The inevitability of energy inconsistency among batteries within a battery pack poses operational challenges and potential safety hazards. It is imperative to swiftly harmonize the state of charge across all batteries to mitigate these issues. Addressing this concern, a dual-layer hybrid equalization topology is introduced, leveraging the Cuk circuit and flyback transformer. The battery pack is segmented into modules, with the Cuk circuit employed for intra-module equalization. Subsequently, the flyback transformer facilitates inter-module equalization. A multimodal equalization control strategy is devised, considering the status of individual modules to minimize repeated energy transfers between batteries. Simulation and experimental findings affirm that the proposed dual-layer active equalization control markedly mitigates the inconsistency among series-connected batteries, demonstrating rapid equalization and heightened efficiency.
I. INTRODUCTION
In light of the escalating scarcity of fossil fuels and the heightened environmental consciousness, electric vehicles (EVs) are experiencing a rapid surge in popularity.1 Due to their elevated energy density and prolonged cycle life, lithium batteries have become ubiquitous in energy storage systems for EVs.2,3 At present, scholars are extensively investigating materials such as the anode,4 cathode,5 and electrolyte6 of lithium batteries to optimize their performance, consequently enhancing the overall lifespan of battery packs. However, typical EV battery packs comprise numerous batteries interconnected in series, inevitably leading to variations in voltage, capacity, and internal resistance among individual batteries. These discrepancies tend to magnify over time, adversely affecting the performance and longevity of the battery pack.7,8 Therefore, ensuring effective equalization of the battery pack and mitigating such inconsistencies are paramount for prolonging battery life and bolstering safety.9
The attainment of battery equalization primarily relies on two key aspects: equalization strategy and equalization topology. Equalization strategy involves algorithmic control; however, its widespread implementation faces challenges due to the intricate nature of intelligent algorithms, often limiting applications to laboratory simulations for validation. In contrast, equalization topology has achieved a relatively advanced stage, with established current paths formed through component connections. Despite this maturity, existing topologies encounter certain issues, and ongoing research primarily focuses on optimizing these well-established structures.10
Equalization topology can be categorized into passive equalization and active equalization.11 Passive equalization commonly employs parallel resistors to dissipate excess energy, offering advantages, such as low cost, simplicity, and ease of control.12,13 However, the release of energy through resistors not only leads to energy wastage but also generates significant heat, posing safety risks, such as fire and explosion.14,15
Active equalization predominantly utilizes inductors, capacitors, transformers, and DC–DC converters as energy storage elements for power transfer.16 Zhang et al. introduced an inductance-based equalization topology, characterized by straightforward control but requiring an extended equalization time.17 Xu et al. devised a direct cell-to-cell equalization topology based on LC resonant transformation, achieving high equalization efficiency.18 Peng et al. implemented an equalization topology based on the LLC converter, employing zero-voltage switching control to reduce power loss and enhance efficiency.19 Several limitations are associated with these methods, as they primarily achieve equalization between two batteries simultaneously, making them less suitable for long series-connected batteries. Ouyang et al. introduced a Cuk circuit-based topology for rapid energy transfer between adjacent batteries,20 while Wu et al. proposed a topology using a Cuk circuit combined with double-layer selector switches to transfer energy between any batteries.21 However, these approaches still face challenges in terms of energy loss, equalization time, and switching losses.
A distinctive approach is the flyback equalization topology, known for its distance-unrestricted equalization, high equalization current, and effective electrical isolation.22,23 Cao et al. connected each battery to the battery pack through a flyback transformer to transfer energy between non-adjacent batteries.24 Pan et al. introduced a dual-layer topology for series-connected modules, utilizing multi-winding flyback transformers within modules and resonance-switched capacitors between modules to enhance equalization efficiency.25 However, flyback equalization topologies can only achieve bi-directional energy transfer between a battery and the battery pack, resulting in repeated energy transfers and a longer time to reach equalization.
Despite some advancements, commonly used single-layer equalization topologies exhibit inherent defects, leading to the degradation of equalization performance. The dual-layer equalization topology emerges as a promising solution, combining the strengths of single-layer topologies while addressing inherent limitations, thereby enhancing overall equalization performance.
