Active cell balancing for electric vehicle battery management system

ABSTRACT


INTRODUCTION
Battery frameworks having Li-ion cells are inclined to fast cell deterioration and burst into flames or cause to detonate. Intelligent circuits and programming advancements are set up to moderate these impending threats. In a multi cell battery pack, which is normally utilized in EVs restorative hardware, setting cells in arrangement leads to the likelihood of the cell imbalance, a slow, however, constant degradation of the battery. Cells are of indistinguishable nature. There is constantly slight change in the condition of the State of Charge (SoC), capacity, self discharge, temperature and internal impedance qualities, notwithstanding for cells that are a similar model from a similar maker and even from a similar cluster of creation [1]. When building multi cell packs, makers typically separate cells with comparative SoC based on the voltage. Voltage dissimilarities between the cells arise after some time due to significant change in the battery variables. By checking the voltage-guideline point of the battery pack the charger recognizes the fully-charged state. Due to internal parameter variations, any cell in the battery pack may reach the overvoltage limit compares to other cells in the string. The condition of the overcharged cell worsens further by repeated charge and discharge cycles and its leads to capacity deterioration.
To give a dynamic solution for this issue while considering the age along with the working states of the batteries, the BMS would attempt to integrate one of the two cell equalization techniques to balance the cells and to prevent them from getting overemphasized [2]: the passive and active cell equalization. The high voltage cell is identified and the excess energy is dissipated across the resistor until its voltage matches with the weak cells in the passive scheme [3]. In the active topology the over charged cell energy is evacuated and given to the undercharged cells by using energy transferring elements [4]. The Figure 1 shows the different cell balancing topologies which is available for the battery powered system. This paper discusses about the various active balancing topologies along with its equalization speed, application, size, cost, design and efficiency. Section 2 explains the various cell balancing topologies and the balancing techniques are compared. Section 3 represents the inductor-based topology implemented for 60V Li-ion battery system in MATLAB/Simscape and the simulation results are discussed followed by the conclusion.

BALANCING TOPOLOGIES 2.1. Active cell balancing method
In this balancing scheme, the excess energy is shifted from the overcharged cell to the weaker cell in the pack. Unlike passive balancing scheme, here energy is not wasted as heat instead it is transferred to other cells.

Capacitor based topologies
In a capacitor-based topology as represented in Figure 2, the capacitors are used for exchanging unequal charge among numerous cells in the battery pack. Single Switched Capacitor utilizes a single capacitor, makes the system more cost-productive than double tiered switched capacitor and switched capacitor [5].
Switched Capacitor transfers energy between adjacent cells. In order to reduce the balancing time a chain structured switched capacitor topology was introduced [6] and series-parallel switched capacitor and equalizer for various speed requirement [7]. A couple of changes made to switched capacitor strategy results in Double-Tiered Switched Capacitor (DTSC). The uniqueness in this is, 2 capacitors are used for energy shuttling purposes. For more series connected cells the balancing speed of DTSC is slower and it is overcome in [8] by using resonant topologies.
Modularized switched capacitor is a different type of capacitive storage device topology which also includes shuttling capacitor techniques [9][10][11]. It includes battery pack modularization, splitting the battery string into modules or groups. To reduce the switching losses, conduction losses and to improve the balancing speed resonant switched capacitor, based on that the balancing is used [12,13].

Inductors/transformers-based topology
Inductor/transformer based active equalizer as represented in Figure 3 transfers energy among multiple cells with the help of magnetic components such as inductors and transformers. Single Inductor Cell Balancing is based on the buck boost techniques where energy transfer takes place from highly charged battery to low charged battery and to achieve this voltage, monitoring circuits are required for every battery [14]. Multiple inductor cell balancing topology using more than one inductor to balance the multiple cells simultaneously. Cell balancing based on optimized cell to cell, any cell to any cell and parallel inductor architecture is discussed in [15][16][17][18].
In the single transformer cell topologies only one transformer is used to share the charge between the batteries with fast equalization speed and low magnetic losses [19,20]. It transfers energy from a pack to the undercharged cell called as pack to cell method and overcharged cell to the entire pack called as cell to pack method. Multiple transformer topologies can balance more than one cell at the time of imbalance.

Converter based topology
This dynamic cell equalization circuit as represented in Figure 4 balances the imbalanced cells in the pack through a DC-DC converter [21,22], for example, buck-boost or forward converter. Converter based topologies are very efficient and have a good control over the power flow [23][24][25]. In a forward converter, in presence of voltage difference, the cell which is having high energy is transferred to the other weak cells through transformer windings, diodes and switches [26]. In the Fly back converter-based cell balancer, when the auxiliary side switch is turned on, the energy gets delivered to cells with low voltage from high voltage cells through the diode since the estimation of high voltage on any cells gets put away in the transformer [27].
Full-bridge Converter is also called as fully controlled converter. Boost and buck mode are two modes of operation of full-bridge converter [28]. The cuk converter is like buck-boost cell balancer. It can increment or decrease the output voltage. It can be applicable for balancing only the neighbouring cells [29,30]. The buck boost topology is used where the voltage deviations among the cells are minimum and the applications need high balancing currents [31].
Quasi-Resonant converter is a highly efficient converter because the switches are on & off at zero voltage and/or zero current. In this equalizer inductors & capacitors are made as the resonant tank to attain the ZCS function [32]. Ramp Converter is additionally an enhancement for the multiple winding transformer equalization circuit. This topology just needs one additional (optional) twisting for the pair of cells rather than one for every cell [33].

