Overview of Battery Impedance Modeling Including Detailed State-of-the-Art Cylindrical 18650 Lithium-Ion Battery Cell Comparisons

Electrical models of battery cells are used in simulations to represent batteries\u27 behavior in various fields of research and development involving battery cells and systems. Electrical equivalent circuit models, either linear or nonlinear, are commonly used for this purpose and are presented in this article. Various commercially available cylindrical, state-of-the-art lithium-ion battery cells, both protected and unprotected, are considered. Their impedance properties, according to four different equivalent circuit models, are measured using electrochemical impedance spectroscopies. Furthermore, the pricing, impedance, specific energy, and C-rate of the chosen battery cells are compared. For example, it is shown that the energy density of modern 18650 cells can vary from a typical value of 200 to about 260 Wh kg(-1), whereas the cell price can deviate by a factor of about 3 to 5. Therefore, as a result, this study presents a concise but comprehensive battery parameter library that should aid battery system designers or power electronic engineers in conducting battery simulations and in selecting appropriate battery cells based on application-specific requirements. In addition, the accuracies and computational efforts of the four equivalent circuit models are compared

Capacitor Voltage Balancing of a Grid-Tied, Cascaded Multilevel Converter with Binary Asymmetric Voltage Levels Using an Optimal One-Step-Ahead Switching-State Combination Approach†

This paper presents a novel capacitor voltage balancing control approach for cascaded multilevel inverters with an arbitrary number of series-connected H-Bridge modules (floating capacitor modules) with asymmetric voltages, tiered by a factor of two (binary asymmetric). Using a nearest-level reference waveform, the balancing approach uses a one-step-ahead approach to find the optimal switching-state combination among all redundant switching-state combinations to balance the capacitor voltages as quickly as possible. Moreover, using a Lyapunov function candidate and considering LaSalle\u27s invariance principle, it is shown that an offline calculated trajectory of optimal switching-state combinations for each discrete output voltage level can be used to operate (asymptotically stable) the inverter without measuring any of the capacitor voltages, achieving a novel sensorless control as well. To verify the stability of the one-step-ahead balancing approach and its sensorless variant, a demonstrator inverter with 33 levels is operated in grid-tied mode. For the chosen 33-level converter, the NPC main-stage and the individual H-bridge modules are operated with an individual switching frequency of about 1 kHz and 2 kHz, respectively. The sensorless approach slightly reduced the dynamic system response and, furthermore, the current THD for the chosen operating point was increased from 3.28% to 4.58% in comparison with that of using the capacitor voltage feedback

Multi-Agent Reinforcement Learning-Based Decentralized Controller for Battery Modular Multilevel Inverter Systems

The battery-based multilevel inverter has grown in popularity due to its ability to boost a system’s safety while increasing the effective battery life. Nevertheless, the system’s high degree of freedom, induced by a large number of switches, provides difficulties. In the past, central computation systems that needed extensive communication between the master and the slave module on each cell were presented as a solution for running such a system. However, because of the enormous number of slaves, the bus system created a bottleneck during operation. As an alternative to conventional multilevel inverter systems, which rely on a master–slave architecture for communication, decentralized controllers represent a feasible solution for communication capacity constraints. These controllers operate autonomously, depending on local measurements and decision-making. With this approach, it is possible to reduce the load on the bus system by approximately 90 percent and to enable a balanced state of charge throughout the system with an absolute maximum standard deviation of 1.1×10−5. This strategy results in a more reliable and versatile multilevel inverter system, while the load on the bus system is reduced and more precise switching instructions are enabled

Battery Impedance Modeling and Comprehensive Comparisons of State-Of-The-Art Cylindrical 18650 Battery Cells considering Cells\u27 Price, Impedance, Specific Energy and C-Rate

Within different areas of research and development of battery cells and systems, it is necessary to include electrical models of battery cells in simulations to reflect batteries\u27 behavior. Linear or nonlinear electrical equivalent circuit models are commonly used for this purpose, such as the Warburg impedance and the three RC-pair model. In this paper, various protected and unprotected commercially available cylindrical (18650), state-of-the-art lithium-ion battery cells are considered. Their impedance parameters, according to the aforementioned models, are estimated using electrochemical impedance spectroscopies. Furthermore, the selected battery cells are comprehensively compared in terms of their price, impedance, specific energy and C-rate. Therefore, this paper provides a brief but comprehensive battery parameter library, which should assist battery system designers or power electronic engineers to conduct battery simulations and, thus, to properly choose battery cells with respect to application specific requirements