To expedite the equalization process among series-connected batteries, a layered approach is incorporated into the equalization topology, complemented by a corresponding equalization control strategy. The primary contributions encompass the following facets:
Introduction of a dual-layer hybrid topology, leveraging the Cuk circuit and flyback transformer. The bottom layer, comprising Cuk circuits, is employed for intra-module equalization, while the top layer, featuring a flyback transformer, facilitates inter-module equalization. This design allows for the direct transfer of energy between batteries.
Implementation of a control sequence based on state of charge (SOC) comparison, involving intra-module equalization followed by inter-module equalization. A multimodal equalization control strategy applicable to inter-module equalization is devised according to the SOC status of modules. This strategy aims to minimize the involvement of non-equalization batteries and reduce repeated energy transfers, thereby enhancing the overall equalization speed.
Establishment of simulation and experimental platforms utilizing six batteries to validate the effectiveness of the proposed dual-layer active equalization control. Comparative analysis with other equalization methods demonstrates its notable features, including rapid equalization speed and heightened efficiency.
This paper is organized as follows: Sec. II delves into the analysis of the proposed equalization topology and elucidates its operational principles. Section III expounds on the equalization strategy, providing a detailed introduction to the multimodal strategy. Section IV presents simulation and experimental results, offering a comparative analysis with outcomes obtained through alternative methods. Finally, Sec. V provides a comprehensive summary of this paper.
II. TOPOLOGY AND WORKING PRINCIPLE
A. Equalization topology
Combining Cuk circuits and a flyback transformer, a dual-layer equalization topology is designed and shown in Fig. 1.
The bottom layer is consisted of Cuk circuits. Since energy can be rapidly transferred between two adjacent batteries in a Cuk circuit, two batteries Cell2i−1 and Cell2i are treated as a module Pi (i = 1, 2, 3, …, n). The two batteries in a module are equalized by a Cuk circuit, and it is called intra-module equalization.
The intra-module equalization topology is composed of a switch Ki, two MOSFET switches Ki1 and Ki2, two inductors Li1 and Li2, and one capacitor Ci. Ki is used to control the on/off of the intra-module equalization. The on/off of Ki1 and Ki2 are controlled by PWM waves, which can realize the bi-directional transfer of energy between two batteries.
However, the Cuk circuit cannot achieve direct energy transfer between non-adjacent batteries. To address this issue, a flyback transformer is applied in the top layer to equalize batteries in different modules, and it is called inter-module equalization.
In the inter-module equalization topology, Qi1 is connected to the positive terminal of Pi and the primary side of the flyback transformer, while Qi2 is connected to the negative terminal of Pi and the primary side of the flyback transformer. This creates a discharge path for Pi. Qn+i,1 is connected to the positive terminal of Pi and the secondary side of the flyback transformer, while Qn+i,2 is connected to the negative terminal of Pi and the secondary side of the flyback transformer. This creates a charge path for Pi. The RCD buffer circuit is used to absorb the energy of the transformer leakage inductance to reduce the spike voltage on the MOSFET.
B. Working principle
1. Intra-module equalization
The SOCs of Cell1 and Cell2 are SOC1 and SOC2, respectively. If SOC1 > SOC2, K1 is turned on, and the intra-module equalization between Cell1 and Cell2 will be carried out.
Stage I (0 - D1T1): K11 is on, and K12 is off. L11 is charged by Cell1 and iL11 increases. Simultaneously, C1 releases energy to charge Cell2 and iL12 increases, as shown in the red loop in Fig. 2. ΔiL11+ and ΔiL12+ represent the current variations of iL11 and iL12, respectively,
Stage II (D1T1 - T1): K11 is off, and the current flows continuously through the body diode of K12. Cell1 and L11 release energy to C1 and iL11 is reduced to zero. At the same time, the energy stored in L12 is released to charge Cell2, and iL12 is reduced to zero, as shown in the blue loop in Fig. 2. In this stage, ΔiL11− and ΔiL12− stand for the current variations of iL11 and iL12, respectively,
Clearly, the magnitude of the current can be varied by changing D1.
Once Cell1 and Cell2 have been equalized, K11 is turned off and the process of intra-module equalization ends.
