Inductor Based Active Cell Equalization for Ultracapacitor Energy Storages

. Ultracapacitors, known for their high power density and long cycle life, are widely used in various applications. However, when ultracapacitor cells are connected in series, voltage imbalances can occur, limiting overall energy storage capacity and system performance. This paper presents an investigation into inductor-based active cell equalization techniques for ultracapacitor energy storage systems. The proposed approach utilizes inductors, switching devices, and control circuitry to efficiently balance cell voltages. By monitoring cell voltages and activating switching devices when predetermined thresholds are exceeded, energy is transferred from higher voltage cells to inductors during the charging phase. In the subsequent discharging phase, the stored energy is released, equalizing the cell voltages. This iterative process continues until voltage balance is achieved. Inductor-based active cell equalization offers advantages such as rapid voltage equalization, wide voltage range operation, and electrical isolation between cells. However, challenges include system complexity, cost, and losses introduced by switching devices. Ongoing research focuses on optimizing design and control strategies to improve energy efficiency and address these challenges. The proposed technique shows promise in maximizing energy storage capacity and enhancing the performance and lifespan of ultracapacitor systems. This circuit could balance the capacity of the ultracapacitor in 2.3 seconds with a voltage ripple of 0.0038 V (0.18 %). Further advancements are expected to promote the widespread adoption of inductor-based active cell equalization in diverse applications. (Abstract)


Introduction
Ultracapacitors, also known as supercapacitors or electric double-layer capacitors (EDLCs), have gained significant attention as energy storage devices due to their high power density, long cycle life, and fast charge/discharge characteristics.They find applications in various fields such as electric vehicles, renewable energy systems, and portable electronics.However, when ultracapacitor cells are connected in series to achieve higher voltage levels, voltage imbalances among the cells can arise.These imbalances result from variances in cell capacitance, internal resistance, and aging effects, leading to reduced energy storage capacity and system performance [1].To overcome this limitation, active cell equalization techniques have been developed, with inductorbased equalization emerging as a promising solution [1,2].
Voltage imbalances among ultracapacitor cells connected in series can have detrimental effects on the overall performance and lifespan of the energy storage system.When cells have unequal voltages, the capacity of the system is limited by the lowest voltage cell, while other cells may not be fully utilized.Additionally, the imbalances can lead to accelerated aging and increased Corresponding author: ekirovianto@staff.uns.ac.id internal resistance, further degrading system performance over time.
Active cell equalization techniques have been proposed as a means to address these voltage imbalances.These techniques aim to redistribute charge among the cells, ensuring that each cell operates within an acceptable voltage range.Various active equalization methods have been explored, including resistor-based, capacitor-based, and inductor-based techniques.Among these, inductor-based active cell equalization has shown promise for its efficiency and effectiveness [3,4].
Inductor-based active cell equalization utilizes inductors as energy storage elements to transfer charge between cells.The principle of operation involves monitoring the individual cell voltages and activating switching devices when predetermined voltage thresholds are exceeded.During the charging phase, energy is transferred from higher voltage cells to inductors, which store the excess charge.Subsequently, during the discharging phase, the stored energy is released from the inductors, equalizing the voltage across the cells.This iterative process continues until voltage balance is achieved [5].
Inductor-based active cell equalization offers several advantages over passive balancing methods.It enables rapid voltage equalization, resulting in improved energy utilization and extended system lifespan.Moreover, it operates effectively over a wide range of cell voltages and is less susceptible to variations in cell capacitance and internal resistance.Additionally, the use of inductors provides electrical isolation between cells, preventing undesired current flow during normal operation [5,6].
However, practical implementation of inductorbased active cell equalization systems requires careful consideration of various factors, such as component selection, control algorithms, and system efficiency.Challenges include increased system complexity, additional cost, and potential losses introduced by the switching devices [5].
Ongoing research and development efforts are focused on optimizing the design and control strategies of inductor-based active cell equalization systems.The aim is to improve energy efficiency, reduce system complexity, and enhance the overall effectiveness of voltage balancing techniques.The proposed technique shows promise in maximizing energy storage capacity and enhancing the performance and lifespan of ultracapacitor systems.Further advancements are expected to promote the widespread adoption of inductor-based active cell equalization in diverse applications [7].
In this paper, we delve into the detailed analysis of inductor-based active cell equalization for ultracapacitor energy storage systems.This paper contributes to balancing the capacity of ultracapacitor with a series of inductor-based active cell equalization.We investigate the principles of operation, advantages, limitations, and ongoing research efforts.Through this study, we aim to contribute to the understanding and advancement of inductor-based active cell equalization techniques, providing valuable insights for optimizing the design and control strategies for improved ultracapacitor system performance and longevity.

