Active equalization is an efficient method to improve the performance of series-connected battery strings for the electric vehicles. However, the large-scale applications of the active equalization are restricted by many factors, such as system cost, control complexity, and equalization speed. In this paper, a unitized multiwinding transformer-based equalization method is proposed. The forward converter and flyback converter are realized, simultaneously. The forward converter principle is used to equalize the battery cells in the same subunit, while the flyback converter principle is applied for the equalization of the different subunits. Fewer metal-oxide-semiconductor field-effect transistors and transformer windings are needed for lower control complexity and lower system cost. Furthermore, different subunits of the battery string can be simultaneously equalized, resulting in higher time efficiency. To verify the proposed method, the simulation and experimental workbenches are established. Results indicate that the proposed method can equalize well both for the cells in different subunits and in the same subunit. The urban dynamometer driving schedule tests are further applied and the unbalanced problem is covered in about one hour. The results show that the proposed method has achieved a good performance of equalization in terms of system cost, control complexity, and speed.

A Unitized Multiwinding Transformer-Based Equalization Method for Series-Connected Battery Strings

A comprehensive review on inconsistency and equalization technology of lithium‐ion battery for electric vehicles

With the development of electric vehicle (EV) technologies, efficient and reliable use of rechargeable batteries (RBs) has become the key subject for automotive manufacturers. Among RBs, lithium-ion battery technology is the most prominent about high specific energy and specific power. The lithium-ion battery pack consists of battery cells with low terminal voltage connected in series to meet the voltage requirement of the EV system. However, the useable capacity of the battery pack is restricted by the low charge cell among the string. The manufacturing inconsistency and different operating conditions of each cell cause the charge imbalances in the battery pack. These imbalances lead to the reduction of the useable capacity, acceleration of cell degradation, and, more importantly, serious safety hazards like overcharging. In all EVs and hybrid electric vehicles (HEVs) using lithium-ion battery systems, the cell balancing controller is an essential task which managed by the battery management system (BMS) to improve battery life cycle and safety. This paper presents a comprehensive overview of the DC-DC converter-based battery balancing system because of the impactful contribution to the charge balancing control and the design of balancing control algorithms. Moreover, other BMS functions such as state of charge (SOC) estimation, which affects balancing performance, are handled in this review.

A comprehensive overview of the dc-dc converter-based battery charge balancing methods in electric vehicles

A controller is presented for an open-ended winding motor dual-inverter drive (DID), where main and floating inverters are supplied from a dc power source and a floating dc capacitor, respectively. The controller utilizes the efficiency characteristic of induction machines, where high power conversion efficiencies are obtained when operating the machine with a constant fundamental power factor, typically around 0.70–0.75, over a wide load range and under variable frequency. The controller uses the drive's topology to maintain the motor's desired power factor. In essence, the main inverter's output voltage is used to control the floating inverter's dc capacitor voltage to keep the injected fundamental voltages of both inverters at a desired ratio. The floating inverter's voltage is operated with a 90° lead relative to the main inverter and uses a constant maximum amplitude modulation depth to minimize the capacitor's operating voltage. This approach updates the motor's voltage automatically to ensure constant power factor operation and improves the floating capacitor's voltage stability during transient conditions. The inherent voltage-boosting capability of this topology is especially beneficial in extending the constant torque region of the motor and improving performance in the speed range extension region. Simulation and experimental results verify the predicted motor efficiency gains and stability under speed and load transients.

High-Efficiency Operation of an Open-Ended Winding Induction Motor Using Constant Power Factor Control

Equalization of series connected lithium‐ion batteries based on back propagation neural network and fuzzy logic control

A new modular topology and control method is presented for balancing the voltages of a series-connected string of battery cells. The proposed topology has fewer components compared to similar methods and is characterized as “cell to string to cell” category since charge transfer occurs between all cells during a cycle: this increases the speed of the charge equalization process. Two battery cells are connected to a bridge module and bridge modules are connected in series through transformer tertiary windings. This series connection causes all bridges to have the same effect on each other and makes the system operation and analysis more predictable. As waveforms and transferred charge can be predicted accurately through analysis, the equalization time is also optimized through a design procedure. The easy-to-implement distributed controller described requires only one voltage sensor per two battery cells.

