model 3.5.4 Dynamic model

3.6 Proposed battery models 3.6.1 Empirical model for the charge/ discharge characteristic of a lihium-ion battery 3.6.2 RC model- mathematical description of the relaxation phenomenon 3.6.3 Spice model for the discharge characteristic of a lithium-ion batery

3.6.4 Simulink model for the charge of a lead-acid battery 3.7 Conclusions REFERENCES

Chapter 4 Battery charging algorithms

4.1 Battery charging algorithms analysis 4.1.1 Charging algorithm for lead-acid batteries 4.1.2 Charging algorithm for Ni-Cd/Ni-MH batteries 4.1.3 Charging algorithm for Li-Ion batteries

4.1 Induction charging 4.2 Problems that can appear during charging

4.2.1 Reverse discharging 4.2.2 Load connected to the charger 4.2.3 Protection circuits

4.3 Converters used for battery charging 4.3.1 Buck converter 4.3.2 Boost converter 4.3.3 SEPIC converter

4.4 Circuit designed to charge a Ni-MH battery pack 4.1 Circuit designed to charge a lithium-ion battery 4.2 Charger design based on LLC converter 4.3 Powering a home using renewable energy sources 4.4 Conclusions REFERENCES

Chapter 5 Battery state of charge determination

5.1 Methods to estimate the state of charge 5.1.1 Direct measurement 5.1.2 Array methods 5.1.3 Adaptive methods

5.2 Parameters that influence the measurement of a battery’s state of charge 5.2.1 Battery’s internal resistance 5.2.2 Electrods reactions 5.2.3 Overpotential

5.3 Proposed methods for state of charge detrmination of a lithium-ion battery 5.3.1 Method based on direct measurement of the electromotive force 5.3.1.1 Voltage calculation

5.3.1.2 Voltage measurement 5.3.1.3 Integrator circuit 5.3.1.4 Switching during charging 5.3.1.5 Switching during discharging 5.3.1.6 Determining the charging/discharging threshold of the capacitor 5.3.1.7 Measurement of the current through the battery 5.3.1.8 Measurements

5.3.2 Adaptive method for state of charge estimation 5.3.2.1 Hardware description

5.3.2.2 Software description 5.3.2.3 Experimental and simulation results

5.3.3 Comparison between the proposed methods 5.4 Conclusions REFERENCES

Chapter 6 Balancing battery packs

6.1 Types of imbalances in battery packs 6.2 Influence of imbalance over the battery pack’s performance 6.3 Methods of battery equalisation

6.3.1 Passive methods for battery cell equalization 6.3.2 Active methods for battery cell equalization

6.4 Proposed methods for battery cell equalization 6.4.1 Simulation of battery balancing circuits 6.4.2 Study of a specialized passive equalization circuit 6.4.3 Proposed circuit for battery cell equalization 6.4.4 Optimization of the proposed system using fuzzy logic

6.4.4.1 General aspects regarding fuzzy logic 6.4.4.2 Member functions 6.4.4.3 Linguistiv variables and fuzzy rules 6.4.4.4 Inference system

6.4.5 Design of the fuzzy control system 6.4.6 Practical implementation of the battery cell equalizer

6.5 Conclusions REFERENCES

Chapter 7 Final conclusions and contributions

List of articles

APPENDIX

The need for more energy led to the development of newer fabrication technologies both from the chemical compounds used point of view but also from the physical dimensions. The study of battery systems is an important aspect due to the continous need for mobility. It is a complex process, it has a lot of factors involved but it is an important process in the development of battery management systems.

The thesis was ment to analize and bring contributions regarding batteries and aspects related to them:

• Researching new technologies regarding the new chemial compounds used for battery development.

• Analysis and comparison of different battery types regarding their performance. • Modelling battery behaviour in order to analyse the charge/dischage characteristics

of each battery. • Analyse the optimum charging algorithm for several types of battery • Analyse newer methods for battery charging like: wireless charging, USB charging,

charging using renewable energy sources. • Analyse existing methods for battery state of charge determination and designing

newer methods. • Analyse existing methods for battery cell equalization and designing newer methods.

In the second chapter a detailed analysis of the most known battery types is presented. In

the description of eah battery the following aspects where taken into account: evaluation according to their physical properties and chemical compouns used, listing of the main advantages and disadvantages, analysis of long therm behavior. The necessity of this analysis came from the continuous innovations in the field of batteries, innovations that want to provide a battery that can keep track with the requirements of mobile power nowadays. The main contribution of this chapter is relatd to the detaild analysis of the most known battery types

In the third chapter a detailed analysis of the existing battery models and the mathematical analysis of the battery during several operation modes: charging, discharging or relaxation. There is no general accepted battery model, realible enough to be used in any application. In this chapter several battery models were realised in order to be used in the applications presented in this thesis. The proposed empirical model used MathCad predefined functions in order to simulate the charge/discharge characteristic of a lithium-ion battery. This model can provide the characteristics using a small amount of samples of the voltage and current. The major advantage of this model is its simplicity and adaptability.