Principles of Inductor-Based Active Cell Equalization
Inductor-based active cell equalization is a technique that utilizes inductors, switching devices, and control circuitry to achieve efficient voltage balancing among ultracapacitor cells.The fundamental principles underlying this approach involve monitoring individual cell voltages and transferring energy between cells to maintain voltage balance [7].
The active cell equalization system continuously monitors the voltage levels of each ultracapacitor cell within the seriesconnected configuration.Monitoring can be performed using dedicated voltage sensing circuitry or by measuring the voltage across each cell using voltage dividers or other voltage measurement techniques.The monitored voltage information is then used to determine if any voltage imbalances exceed predetermined thresholds [6,7].
To enable the transfer of energy between cells, switching devices such as MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) or other suitable electronic switches are employed.These devices are controlled by the equalization control circuitry based on the voltage imbalance detected.When a predetermined threshold is exceeded, the switching devices are activated to facilitate the energy transfer process [8][9][10].
During the charging phase of the equalization process, the switching devices are activated to connect the higher voltage cells to inductors.Energy from these cells is transferred to the inductors, which act as energy storage elements.The inductors store the excess charge from the higher voltage cells, thus reducing their voltage levels [10,11].
In the subsequent discharging phase, the stored energy in the inductors is released back into the lower voltage cells.The switching devices are operated in a way that allows the energy stored in the inductors to flow back into the cells with lower voltages.This energy transfer equalizes the voltage levels across the cells, gradually reducing voltage imbalances.The discharging and charging phases are repeated iteratively until the voltage balance is achieved within an acceptable range [12].
The equalization control circuitry plays a crucial role in the overall operation of the system.It receives input from the voltage monitoring circuitry, determines the appropriate timing and duration of energy transfer, and controls the activation of the switching devices.The control circuitry ensures that energy is transferred when voltage imbalances exceed the predetermined thresholds and stops the equalization process once the desired voltage balance is achieved [13,14].
The inductor-based active cell equalization technique offers several advantages.It enables fast voltage equalization, leading to improved energy utilization and extended system lifespan.It can operate effectively across a wide range of cell voltages and is less affected by variations in cell capacitance and internal resistance.Additionally, the use of inductors provides electrical isolation between cells, preventing undesired current flow during normal operation.
Ongoing research focuses on optimizing the design and control strategies of inductor-based active cell equalization systems.The goal is to enhance energy efficiency, reduce system complexity, and further improve the overall effectiveness of voltage balancing techniques.Advancements in this field will contribute to maximizing the energy storage capacity and enhancing the performance of ultracapacitor systems in various applications.The ultracapcitor used is shown in Fig. 1 below.While Table 1 shows the ultracapacitor datasheet.ULTRACAPACITOR CELL [15].
Rated voltage,  2.7 VDC Surge voltage 1  2.85 VDC Rated capacitance, C 3  10 F Min. / Max.Capacitance, Initial 9 F / 12 F Typical Capacitance, Initial 2, 3  10.6 F Rated (Max.)ESRDC, Initial 3  30 mΩ Typical ESRDC, Initial 2,3  25 mΩ Typical ESRDC, Initial, 5 sec 2,3  46 mΩ Maximum Leakage Current 4  23 μA The block diagram of the proposed circuit is shown in Fig. 2.This topology consists N-1 number of inductors per N number of cells.All switches are MOSFETs with body diodes and controlled by a pair of complementary signal in synchronous trigger pattern with the duty cycle of 50%.The operational principle of the circuit can be divided in two modes.