A Modular Battery Voltage-Balancing System Using a Series-Connected Topology

A new distributed control scheme and charge flow analysis is presented for voltage balancing of series connected battery cells using nondissipative modular power electronics. Each modular bridge is connected across two battery cells using a high frequency transformer and an asymmetrical half-bridge. This results in using one switch and one diode per battery cell. Intrabridge charge transfer equalizes the voltage of two battery cells within a module using coupled transformer windings. Each modular bridge is connected to adjacent bridges by connecting transformer windings within each module. This allows interbridge charge transfer and the balancing of pairs of battery cells, both within adjacent bridge modules and modules more removed. The proposed controller uses a distributed control strategy whereby the control of each modular bridge monitors its own battery cell voltages and also those of adjacent bridges, thus reducing the number of feedback sensors. Detailed analysis is presented that quantifies the flow of charge between a number of series connected battery cells ( $ N$ battery cells). A per-unit design methodology is used to illustrate the system charge flow characteristics. Simulations, design guidelines and experimental results are presented to validate the proposed method.

Control and Analysis of a Modular Bridge for Battery Cell Voltage Balancing

A novel control scheme is presented for an open-ended winding motor dual inverter drive, where a primary inverter is supplied from a dc power source and a secondary inverter is supplied from a floating dc capacitor with no power source Examination of the basic single phase equivalent circuit model of an induction machine reveals that near optimal motor efficiency is achieved at a relatively constant motor fundamental power factor over a wide range of motor loads and drive operating frequencies, eg 071 for the motor used in this work The drive control described uses the phase difference between the fundamental output voltages of the primary and secondary converters in order to control the motor's terminal voltage and to maintain a motor power factor of 071 This control is maintained over the full range of the motor's load and drive frequency settings The floating capacitor bridge voltage is regulated using PI feedback control, with the capacitor voltage error signal as the input and the amplitude modulation index of the primary inverter as the output The capacitor voltage reference signal is related to the per-unit drive frequency and measured current: as the motor's current or frequency changes, the capacitor's voltage reference is updated in a proportional manner to both parameters This control approach keeps the fundamental phase difference between the two inverters within a stable operating range, maintains the power factor of the main bridge close to unity, and improves the floating bridge's dc capacitor voltage stability during transient load changes The drive control performance is presented using detailed simulations and experimental testing; verifying the regulation of the floating capacitor's voltage as well as the predicted efficiency gains for the induction motor under constant power factor operation as compared with conventional drive control

Power factor control for high efficiency operation of an open-ended winding motor using a dual inverter drive with a floating bridge

Two switching patterns are described for a non-dissipating modular battery voltage balancing topology. Each balancing bridge or cell consists of an asymmetrical half-bridge and a high frequency transformer connected across two batteries rated at around 4V each. This topology has the advantage of having one switch and one diode per a battery. There are two charge transfer paths, Intra-bridge charge transfer and interbridge charge transfer. The intra-bridge charge transfer happens automatically when two switches in a bridge are turned on and off simultaneously while, the inter-bridge charge transfer needs a control method. Two fundamental control methods are described for transferring electrical charge between cells: “2-switch flyback control” and “phase-shifted control”. Both methods assume that each module determines its own switching pattern by monitoring the dc voltages in each of its adjacent modules, therefore, avoids a central controller requiring to monitor each battery using isolated feedback signals. Simulation and experimental results are presented for both controllers to describe methods advantages and validate the charge transfer methods used.

Performance comparison of two controllers for a modular voltage balancing circuit

A modular multi-layer voltage equalizer with a distributed controller is proposed to equalize the voltage of series-connected energy storage cells. In the cell-level equalizer, every two cells are connected in series through a module, and in the module-level equalizer, all modules are connected through the AC-bus established with the transformer’s secondary side windings. As the module-level equalizer topology is an AC fully coupled structure, based on the proposed controller method, all cells can transfer or receive energy in a switching cycle simultaneously. The implemented distributed controller requires only one voltage sensor per two cells, and each module benefits from using only one switch without the need for current feedback. Therefore, this method has fewer components than existing methods with similar features and prevents increasing component rates as the number of battery cells increases. The proposed circuit is comprehensively analyzed, and its feasibility is verified by experimental results. 

Atrin Tavakoli

Papers

A Modular Battery Voltage-Balancing System Using a Series-Connected Topology

Control and Analysis of a Modular Bridge for Battery Cell Voltage Balancing

Power factor control for high efficiency operation of an open-ended winding motor using a dual inverter drive with a floating bridge

Performance comparison of two controllers for a modular voltage balancing circuit

Modular Voltage Equalizer Circuit with AC-bus Inter-Modules Connection