The RC model was chosen in order to simulate the recovery effect once the battery is disconnectd from a load. This subject is not as debated in literature as it is really difficult to calculate the amount of charge that is recovered by the battery. The purpose of this study was to confirm this model for the use in future projects that require information regarding charge

recovery of a battery. For this model the mathematical equatios were described and its behaviour was simulated in LtSpice and Matlab Simulink.

The need of Spice simulation of several charging circuits led to the development of the Spice model proposed. The model relies on the correspondence between the battery’s voltage and its state of charge. One advantage of this model is the fact that it takes into account the rate of discharge, rate defined with the help of the current flowing into the battery. The development of the model as a subcircuit allows its use in any type of Spice simulations.

In the final part of the third chapter the Simulink model of a lead acid battery was studied. The adavantages of this model are: adaptability, ease of parameter use, large number of variables considered in the description of the model. This model was chosen for the simulation of a hybrid power system for a home using renewable energy sources.

In the fourth chapter different charging methods were studied. The charging methods were detailed according to the type of battery charged (lead acid, metal hydrid, Lithium-ion) and the type of charging source: solar, wind, etc. For the design of such chargers several topologies of dc converters were studies and also some protection circuits. The fourth chapter also contains a detailed analysis of possible damaging situations that can appear during charging. Starting from the analysis of the charging methods several chargers were designed for three types of batteries. A method of designing a battery charger using a resonant converter was described in section 4.7. The advanteges of using such a converter consist of the low electromagnetic emissions and the possibility of achieving high levels of efficiency. During this chapter a novel method of design and analysis as proposed, method based on two control loops. This method allowed the use of this converter as a battery charger.

A battery charger fr a Ni-Mh battery pack was designed. The chaging algorithm fr this type of batteries is rather classic: a slow charge at low values of charging current followed by a fast charge at high values of the current. The method ofdetermining the end of charge is what actually makes a difference. The charger described in section 4.5 has a detection method based on temperature detection. The proposed method led to a fast and safe charging, thing proved both throught simulation and expriment.

A battery charger for a single cell lithium-ion battery was also designed. Due to its instability, charging for this type of battery is a really important aspect. The charging algorithm proposed is based on two control loops. The charging algorithm has the two stages recommended for lithium-ion batteries: constant current, constant voltage. The converter chosen for this charger was based on SEPIC converter due to its high range of output voltages and high operating frequencies.

The last charger was designed for a lead acid battery pack. The interesting aspect of this system consists of the hybrid system that powers the isolated home: solar panels and wind turbine.

In the fifth chapter a detailed analysis of the methods of state of charge determination was presented. Several aspcts that influence the accuracy of the stae of charge determination were studied. Two method were proposed for the determination of the state of charge of a battery.The first method is basd on direct measurement an relies on the relationship[ between a battery’s stae of charge and its electromotive force. The second method is an adaptive one, method that combines coulomb counting method with direct measurement. Two prototype circuits were implemented and both of the proposed methods were tested on the same type of lithium battery, the results being really close. The main contribution are represented by the prototype circuits, the method of finding a battery’s overpotential and the adaptive method proposed.

In chapter six a detailed analysis is presented regarding ways of balancing battery packs. Battery equalization is a relative new aspect that appeared along with the development of the new lithium battery packs. In this chapter several topologies were presented that take care of both actve and passive equalization. A specialized integrated circuit was studied to enhance the principales of passive battery equalization. A novel balancing topology was proposed, circuit that was first simulated ,then described throughout mathematical equations and the implemented as a prototype. In order to improve the equalization time, a original method of optimization was proposed, method that releis on fuzzy logic. Both of the two control bethods: classic and fuzzy logic optimized were implemented and compared.

In this thesis a extened study of batter behavior was studied in different systems. The most important aspects related to batteries were studied: specific characteristics of the most important battery types, study of the existent battery models together with the development of novel models, the study of the best charging methods, methods to determine a battery’s stae of charge and methods to equalize a battery pack. But because of the high level of complexity the presented solution are just a small part of this theme.

Contributions1. The detailed study of the following types of batteries: lead-acid, Ni-Cd, Ni-Mh,

lithium-ion, lithium-polymer and vanadum-redox, showing th advantages and disadvantages of each one. The analysis brings new and important data related to these batteries.

2. The synthesis of the existing battery models and simulation of some of these models in simulation environments like Mathlab/Simulink.

3. Development and simulation in MathCad of a empirical battery model for the reconstruction the discharge/charge characterisic of a lithium-ion battery.

4. Development and simulation in LtSpice of a battery model used for simulation of several battery chagers.

5. Analysis of the relaxation phenomenon: the mathematical description of the model; comparison of the simulation in LtSpice and equations solving in Simulink.