2. Inter-module equalization
Assuming that the high-energy part consists of a (1 ≤ a < n) modules and the low-energy part consists of b (1 ≤ b < n) modules, the inter-module equalization between the high-energy part and the low-energy part will be carried out.
To prevent hysteresis saturation, the flyback transformer must operate in DCM. In one period T2, the inter-module equalization process has two stages, as shown in Fig. 4. The operating sequence is shown in Fig. 5, and D2 is the duty cycle of control signals of switches Q11 and Qa2.
Stage I (0 - D2T2): Q11 and Qa2 are on, and the other switches are off. The high-energy part composed of a modules forms a loop with the primary winding of the transformer, as shown in the red loop in Fig. 4.
Stage II (D2T2 - T2): Qn+a+1,1 and Q2n,2 are on, and the other switches are off. The low-energy part composed of b modules forms a loop with the secondary winding of the transformer, as shown in the blue loop in Fig. 4.
In the specific design process, the intra-module equalization needs to first determine the maximum current and switching frequency, and the inter-module equalization also needs to determine the transformer turns ratio. Based on this, the relevant circuit parameters can be derived from the aforementioned equations.
III. EQUALIZATION CONTROL STRATEGY
Generally, the SOCs of batteries are adopted to evaluate their consistency. However, SOC cannot be measured directly. Hence, it is necessary to measure the voltage and other parameters of the battery and use Kalman filtering to estimate the SOC of each battery.27,28 SOCs of Cell2i−1 and Cell2i are SOCi1 and SOCi2, respectively, and ΔSOC represents the equalization threshold.
In module Pi, if |SOCi1 − SOCi2| > ΔSOC, the intra-module equalization between Cell2i−1 and Cell2i will be conducted until |SOCi1 − SOCi2| ≤ ΔSOC. After intra-module equalization, the SOCs of two batteries in a module become similar, making them equivalent.
where SOCmax and SOCmin are the maximum and minimum of all SOCPi, respectively. If SOCmax − SOCmin > ΔSOC, the inter-module equalization is carried out until SOCmax − SOCmin ≤ ΔSOC. The energy is transferred from the module with SOCmax to that with SOCmin.
Defining ΔSOCP as the equalization threshold between modules, if , then Pi is regarded as the module corresponding to SOCmax. Similarly, if , then Pi is regarded as the module corresponding to SOCmin. During the two stages of the inter-module equalization, the first stage is to release energy to the primary winding of the flyback transformer by the serial modules with SOCmax, and the second stage is to transfer energy to the serial modules with SOCmin by the secondary winding of the transformer. In this process, the SOC of the module is variable and there may be multiple modules with SOCmax or SOCmin. According to the status of modules, a multimodal equalization strategy is proposed and it has four working modes.
OH/OL mode. It refers that, at the same time, there are one module with SOCmax and one module with SOCmin.
OH/ML mode. It refers that, at the same time, there are one module with SOCmax and multiple modules with SOCmin.
MH/OL mode. It refers that, at the same time, there are multiple modules with SOCmax and one module with SOCmin.
MH/ML mode. It refers that, at the same time, there are multiple modules with SOCmax and multiple modules with SOCmin.
Taking MH/ML mode as an example, and assuming that the modules with SOCmax and SOCmin are Pmax1, Pmax2 and Pmin1, Pmin2, respectively, the equalization process is shown in Fig. 6.
In this mode, Qmax1,1 and Qmax2,2 are turned on, and Pmax1 and Pmax2 are connected in series and release energy to the primary winding of the transformer, as shown in the red loop in Fig. 6. Then, Qn+min1,1 and Qn+min2,2 are turned on, and Qmax1,1 and Qmax2,2 are off, and Pmin1 and Pmin2 are connected in series and charged by the secondary winding of the transformer, as shown in the blue loop in Fig. 6.
During the inter-module equalization, the corresponding mode is selected based on the SOC status of each module for equalization operation. The mode selection flag is defined as W. When W equals 1, 2, 3, or 4, the OH/OL, OH/ML, MH/OL, or MH/ML mode is selected, respectively. Two batteries in a module with SOCmax can transfer energy to two batteries in a module with SOCmin using switches and a flyback transformer during the inter-module equalization, enabling the transfer of energy between non-adjacent batteries. The equalization control strategy flow for series-connected batteries is shown in Fig. 7.
