1. Introduction
To solve the problems of energy crisis, sustainable energy has been developed and received extensive attention. As an important player in sustainable energy, lithium-ion batteries have been widely used in many fields, such as electric vehicles [
1,
2] and battery energy storage systems [
3,
4], due to their merits including high energy density, low self-discharge rate, low cost and rechargeable operation. Considering the low-voltage and low-power features of the battery cell [
5], it is necessary to connect multiple cells in series and parallel to increase the capacity of the pack for high-voltage and high-power applications. However, there exists a difference in production process, temperature and others, and the same type of batteries that are produced by the same manufacturer show inconsistency during use [
6]. To address this phenomenon, the SOC difference [
7] of cells in a pack is presented to describe the inconsistency of the battery pack [
8]. The cell with the highest or lowest SOC states plays a decisive role and affects the charging and discharging performance, which changes the loss distribution and the long-term service life of the pack [
9]. In other words, the service life is usually limited by the shortest-life cell, and this phenomenon is called the “barrel effect”.
Although the problem of eliminating the inconsistency of battery packs is a big challenge [
10,
11,
12], advancements have been made regarding the inconsistency mechanism and advanced strategy. By balancing the SOC levels of all cells, the service life and safety performance of the pack need to be improved. In this case, the battery-balancing problem can be equivalently solved by the minimization of the cell SOC difference. The battery balancing methods can be categorized into two types: the passive and the active methods. The former is also called dissipation equalization and achieves pack balancing by means of energy-consuming components such as switch resistors, which is only applicable in the charging phase. The latter is also called the non-dissipative equalization method, which achieves SOC consistency across the battery pack by shifting energy from higher cells to lower cells through equalization circuits [
13]. Compared with the passive method, it attracts more attention, which is attributed to its lower amount of energy used for dissipation and faster equalizing speed. Active balancing methods are transformed into the following forms: cell-to-cell, cell-to-pack, pack-to-cell, and pack-to-pack [
11].
Compared with other balancing topologies, the cell-to-cell balancing structure potentially achieves better efficiency [
14]. This method realizes the equalization of all cells in a pack by installing an ICE among the cells. The ICE has the merits of quick equalization, simple implementation, and low cost. From the perspective of equalization topology, innovative structures such as layered equalization have been developed in the acceleration of equalization speed [
15], but this increases the number of ICEs and the complexity of the topology. Many balancing methods have been proposed, such as the multi-agent consensus control based on multi-objective constraints [
16]. However, it fails to output a reasonable current to suppress the inconsistency during the charging process. In addition, precise balancing requires additional cost, complexity, and strict cooling design. A quasi-sliding mode control strategy with SOC estimation [
17] and an equalization strategy based on predictive control [
18] are proposed. But these types of methods can easily become computationally infeasible and impractical. Some methods have been applied to optimize the battery balancing system, such as genetic algorithm, neural network [
19], and particle swarm [
20]. Moreover, the distributed consensus strategies of battery balancing are growing hot spots and are widely used in factory electronics. Following the consensus spirit, the controlled system converges to a balancing state with high speed and time-saving properties. The “plug and play” function can be realized even when facing an abnormal emergency due to its strong robustness and fast self-solving under the conditions of system faults, showing the system’s flexibility and resiliency. Based on the intelligent reconfigurable battery, a multi-agent voltage balancing strategy is applied [
21], but it requires memory for each battery node and becomes costly. In this sense, the consensus control for battery balancing is proposed in this paper to explore the memory-less scheme, so that the memory for each cell is not required anymore.
Inspired by the study of the distributed consensus method in the secondary control of the DC microgrid [
22], an event-triggered consistency control strategy [
23] with a communication-less design is proposed to solve the problem of SOC balance and power allocation among the distributed energy storages. However, the consensus will not spread to the cell level, which has long-term negative impacts on lithium-ion battery safety. To solve the cell-based inconsistency problem, a novel TTM and ETM balancing strategy with a centralized structure is proposed in this paper.