Comparison of active balancing topologies
The Table.1 compares the various cell balancing topologies based on the balancing speed, reliability, control strategy, efficiency, etc.
The control system and plan perspectives are the two significant criterias for the choice of active cell balancing topologies. Control qualities are identified with dependability and methods of activity, which comprises of charge and discharge control [34,35]. Basic control implies better dependability. In light of the correlation the inductor-based topology is the better topology for charging as well as discharge balancing.

Inductor based active cell balancing
The inductor based active cell balancing topology is actualized for 60V Li-ion battery framework which comprises of 16 cells associated in an arrangement. Regular inductor-based topology sets aside a great deal of effort to exchange the charge from the principal cell to the last cell in the string. So, the effectiveness of the traditional topology isn't satisfactory. In the proposed cell balancing topology a smaller number of switches and inductors are utilized [17,18] contrasted with conventional inductor-based techniques which lessen the power misfortunes and improve the proficiency. The square chart of 16 cell inductor-based cell adjusting topology is appeared in Figure 5

Control methodology
The control methodology executed in this topology is based on the voltage deviation among the cells. Right off the bat, all the cell voltages are estimated. The process of acquiring abnormal battery cells and the application of active cell balancing is described in Figure 6.

First stage
The voltage distinction between the odd numbered cells (C1, C3... C15) with its contiguous even numbered cell (C2, C4.... C16) is more than the threshold voltage (20mV), at that point the overcharged cell is switched on & the inductor is charged. When the voltage distinction is underneath normal then the switch is turned off, the stored energy in the inductor is given to the low charged cell through the body diode of the switch. The voltage across the inductor L1, For instance, if the voltage contrast between odd numbered cells is more than 20mV contrast with even numbered cells, odd numbered switches S1...S15 are on and the energy is stored in the inductor. When it is below the threshold, the switches are turned off, the inductor energy is given to the even cells.

Second stage
In the second stage, the initial two cell voltage (V1 + V2) is contrasted with the next two cell voltage (V3 + V4) in the event that it is more than 20mV, at that point the profoundly charged cell switch is turned on. The strategy is same for the remaining cells.

Fourth stage
In this stage the top 8 cell voltages are contrasted with the bottom 8 cell voltages. The comparing cell switches are turned on dependent on the voltage contrast.
This proposed inductor-based topologies can even out top, centre and base cells at the same time with more prominent equalization speed contrasted with the regular technique.

Simulation results
So  Initially the voltage difference between the cell 1 and the cell 2 is 25mV. The cells are charged at consistent current and steady voltage mode. The cells are charged at 0.5C rate. When the cells are come to threshold charge voltage (3.7), the cell balancing technique is executed. Figure 7 represents the subsystem for cell 1 and cell 2 in MATLAB/Simscape. Shows the voltage and current waveforms of the inductor 1 during balancing. The cell 1 voltage is higher than the cell 2 voltage, switch S1 is turned on. The inductor1 is charged through S1.Once the voltage difference between the two cells are less than 20mV, switch S1 turned off. At 5800s the energy stored in the inductor 1 is given to cell 2 through the body diode of the switch S2. Since the threshold voltage limits between the cells are less, the cell voltage variations are not very high as shown in Figure 9.
This balancing topology took 50 minutes to balance the cell voltage equally and it took around 2 hours for complete charging. Since the whole cell voltages are balanced to the same level, the SoC of the cells are the same at the end of charge which is appeared in Figure 10. The charging procedure happens in an ambient temperature of around 20˚C. Amid charging the temperatures of the cells are increasing up to 24.75˚C appeared in Figure 11.
In  Amid charging, the cells are charged at steady current however the voltage is differing. When the cell achieves the most extreme limit voltage then it moves to the steady voltage mode where it keeps up the voltage as a constant and the current will achieve the base value. This relationship is appeared in Figure 12.

CONCLUSION
Diverse active cell balancing topologies are discussed in this paper. Inductor based topology is picked up for utilization based on the assessment and analysis. The proposed topology can alter top, middle and base cells simultaneously with a smaller number of parts than standard inductor-based systems. The control strategies implemented on the switches are based on the voltage difference between the cells. Simple

ACKNOWLEDGEMENT
This work has been supported by Robert Bosch Engineering and Business Solution Limited, Bangalore, India.