IV. SIMULATION AND EXPERIMENT
A. Simulation and analysis
MATLAB/Simulink is used to carry out the equalization simulations of six batteries Cell1, Cell2, …, Cell6. In order to quickly verify the viability of the proposed equalization control and obtain the results compared with the traditional Cuk and flyback equalization topologies, the initial SOCs of batteries are set in a small range, as given in Table I. The other parameters of the simulation are given in Table II, and the MOSFETs and diodes are set as default parameters.
Battery parameters.
Module . | Battery . | SOC (%) . | Voltage (V) . | Capacity (Ah) . |
---|---|---|---|---|
P1 | Cell1, Cell2 | 87.0, 86.6 | ||
P2 | Cell3, Cell4 | 86.3, 86.7 | 3.7 | 3.4 |
P3 | Cell5, Cell6 | 86.4, 86.0 |
Module . | Battery . | SOC (%) . | Voltage (V) . | Capacity (Ah) . |
---|---|---|---|---|
P1 | Cell1, Cell2 | 87.0, 86.6 | ||
P2 | Cell3, Cell4 | 86.3, 86.7 | 3.7 | 3.4 |
P3 | Cell5, Cell6 | 86.4, 86.0 |
Simulation parameters.
Parameter . | Value . |
---|---|
Li1, Li2 | 100 µH |
Ci | 47 µF |
N1: N2 | 30:30 |
Lp, Ls | 260 µH |
f | 30 kHz |
D1 | 0.48 |
D2 in OH/OL mode | 0.6 |
D2 in MH/OL mode | 0.3 |
ΔSOC | 0.5% |
ΔSOCP | 0.2% |
Parameter . | Value . |
---|---|
Li1, Li2 | 100 µH |
Ci | 47 µF |
N1: N2 | 30:30 |
Lp, Ls | 260 µH |
f | 30 kHz |
D1 | 0.48 |
D2 in OH/OL mode | 0.6 |
D2 in MH/OL mode | 0.3 |
ΔSOC | 0.5% |
ΔSOCP | 0.2% |
The traditional Cuk equalization topology consists of interconnected Cuk circuits. During the transfer of energy from battery Cell1 to battery Cell6, the intermediate batteries Cell2 to Cell5 will transfer energy multiple times due to the fact that the Cuk circuit only allows for energy transfer between two adjacent batteries. As a result, the SOCs of these batteries will fluctuate repeatedly, as shown in Fig. 8. At t = 95.065 s, all batteries complete the equalization operation.
In the traditional flyback equalization method, energy transfer typically occurs between batteries and the battery pack. The battery pack first transfers energy to battery Cell6 with the lowest SOC. When the SOC of Cell6 increases to match that of battery Cell3, the battery pack simultaneously transfers energy to both Cell6 and Cell3. This process continues until all SOCs of batteries are equalized to the same level, as shown in Fig. 9. However, this mode of energy transfer leads to a problem of repeated energy transfers for batteries involved in equalization, and it also involves batteries that do not need to participate in equalization. As a result, it takes a long time for all batteries to complete the equalization operation, and t = 112.431 s.
The equalization result based on the proposed dual-layer equalization control is shown in Fig. 10. Approximately at t = 13.723 s, each module completes intra-module equalization. When performing inter-module equalization, the energy is transferred from P1 to P3, and SOCP1 gradually decreases and SOCP3 rises. Approximately at t = 45.374 s, SOCP1 ≈ SOCP2, then P1 and P2 release energy to P3. At t = 48.025 s, all batteries achieve the equalization.
Table III gives the main performance of three equalization methods. Our equalization method notably emerges as the fastest in achieving equalization between batteries. It outperforms the traditional Cuk and flyback equalization methods by significantly reducing the time by 47.04 and 64.406 s, respectively.
Performance of three equalization methods.
Method . | Time (s) . | Efficiency (%) . |
---|---|---|
Our proposed | 48.025 | 79.77 |
Traditional Cuk | 95.065 | 68.42 |
Traditional flyback | 112.431 | 76.49 |
Method . | Time (s) . | Efficiency (%) . |
---|---|---|
Our proposed | 48.025 | 79.77 |
Traditional Cuk | 95.065 | 68.42 |
Traditional flyback | 112.431 | 76.49 |
Compared with the two traditional equalization methods, the equalization efficiency of our method has been improved by 11.35% and 3.28%, respectively.