The contributions in this paper are as follows: the balancing system is built as a multi-agent system, where each battery cell is regarded as an agent and multi-agent control is adopted during the balancing process. In addition, an event-triggered mechanism is added in multi-agent system control, which is greatly beneficial to reducing the balancing convergence time and the update frequency of the actuator. To accelerate the balancing speed, the method of adjusting equalizing current is proposed for achieving cell consensus. The equalizing current becomes time-variable with the sampled data instead of a stable current. An improved balancing converter based on bidirectional Cuk [
17] is utilized for bidirectional energy transfer among cells with different SOCs. The process is implemented via PWM applying to MOSFET of the ICE to generate a time-varying equalizing current. An equalization strategy based on multi-agent consensus control with sampled data featuring ETM is proposed in this paper.
The remainder of this paper is organized as follows: In
Section 2, a model of the balancing system for a series-connected battery pack is presented. In
Section 3, a TTM algorithm for balancing the battery pack by adjusting the balancing current is proposed. In
Section 4, an equalization strategy based on ETM towards sampled data is proposed to speed up the equalization speed and reduce the actuator updates. Simulation results are presented in
Section 5. HIL experiments are described in
Section 6. This paper is concluded in
Section 7.
4. Consensus Algorithm with Event-Triggered Mechanism
Since TTM brings frequent updates to the actuator, it is necessary to find a new method driven by less-updates with sampled data, and then ETM is proposed in this paper. This strategy can not only greatly reduce the update frequency and control load, but also effectively accelerate the equalizing speed.
4.1. Battery Balancing under ETM
The measurement error
of the
kth sampling data is expressed by:
The sampled data of all cells in a pack is applied to update the actuator depending on the ETC rather than the fixed-time update in TTM.
The ETC of the control system can be shown as follows:
Denote the
kth trigger the update time of all cells at
, and the next trigger time can be defined as follows:
where
h is the sampling period and
represents the constant parameter greater than zero. As for
, the design process is detailed in
Appendix B.
is the data obtained by measuring and estimating the SOC of cell
i at
kth time, wherein,
. The set of event-triggering time is a subset of sampling-time data. The direction and magnitude of the equalizing current will only be updated when ETC is met, instead of every sampling instant. The event-triggered controller is designed based on the state and error of all cells. When the trigger condition is satisfied and the state error reaches a threshold value, the control inputs of all cells will be updated.
The balancing model of ETM is illustrated in
Figure 8. The sampler collects SOC data of the cell at each sampling instant, and the consensus controller and the CEP receive the data from the sampler. The event detection is sensed in CEP and the signal is sent to the event trigger and controller. If the arrived signal from the event trigger is positive, the control law will be changed with the newest sampled data. And the updated controlled equalizing current will be given to cells with ICE.
If the ETC in Equation (28) is satisfied, the accumulated sampling-data error is still within the allowable range, and the inundated data are adopted by all actuators. The ETC is only periodically calculated and checked at each sampling instant, which greatly reduces the computational and measuring burden compared with other ETM using continuous data. In other words, the ETC in Equation (28) brings the advantages of simplifying the control procedure and saving computing resources. In conclusion, the sampler is time-driven whereas the controller and actuator are event-triggered.
4.2. ETM Consensus Protocol for Cell Balancing
It can be seen from Formula (28) that the ETC of all cells is related to their sampled data and error at the
kth sampling instant. In other words, the central controller collects each cell’s SoC at the sampling instant, and the state is driven at the latest trigger time. The state error is calculated for evaluating ETC to decide whether to update the control input of all cells. Thus, a novel consensus protocol based on the sampling data is proposed as follows:
The event-triggered time interval with
is divided into plenty of
sampling intervals. Therefore, the consensus control protocol
is derived as follows:
Formula (31) can be rewritten into a compact form by:
where
and
.
and
are the recorded SOC of batteries
i and
j. When the ETC is reached and Laplacian matrix
L for the designed equalizing topology is derived, the SOC-based consensus model with ETM over
is presented as follows:
The “back-to-origin” state convergence for most existing consensus algorithms is generally verified by the Lyapunov function design. But in this paper, the LaSalle invariance theorem is proposed to analyze the convergence of event-triggered protocols, which makes it converge to the subspace instead of the origin.