Fig. 2. The proposed circuit based on inductor balancing
During the first stage, energy is stored in the inductors by turning on switches S1-S7 while keeping switches S8-S14 turned off.As switches S1-S7 are activated, the energy from cell 1, cell 3, cell 5, and cell 7 is transferred to the respective inductors (L1-L7).
Also the inductor voltage of L7 can be driven as [16] Eq. 1 can be rewritten as [16] where D is the duty cycle.The inductor current when S1 is turned on can be written as [16] Similarly the inductor current of L2-L4 can be written as [16] where k = 2, 3, 4, and n = 3, 5, 7.
And finally the inductor current of L7 can be expressed as [16] In the subsequent mode, switch S1 is deactivated, causing the body diode of switch S2 to turn on.As a result, the energy previously stored in inductor L1 is transferred to cell 2. Likewise, the voltages across inductors L2-L4 can be represented in a similar manner [16] where k = 2, 3, 4, and n = 4, 6, 8.
And finally the inductor current of L7 can be expressed as [16]  The transfer function H21 can be defined as [16] 3 Method The research paper utilized MATLAB software to implement and evaluate the inductor-based active cell equalization technique for ultracapacitor energy storage.The following methods describe the steps involved in conducting the research and analyzing the results [16].
First, the ultracapacitor energy storage system was modeled in MATLAB to simulate its behavior and characteristics.The model incorporated individual ultracapacitor cells, inductors, switching devices, control circuitry, and the energy transfer algorithm.Parameters such as cell capacitance, internal resistance, inductance value, switching device characteristics, and control algorithm settings were defined based on component specifications and system requirements.
Next, a control algorithm was developed to monitor the ultracapacitor cell voltages and activate the switching devices accordingly.The algorithm initiated the energy transfer process based on predetermined voltage imbalance thresholds.Considerations such as voltage measurement accuracy, control loop stability, and the desired rate of voltage equalization were taken into account during algorithm design.MATLAB's programming capabilities were used to implement the algorithm, ensuring proper synchronization of switching devices and efficient energy transfer.
The model was then employed to simulate the behavior of the inductor-based active cell equalization system.The simulation involved running the control algorithm and monitoring the voltage balancing process over time.Various scenarios, including different initial voltage imbalances, voltage ranges, and load profiles, were tested to evaluate system performance under diverse operating conditions.Simulation results were analyzed to assess voltage balancing effectiveness, energy transfer efficiency, and other relevant performance metrics.
During the simulation, data on voltage imbalances, energy transfer rates, and other relevant parameters were collected and recorded.The data processing and analysis capabilities were utilized to analyze and interpret the collected data.Statistical analysis techniques, visualization tools, and relevant MATLAB functions were employed to extract meaningful insights from the simulation results.
The performance of the inductor-based active cell equalization technique was evaluated based on the simulation results.Metrics such as voltage balancing effectiveness, ripple voltage reduction, energy transfer efficiency, and system stability were assessed and compared against predefined performance targets or benchmarks.This evaluation provided quantitative and qualitative insights into the effectiveness and feasibility of the proposed technique in achieving voltage balance and optimizing energy utilization in ultracapacitor energy storage systems.
Based on the simulation results and performance evaluation, further optimization and sensitivity analysis were conducted.The optimization algorithms and sensitivity analysis tools were utilized to explore the effects of different system parameters, control algorithm modifications, and component selections on system performance.This analysis aimed to identify opportunities for improvement, enhance energy efficiency, reduce losses, and address any limitations or challenges encountered during the simulation.

Result and Discussion
The obtained result of active cell balancing at a frequency of 2000 highlights the effectiveness of the inductor-based active cell equalization technique in maintaining voltage balance among the ultracapacitor cells.This analysis focuses on the key aspects and implications of the achieved voltage balance result as shown in Fig. 3.