6. Analysis of the optimum chaging methods for the design of performant battery chargers.

7. The study of new charging methods: wireless charging and charging from renewable energy sources.

8. Analysis of situations that can apper during charging and offering solution to enhance thoe problems.

9. The detailed study of dc-dc converter that can be used as battery chargers: buck, boost, SEPIC, LLC.

10. Analysis of methods to determine a bater’s state of charge; exemplification of problems that occur in the determination of the battery’s state of charge.

11. Analysis of the balancing methods for battery packs and synthesis of the main causes of cell imbalance. Comparison through simulation of several balancing circuits.

12. Detailed analysis of a specialized passive balncing circuit, in order to establish its equalization principal.

13. Mathematical descripton of a original circuit proposed for a lithium-ion battery pack equalization. Graphical correspondence of the balancing current and the duty cycle for several values of the switch on resistance of the switching transistor.

14. Development of a original optimization algorithm for the control of the equalization circuit, algorithm based on fuzzy logic.

15. Practial implementation of a charging circuit of a Ni-Mh battery pack based on SEPIC converter using a original method for end of charge detection.

16. Practial implementation of a charging circuit of a single lithium-ion cell based on SEPIC converter using a original charging method based on two control loops.

17. Practial implementation of a charging circuit of a lithium-ion battery pack based on resonant LLC converter using a original charging method based on two control loops.

18. Implementation of a testing setup for the GP18500 lithium-ion battery, for the study of its charging/discharging characteristic and recovery phenomenon.

19. Practial implementation of a circuit for state of charge determination for the method based on direct measurement and overpotential.

20. Practial validation of the second method for state of charge determination based on coulomb counting.

21. Practical implementation of a original balancing circuit for lithium-ion battery packs.

22. Practial validation of the balancing circuit using two control methods: classical one and optimized using fuzzy algorithm.

List of articles

1.Petreuş D., Ciocan I., Rusu A., Cadar D. „Maximum power point tracking simulator in charging photovoltaic systems”, Acta Tehnica Napocensis, Cluj-Napoca, 2009, Vol. 50, No. 2, ISSN 1221-6542 pp. 43-48

2. Dorin Cadar, Dorin Petreuş, Ionuţ Ciocan, Petru Dobra- “An improvement on empirical modelling of the batteries”, IEEE 32nd International Spring Seminar on Electronics Technology, 2009, 13-17 May 2009, ISBN 978-1-4244-4260-7

3. Dorin Cadar, Ionut Ciocan “An empirical model of a lithium-ion battery”, Novice Insight , ISSN1842-6085, pp 26-33

4. Dorin V. Cadar, Dorin M. Petreuş, Cristian A. Orian “A method of determining a lithium ion battery’s state of charge“, SIITME 2009 – IEEE International Symposium for Design and Technology of Electronic Packages, 15th Edition, 17-20 September 2009, Gyula, Ungaria, 978-1-4244-50330309, 2009

5. Cadar D., Petreus D., Patarau T., Palaghita N., “Active Balancing Method for Battery Cell Equalization”, Acta Technica Napocensis - Electronics and Telecommunications, ISSN 1221-6542, Volume 51, Number 2, 2010, pp. 1-5

6. Cadar D., Petreus D., Patarau T, „An Energy Converter Method for Battery Cell Balancing”, IEEE 33rd International Spring Seminar on Electronics Technology (ISSE), 12-16 May 2010, Var_ovia, Polonia, pp. 290-293, ISBN 978-1-4244-7849-1

7. Etz R., Cadar D., Petreus D., Daraban S., “Comparison between Analog and Fuzzy Logic Control for a Buck Converter”, SIITME 2010 – IEEE International Symposium for Design and Technology of Electronic Packages, 16th Edition, 23-26 Septembrie 2010, Piteşti, Romania

8. Dorin Cadar, Radu Etz, Dorin Petreus, „Fuzzy control of battery cell active balancing circuit”, ISBN 978-973-1890-27-2 (SATEE 2010), 9-11 Septembrie 2010, Alba-Iulia, Romania

9. D. Cadar, D. Petreus, T.Patarau, R.Etz “Fuzzy controlled energy converter equalizer for lithium ion battery packs”, PowerEng 2011, 11-13 Mai, Malaga, Spania

10. D. Cadar, A. Rusu, D. Petreuş , “A Ni-Mh battery charger with dT/dt end-of-charge detection” publicat in volum conferinta PRODOC, Cluj-Napoca, 24 iunie 2011

11. A. Rusu, D. Cadar, D. Petreuş , “Fuzzy Logic Control in Maximum Power Point Tracking Photovoltaic Systems” publicat in volum conferinta PRODOC, Cluj-Napoca, 24 iunie 2011

Accepted for review1. D Cadar, D. Petreuş, E. Cioran, „Combined State-of-Charge Estimation Methods for Lithium-Ion Batteries”, SIITME 2011 IEEE International Symposium for Design and Technology of Electronic Packages, 17th Edition, Timişoara, Romania