The proposed ETM balancing strategy has some outstanding advantages such as the slow updating frequency of the equalizer actuator attributed to the event-triggered sampling, which had never been reported in a battery balancing system previously. The central controller is designed to decide the update time of the actuator according to the latest sampling data derived from the ETC. It is concluded that this strategy can effectively reduce the updated number of actuators. Overall, the ETM control strategy consumes less equalization time than TTM’s, and the speed of cell equalizing is remarkably improved.
The proof of consensus convergence of ETM can refer to
Appendix B.
In order to avoid the Zeno effect in the event-triggered equilibrium, the ETC is judged based on the periodic sampling data. The lower bound of the event piece time is at least the sampling period h, so the Zeno effect is naturally eliminated in the proposed ETM.
5. Simulation Results
The test of a series-connected pack with four battery cells is simulated in MATLAB. Under the traditional chain-topology pack, the TTM and ETM consensus controls are verified in standby mode and the charging/discharging mode. A series of comparisons were made for the equalizing strategy of TTM and ETM. The sampling period
h equals 0.2 s. In order to validate the effectiveness of the proposed methods for convenience, the capacity of each cell is designed to be about 0.1 Ah, and the voltage of each cell is set to 3.6 V. Also, two groups of batteries with different initial values of SOC are designed, and the initial SOCs are specified as Case 1 and Case 2 in
Table 2.
It can be characterized that the arrival of convergence state for the battery pack is obtained when the SOC difference in all cells and the average satisfies , where is greater than zero and represents the average SOC at each sampling instant. In practice, is designed to avoid over-equalization after the battery pack has already reached convergence.
In order to verify the equilibrium state of the system under dynamic external current, the external current is imposed as 0.05 A, the period of the external current is set as 100 s, the duty cycle is 0.5, and the SOC changes in the equalizing process of TTM and ETM are shown in
Figure 9. As observed, the equalizing system under external dynamic current can also converge to consensus. The TTM system converges at 504 s, the ETM system convergences at 450 s, and the equalizing speed has been accelerated by 54 s; it is about 12% faster than TTM. The result validates the effectiveness of ETM under an external variable current.
From
Figure 10,
Figure 11,
Figure 12,
Figure 13 and
Figure 14, it can be concluded that the current and voltage of each cell will reach zero, and the voltage of each cell convergences to a consensus value. The ETM system has a faster convergence time.
The simulation results with TTM are presented in
Figure 11,
Figure 12 and
Figure 13. As shown in
Figure 11a, four cells with different initial SOCs finally reach an equalization point, and the convergence time takes more than 470 s. The blue line in
Figure 11b indicates the pre-defined equalization upper bound
. The charging and discharging current are designed to be 0.05A/−0.05A considering the simulated low-capacity battery. The SOC changes when charging/discharging are shown in
Figure 12a and
Figure 13a, and the SOC differences are presented in
Figure 12b and
Figure 13b. Whether in the standby state or the charging and discharging state, the SOC in the pack eventually reaches the same equilibrium point and it validates the theoretical analysis. Also, the difference between the pack SOC and the mean SOC decreases in a quasi-exponential trend and eventually converges to zero. More information containing equalizing current and voltage of each cell in different modes is summarized and shown in
Appendix C.