B. Experimental and analysis
To further verify the viability of the proposed dual-layer equalization control, an experimental platform for equalizing six batteries is established, as shown in Fig. 11. The experimental platform consists of a controller, the intra-module equalization circuit, and the inter-module equalization circuit. Six batteries are Panasonic NCR18650B lithium-ion batteries with the rated voltage of 3.7 V and the capacity of 3.4 Ah. The controller adopts TMS320F28335. The MOSFET, gate driver, and diode are IRF3205s, TLP250, and 1N4007, respectively. The other experimental parameters are consistent with the simulation parameters.
The initial SOCs of six batteries are 95.5%, 89.9%, 84.01%, 87.83%, 73.9%, and 69%, respectively. The changes in SOC during the equalization process are shown in Fig. 12.
The intra-module equalization is performed first, with Cell1 transferring energy to Cell2, Cell4 transferring energy to Cell3, and Cell5 transferring energy to Cell6. At about t = 240 s, three modules have achieved intra-module equalization. During this process, the iL11 in P1 is shown in Fig. 13. When K11 is on, Cell1 releases energy to L11 and iL11 gradually increases. When K11 is turned off, Cell1 and L11 release energy to C1, and iL11 gradually decreases. The change of iL11 is consistent with the theoretical analysis.
After achieving intra-module equalization, SOCP1 = 92.125%, SOCP2 = 85.465%, and SOCP3 = 70.925%. The inter-module equalization will operate in the OH/OL mode. Module P1 releases energy to P3, while P2 is not involved in the equalization process. Figures 14 and 15 show profiles of ip and is during this process. During a period, Q11 and Q12 are first turned on, and the energy is transferred from P1 to the flyback transformer. SOCP1 drops and ip gradually increases. Then, Q61 and Q62 are turned on, and the transformer charges P3 through the secondary winding and is gradually decreases.
At about t = 1160 s, SOCP1 drops to the same level as SOCP2, but still greater than SOCP3, and the inter-module equalization will operate in the MH/OL mode. P1 and P2 are in series to release energy to the flyback transformer through Q11 and Q22. P3 is charged by the transformer through Q61 and Q62, thereby achieving energy transfer from P1 and P2 to P3. Until t = 1960 s, SOCP1, SOCP2, and SOCP3 are at the same level, the inter-module equalization is achieved and ends.
The SOC of each battery before and after equalization is given in Table IV. The extreme difference of SOC between batteries is reduced from 26.5% before equalization to 0.43% after equalization, greatly reducing the inconsistency between batteries.
SOC of each battery before and after equalization.
Battery . | SOC(%) . | |
---|---|---|
Before . | After . | |
Cell1 | 95.50 | 82.33 |
Cell2 | 89.90 | 82.17 |
Cell3 | 84.01 | 82.17 |
Cell4 | 87.83 | 82.08 |
Cell5 | 73.90 | 81.99 |
Cell6 | 69.00 | 81.90 |
Battery . | SOC(%) . | |
---|---|---|
Before . | After . | |
Cell1 | 95.50 | 82.33 |
Cell2 | 89.90 | 82.17 |
Cell3 | 84.01 | 82.17 |
Cell4 | 87.83 | 82.08 |
Cell5 | 73.90 | 81.99 |
Cell6 | 69.00 | 81.90 |
To address the characteristics of our equalization method, this paper takes the equalization circuit composed of six batteries as an example, analyzes, and compares our equalization method with common equalization methods in terms of cost, speed, and efficiency. The results are given in Table V.
Comparison of equalization methods.