Fig. 3. Cell voltages of the proposed circuit with switching frequency of (2kHz)
The active cell balancing approach at a frequency of 2000 has successfully equalized the voltages across the ultracapacitor cells.This indicates that the inductorbased equalization system effectively transferred energy from higher voltage cells to lower voltage cells, resulting in a reduced voltage imbalance.The achieved voltage balance demonstrates the capability of the system to mitigate the effects of cell capacitance variations, internal resistance discrepancies, and aging effects that can cause voltage imbalances in the first place.
The voltage balance achieved through active cell equalization at a frequency of 2000 enables improved energy utilization within the ultracapacitor system.With balanced voltages across all cells, the system can fully utilize the energy storage capacity of each individual cell.This enhanced energy utilization leads to increased overall system efficiency and improved performance in applications requiring rapid and high-power energy delivery.
Maintaining voltage balance through active cell equalization helps to extend the lifespan of the ultracapacitor system.Voltage imbalances can accelerate cell degradation and increase internal The chosen frequency of 2000 for the active cell balancing process plays a crucial role in achieving efficient voltage balance.The frequency selection should strike a balance between the time required to achieve voltage balance and the system's energy efficiency.Higher frequencies may allow for faster voltage equalization but could lead to increased switching losses and reduced overall efficiency.Lower frequencies, on the other hand, may result in longer equalization times but with improved energy efficiency.Therefore, the chosen frequency should be optimized based on specific system requirements and trade-offs between equalization speed and energy efficiency.
The achieved voltage balance at a frequency of 2000 opens up opportunities for further optimization and improvement.Future research efforts can explore optimizing the control algorithm, component selection, and system design to enhance energy efficiency and reduce losses associated with the active cell equalization process.Moreover, investigations into dynamic frequency adjustment or adaptive control algorithms can be conducted to adaptively adjust the equalization frequency based on the real-time voltage imbalance conditions, thereby improving the overall performance and effectiveness of the equalization technique.
The obtained data on ripple voltage at a frequency of 2000 provides valuable insights into the performance and stability of the inductor-based active cell equalization technique in the ultracapacitor energy storage system.This analysis of the observed ripple voltage results is shown in Fig. 4.
The analysis of ripple voltage at a frequency of 2000 reveals the effectiveness of the inductor-based active cell equalization technique in reducing voltage ripple within the ultracapacitor system.Ripple voltage refers to the small fluctuations or oscillations superimposed on the DC voltage.By actively equalizing the cell voltages, the technique minimizes voltage differentials among the cells, thereby reducing the amplitude of the ripple voltage.The reduced ripple voltage contributes to improved system stability and enhances the quality of power delivery.
The reduction in ripple voltage achieved through active cell equalization at a frequency of 2000 leads to enhanced voltage stability within the ultracapacitor system.Voltage stability is crucial in maintaining consistent and reliable operation of electrical devices and systems.By minimizing the fluctuations in voltage, the active equalization technique ensures a more stable power supply, which is particularly beneficial for applications with sensitive electronic components or devices that require precise and steady voltage levels.
The analysis of ripple voltage also provides insights into the energy efficiency of the inductor-based active cell equalization technique.Higher ripple voltages indicate greater energy losses within the system.By reducing ripple voltage, the equalization technique helps to improve overall system efficiency by minimizing energy dissipation and maximizing energy utilization.This translates into improved performance, reduced power consumption, and potentially longer operating times in battery-powered or energy-limited applications.
The selected frequency of 2000 for the analysis of ripple voltage is an important parameter that affects the observed results.Different frequencies can impact the magnitude and characteristics of the ripple voltage.Generally, higher frequencies can lead to smaller ripple voltages, as they enable faster energy transfer and voltage equalization among the cells.However, it is important to strike a balance between achieving low ripple voltage and minimizing losses associated with the switching devices and control circuitry.Therefore, the choice of frequency should consider the specific requirements and trade-offs of the system.
The analysis of ripple voltage at a frequency of 2000 suggests potential avenues for further optimization and improvement.Future research and development efforts can focus on refining the control algorithms, exploring alternative component selections, and enhancing the system design to further minimize ripple voltage and improve overall system efficiency.Additionally, investigations into dynamic frequency control or adaptive control techniques can be explored to adaptively adjust the equalization frequency based on real-time system conditions, enabling optimal ripple voltage reduction while maintaining energy efficiency.
With a duty cycle of 50%, the equalization system achieves an equal amount of on-time and off-time for The observed 50% duty cycle without dead time indicates that the equalization system operates in a continuous mode.This means that energy transfer between the cells occurs continuously without any interruptions or dead time.The continuous operation contributes to effective voltage balancing by ensuring a constant flow of energy between cells.The balanced duty cycle prevents voltage imbalances from deteriorating or increasing over time, leading to improved voltage stability and extended lifespan of the ultracapacitor system.
The duty cycle of 50% without dead time helps to minimize switching losses in the equalization system.Switching losses occur during the transitions between the onstate and the off-state of the switching devices.With a 50% duty cycle, the equalization system achieves symmetrical switching patterns, resulting in reduced switching losses.Minimizing switching losses enhances the overall energy efficiency of the system, as less energy is dissipated as heat during the switching process.
The balanced duty cycle and absence of dead time also contribute to improved thermal performance of the equalization system.Without dead time, the equalization process avoids unnecessary interruptions or gaps between energy transfer cycles.This helps to reduce thermal stress on the switching devices and other components, leading to improved reliability and longevity of the system.The balanced duty cycle further aids in thermal management by minimizing excessive heating and temperature fluctuations during operation.
The selection of a 50% duty cycle without dead time should be considered in the context of the specific control strategy and system design.The duty cycle can be adjusted based on the desired rate of voltage equalization and the characteristics of the ultracapacitor cells.Different duty cycles may be more suitable for specific application requirements or system constraints.It is important to optimize the control strategy and system parameters to achieve the desired voltage balancing performance while considering factors such as energy efficiency, component stress, and system stability.