The simulation results with ETM are captured in
Figure 15. As shown in
Figure 15, the battery pack balancing is terminated while the SOC difference is slowly brought down to 0.2 to prevent excessive battery balancing. The comparisons between TTM and ETM can be conducted by distinguishing the results of
Figure 11a and
Figure 15a. The balancing process is completed in 488 s under the TTM strategy, whereas it only takes 402 s while using the ETM strategy. As a result, the balancing speed is accelerated and the number of actuator updates is decreased, which is unquestionably beneficial to the battery health. In
Figure 15c, when the red-marked
and the blue-labeled
intersect, the error will be instantly reset to zero and an update of the actuator is triggered simultaneously. The equalization system will be guided by using the latest sampled data. Compared with the TTM strategy with an
hs update period, much more than
hs is available for the update in the ETM strategy, which has significantly released the computational complexity.
Figure 16 shows the ETC relation under charging and discharging mode.
Figure 17a,b show the battery balancing dynamics under the ETM strategy during charging and discharging operations, respectively. The charging and discharging current are set to be 0.05A/−0.05A—the same as the TTM strategy. Compared with the TTM strategy, the time required for battery equalization to reach consensus is significantly decreased by using the ETM strategy in both charging and discharging modes. The comparison is statistically recorded in
Table 3.
The SOC difference responses in the charging and discharging mode by using ETM with initial Case 1 are presented in
Figure 18. As shown, the SOC differences reach zero rapidly and it shows non-linear features over time, attributed to the inherent characteristics of ETM.
As shown in
Table 3, the equalizing speed of the ETM strategy is about 22.1% faster than the TTM strategy in standby mode; this advantage will result in about a 13.1% faster equalizing speed in the charging and discharging mode, respectively. These data indicate that the ETM strategy effectively speeds up the balancing speed of the battery pack and obtains a 10.9% increase over the TTM strategy.
6. Hardware-in-the-Loop Results
To investigate the cell balancing performance after using the proposed ETM strategy, the experimental verification by using CHIL is implemented. The CHIL experimental hardware platform is shown in
Figure 19. The battery balancing topology is generated in Typhoon HIL604. The sampling frequency is set to 10 kHz, the battery capacity is designed as about 1.2 Ah, the initial voltage of the battery while the cell is fully charged is set as 3.6 v, and the parameters of 18,650 lithium batteries are adopted in HIL604 experiments. The initial SOC value of the battery follows the specification of Case 1 in
Table 2.
The definition of equalizing current in the experiments is designed as follows:
In order to protect the battery, the upper bound of the balancing current Imax is necessary to be designed to prevent overcharging, over-discharging, and short-circuiting.
Figure 20a shows the SOC dynamical trajectory in the TTM equalizing process and the convergence time is 2855s. The convergence time of the ETM equalizing method goes to 2361 s, as shown in
Figure 20b. It is obvious that this equalization time by using ETM is much shorter than TTM’s. From this perspective, the pack balancing obtained by ETM is superior to the TTM one in terms of equalization efficiency.
It can be seen from
Figure 21,
Figure 22 and
Figure 23 that the ETM strategy stimulates a faster balancing speed than the TTM strategy. In the case of standby, charging, and discharging, the equalizing speed is increased by about 9% and the equalization time is shortened.
In the SOC-based consensus strategy, the control implementation and ETM are mostly based on simulation, rarely reported by experiment. This paper verifies the theory and simulation through hardware-in-the-loop experiments and provides an effective way to demonstrate the large-scale battery system.
In conclusion, the effectiveness and rapidity of the proposed ETM strategy of cell balancing are validated by CHIL experiments. Compared with the traditional TTM strategy, the equalizing time and the update frequency of the actuator have been remarkably reduced.
From
Table 3 and
Figure 20,
Figure 21,
Figure 22 and
Figure 23, it can be concluded that the ETM strategy has a faster equalizing speed than that of the TTM strategy, which can save the balancing time so that better consistency is obtained for all cells in a pack.
Figure 15c and
Figure 16 show that the PWM signal input into the MOSFET of ICE will change its value to control the balancing system only when the red-marked
and the blue-labeled
intersect. Whereas The update frequency of TTM is related to the sampling time h instead. It is much less than that of TTM, which can reduce the computing burden of the PWM signal.