Proactive SoC Balancing Strategy for Battery Modular Multilevel Management (BM3) Converter Systems and Reconfigurable Batteries

The battery modular multilevel management converter topology and different types of reconfigurable battery topologies have been proven to be a viable option for various electric power applications. This paper presents a unique SoC balancing approach for the integrated battery strands/packs of BM3 converter systems or reconfigurable batteries. The suggested approach alternately utilizes different redundant switching state combinations to balance and to keep the battery strands\u27 SoCs balanced. Furthermore, the suggested algorithm attempts to utilize all converter modules, because the parallel connection of adjacent modules reduces the phase-strand\u27s battery impedance. Furthermore, the presented approach tries to reduce the number of switching events when changing the switching state combination. Thereby, the ohmic battery losses and switching losses are kept as low as possible. Since no power is dissipated in designated bleeder resistors and no designated active balancing circuitry is required, the suggested approach can be categorized as a proactive balancing approach. Simulations are used to verify the algorithm\u27s validity

Battery Emulation for Battery Modular Multilevel Management (BM3) Converters and Reconfigurable Batteries with Series, Parallel and Bypass Function

This paper deals with the emulation of lithium-ion battery cells/modules for the development and testing of battery modular multilevel management converters and any kind of reconfigurable battery systems with series, parallel and bypass function. The developed emulator is based on a buck converter type with an isolated input voltage supply. A circuit board with the form factor of two cylindrical 18650 battery cells was developed, which can function as a replacement of a real battery cell/module for a laboratory setup. In addition to the implemented safety mechanisms, such as over-current, over-voltage and short-circuit protection, a simplified electrical equivalent circuit model is implemented on the integrated micro controller. Thereby, the dynamic electrical behavior of any battery cell can be emulated with low deviations from its real battery behavior

Bidirectional Charging for BEVs with Reconfigurable Battery Systems via a Grid-Parallel Proportional-Resonant Controller

This paper investigates the potential of bidirectional charging using modular multilevel inverter-based reconfigurable battery systems via grid-parallel control. The system offers several advantages such as modularity, scalability, and fault-tolerance over conventional battery electric vehicle systems. It is designed for seamless integration with the grid, allowing bidirectional power flow and efficient energy storage. Within this study, the battery system is first simulated in Matlab/Simulink and later implemented into a hardware setup. Eventually, the simulation results and the measurements have been compared and evaluated. Thereby, startup sequences and constant current scenarios were investigated. It has been shown that the system is fully capable to charge and discharge the batteries in the grid-parallel connection, thus enabling bidirectional charging with close to full drive system power. The current total harmonic distortion complies with grid regulations and can potentially improve the grid quality. The proposed system offers significant potential for grid-integrated energy storage systems, addressing the challenges associated with renewable energy integration, grid stability, and energy management. In comparison to other publications on this topic, the proposed approach does not need additional dedicated power electronic hardware and has more degrees of freedom for current control

Charging Strategy for Battery Electric Vehicles with a Battery Modular Multilevel Management (BM3) Converter System using a PR controller

Modular Multilevel Converters (MMC) with integrated battery packs have been proven to be a viable option for various electric power applications. In novel applications, fully charged battery packs are used to form flexible output voltage waveforms. Nonetheless, the charging of the batteries after the actual usage has previously not been described for MMC based inverters with integrated battery packs. This paper presents an approach of power grid compliant electric vehicle battery charging for Battery Modular Multilevel Management (BM3) converter systems with three-switch modules using a PR controller when directly connected to the AC grid. By using such a system, the conventional battery electric vehicle\u27s on-board charger and any additional battery balancing circuitry become obsolete. It is demonstrated, that the analyzed approach is able to charge the battery modules with a current total harmonic distortion of less than 5% at any charging power level without the usage of a dedicated grid filter. Furthermore, the power factor angle can be freely adjusted