Conclusion
In conclusion, the active cell balancing achieved at a frequency of 2000 demonstrates the effectiveness of the inductor-based active cell equalization technique in maintaining voltage balance among ultracapacitor cells.The achieved voltage balance enhances energy utilization, extends the system's lifespan, and provides a foundation for further optimization.These findings contribute to the advancement and practical implementation of inductor-based active cell equalization techniques in various energy storage applications, paving the way for improved performance and reliability of ultracapacitor systems.The analysis of ripple voltage at a frequency of 2000 demonstrates the effectiveness of the inductor-based active cell equalization technique in reducing voltage ripple and enhancing voltage stability within the ultracapacitor energy storage system.The reduced ripple voltage contributes to improved system efficiency, stability, and power quality.These findings support the ongoing optimization and practical implementation of inductorbased active cell equalization techniques, fostering improved performance and reliability in various energy storage applications.The analysis of the graphic duty cycle at 50% without dead time highlights the energy transfer efficiency, voltage balancing effectiveness, and impact on switching losses and thermal considerations within the inductor-based active cell equalization system.The balanced duty cycle and continuous operation contribute to improved voltage stability, extended system lifespan, and enhanced energy efficiency.The choice of duty cycle should be carefully considered and optimized based on specific system requirements and tradeoffs to achieve optimal performance and reliability in ultracapacitor energy storage applications.

Fig. 4 .
Fig. 4. Ripple voltage frequency 2kHz; (a) full preview; (b) zoom in preview resistance, negatively impacting the overall system performance and longevity.By equalizing the cell voltages, the active equalization technique minimizes stress on individual cells, reducing the risk of premature aging and ensuring a longer lifespan for the ultracapacitor system.The chosen frequency of 2000 for the active cell balancing process plays a crucial role in achieving efficient voltage balance.The frequency selection should strike a balance between the time required to achieve voltage balance and the system's energy efficiency.Higher frequencies may allow for faster voltage equalization but could lead to increased switching losses and reduced overall efficiency.Lower frequencies, on the other hand, may result in longer equalization times but with improved energy efficiency.Therefore, the chosen frequency should be optimized based on specific system requirements and trade-offs between equalization speed and energy efficiency.The achieved voltage balance at a frequency of 2000 opens up opportunities for further optimization and improvement.Future research efforts can explore optimizing the control algorithm, component selection, and system design to enhance energy efficiency and reduce losses associated with the active cell equalization process.Moreover, investigations into dynamic frequency adjustment or adaptive control algorithms can be conducted to adaptively adjust the equalization frequency based on the real-time voltage imbalance conditions, thereby improving the overall performance and effectiveness of the equalization technique.

Fig. 5 .
Fig. 5. Duty cycle 50% without dead time the switching devices.This balanced duty cycle ensures efficient energy transfer between the ultracapacitor cells.During the on-time, energy from the higher voltage cells is transferred to the inductors, while during the off-time, the energy stored in the inductors is discharged back into the lower voltage cells.The 50% duty cycle optimizes energy transfer efficiency and helps to maintain voltage balance among the cells.Fig.5shows the duty cycle 50%.The observed 50% duty cycle without dead time indicates that the equalization system operates in a continuous mode.This means that energy transfer between the cells occurs continuously without any interruptions or dead time.The continuous operation contributes to effective voltage balancing by ensuring a constant flow of energy between cells.The balanced duty cycle prevents voltage imbalances from deteriorating or increasing over time, leading to improved voltage stability and extended lifespan of the ultracapacitor system.The duty cycle of 50% without dead time helps to minimize switching losses in the equalization system.Switching losses occur during the transitions between the onstate and the off-state of the switching devices.With a 50% duty cycle, the equalization system achieves symmetrical switching patterns, resulting in reduced switching losses.Minimizing switching losses enhances the overall energy efficiency of the system, as less energy is dissipated as heat during the switching process.The balanced duty cycle and absence of dead time also contribute to improved thermal performance of the equalization system.Without dead time, the equalization process avoids unnecessary interruptions or gaps between energy transfer cycles.This helps to reduce thermal stress on the switching devices and other components, leading to improved reliability and longevity of the system.The balanced duty cycle further aids in thermal management by minimizing excessive heating and temperature fluctuations during operation.The selection of a 50% duty cycle without dead time should be considered in the context of the specific control strategy and system design.The duty cycle can be adjusted based on the desired rate of voltage equalization and the characteristics of the ultracapacitor cells.Different duty cycles may be more suitable for specific application requirements or system constraints.It is important to optimize the control strategy and system parameters to achieve the desired voltage balancing performance while considering factors such as energy efficiency, component stress, and system stability.