. | Componenta . | |||||||
---|---|---|---|---|---|---|---|---|
Method . | S . | L . | C . | D . | T . | Cost ($)a . | Time1% (s) . | Efficiency (%) . |
Proposed | 21 | 6 | 4 | 13 | 1 | 35.5 | 73.96 | 73.68 |
LC30 | 20 | 1 | 1 | 0 | 0 | 20.4 | 285.36 | 59.52 |
Flyback + MPC31 | 6 | 0 | 6 | 0 | 5 | 37.2 | 140 | 73.28 |
Flyback + VSSGPC24 | 12 | 0 | 0 | 0 | 6 | 48.0 | 46.77 | 38.98 |
Multi-winding transformer32 | 23 | 0 | 0 | 0 | 1 | 29.0 | 450 | 77.25 |
Buck-boost33 | 12 | 6 | 0 | 0 | 0 | 13.2 | 124.46 | 53.39 |
. | Componenta . | |||||||
---|---|---|---|---|---|---|---|---|
Method . | S . | L . | C . | D . | T . | Cost ($)a . | Time1% (s) . | Efficiency (%) . |
Proposed | 21 | 6 | 4 | 13 | 1 | 35.5 | 73.96 | 73.68 |
LC30 | 20 | 1 | 1 | 0 | 0 | 20.4 | 285.36 | 59.52 |
Flyback + MPC31 | 6 | 0 | 6 | 0 | 5 | 37.2 | 140 | 73.28 |
Flyback + VSSGPC24 | 12 | 0 | 0 | 0 | 6 | 48.0 | 46.77 | 38.98 |
Multi-winding transformer32 | 23 | 0 | 0 | 0 | 1 | 29.0 | 450 | 77.25 |
Buck-boost33 | 12 | 6 | 0 | 0 | 0 | 13.2 | 124.46 | 53.39 |
Component cost per unit ($). Switch (S) (1), inductor (L) (0.2), capacitor (C) (0.2), diode (D) (1), and transformer (T) (6).34
As given in Table V, the method utilizing only one LC reduces the overall cost of the circuit, yet results in a significant increase in the equalization time, which is nearly four times longer than ours.30 The method based on flyback transformers reduces energy loss due to the application of a model predictive control (MPC) equalization strategy.31 The cost and efficiency of this approach are comparable to our method, but it requires nearly 89% more time due to the use of flyback transformers for equalization between adjacent batteries. The method with the variable step size generalized predictive control (VSSGPC) strategy achieves the fastest speed among all methods.24 However, the use of a large number of transformers results in the highest cost and the lowest efficiency of all methods. The method in which all batteries share a multi-winding transformer results in an approximate 18% reduction in cost and a 3.57% increase in efficiency compared to ours.32 However, the repeated transfer of energy between the battery and the battery pack causes the slowest speed of all methods. The method using buck–boost converters only utilizes inductors as energy storage devices, resulting in the lowest cost among all methods.33 However, it requires ∼68% more time than ours, and its efficiency is ∼20% lower than ours due to the transfer of energy between inductors. Although the cost may be a bit high, we have achieved a high efficiency of 73.68%, with a lower time consumption. Overall, our approach is characterized by fast equalization speed and high efficiency.
V. CONCLUSION
This paper introduces a novel approach for rapidly mitigating inconsistencies among series-connected batteries by proposing a dual-layer hybrid equalization topology that integrates the Cuk circuit and flyback transformer. The topology comprises a bottom layer and a top layer. The bottom layer employs the Cuk circuit to equalize two batteries within each module, while the top layer uses a flyback transformer to equalize different modules. This dual-layer configuration establishes a direct energy transfer path for non-adjacent batteries, effectively reducing equalization time. In addition, to minimize repeated energy transfers, a multimodal equalization control strategy is presented. An experimental platform is constructed to validate the effectiveness of the proposed equalization control. Results demonstrate a significant reduction in the extreme SOC difference between batteries, from 26.5% to 0.43%. All batteries achieve equalization within 1960 seconds with an efficiency of 73.68%. Comparative analysis with other equalization methods reveals superior speed and efficiency. However, the increased number of components raises costs, prompting future work to optimize the topology for cost reduction.
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Hao Qiang: Conceptualization (lead); Funding acquisition (lead); Methodology (equal); Project administration (equal); Supervision (lead); Validation (equal). Zhengwen Mo: Conceptualization (equal); Formal analysis (lead); Methodology (lead); Project administration (equal); Validation (lead); Writing – original draft (lead). Junhao Xie: Methodology (equal); Software (equal); Writing – review & editing (equal).
DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon reasonable request.