电动自行车锂电池组均衡设计

An Integrated Design of Active Balancing and Redundancy at Module Level for Electric Vehicle Batteries
Wai Chung Lee*, David Drury, Phil Mellor
Department of Electrical and Electronic Engineering, University of Bristol Merchant Venturers Building, Woodland Road, Bristol, BS8 1UB,UK *E-mail: Wai-Chung.Lee@https://www.360docs.net/doc/a612037945.html,
Abstract ─ This paper proposes an integrated design of active balancing and battery redundancy at module level to maximize the performance and the reliability of the battery packs of Electric Vehicles (EVs) economically. Through the bidirectional flyback converter and the selection switches, the charge from the battery pack can be transferred to the selected module or vice versa to perform balancing. The design allows disconnection of faulty modules whilst maintaining the power supplied to the vehicle to avoid the risks of immediate power cut-off. Analytical studies and experimental results from the laboratory prototype are presented to verify the performance of the proposed design. Index Terms ─ cell balancing, module balancing, redundancy, battery management system, lithium-ion battery, electric vehicles.
I.
INTRODUCTION
Cell-to-cell characteristic mismatches such as internal impedance and capacity [1] can limit the performance of the battery pack of an electric vehicle (EV). Firstly, the weakest cell with the lowest capacity determines the capacity of the battery pack that can be discharged. Secondly, the cells having lower capacity will experience a higher change in the state of charge (SOC) during discharge. An overview of the battery ageing mechanisms in [2] indicates that SOC variance would result in a higher rate of capacity fade. Consequently a vital function of a Battery Management System (BMS) is cell balancing in order to match the SOC of the cells in a series string to the same level to avoid the adverse effects of cell imbalance. Passive cell balancing using shunting resistors to drain current from cells prone to overcharge is widely employed in various battery applications but the energy dissipation and heat generation are the concerns for EV applications. Active cell balancing transfers energy between the cells; different topologies of which are overviewed comprehensively in [3] – [5]. It is demonstrated that active cell balancing can yield a higher utilization of the capacity of the battery pack [6] and is more energy efficient [7]. Previous research suggest that a modularised active cell balancing circuit would suit an EV battery consisting of over 80 cells in series architecturally and can reduce the balancing time [5], [8], [9]. Kim et al proposed a modular configuration to select the target cell for balancing [8]. The design employed enhances the packaging however only one cell can be balanced each time resulting in an extended balancing time for multiple cells. An idea of using an intra-module equalizer and an outer-module equalizer was proposed in [9], the tests in which show that a modularized switched capacitor
equalizer achieved a smaller standard deviation of cell voltages than a non-modularized approach within the same balancing time. However, the balancing current capability of the switched capacitor-based approach is generally between the C-rates of C/500 and C/2000, and is limited by the voltage difference of adjacent battery cells/modules [10]. Applying the same concept of module balancing using flyback converters in [5] increased the balancing current although a rather low overall energy efficiency of 56% was recorded. Apart from cell balancing, the safety issues of lithiumbased batteries are critical as shown by the hazardous failures of laptop or mobile phone batteries [11]. The typical safety design for an EV is to isolate the battery pack from the load or charger by opening the protective relay when failures are detected [12]. Since the battery pack is the sole energy source in an EV, disconnecting the battery means an immediate power cut-off of the vehicle. This situation is similar to the engine stall of an internal combustion engine vehicle and can in itself cause a hazardous situation. Weijing Du et al [13] presented a design that can shunt the charge or discharge current of the faulty cell using a bidirectional flyback converter. However the design does not take the scenario of external or internal short circuit of the cells into account. Safer architectures are proposed in [14] – [16] that make use of switches to disconnect the faulty cells from the string and reconnect the remaining cells to deliver power to the load. Implementing these cell-redundant designs on an EV battery requires a large amount of high current rated switches and will result in a substantial increase in the already high cost of the battery pack. In order to satisfy the requirements of electric vehicles for better performance and higher reliability of the battery pack, a novel integrated design capable of balancing battery modules
978-1-4673-1408-4/12/$31.00 ?2012 IEEE

actively whilst simultaneously providing redundancy in an affordable architecture is proposed. Through the bidirectional flyback converter and the selection switches, the energy from the battery pack can be transferred to the selected module or vice versa. The design can also isolate faulty modules and reconfigure the connection of the remaining modules to maintain the power to the load. This integrated solution is based on the module level with half of the selection switches used to serve both the functions of module balancing and redundancy, thereby reducing the number of components. The design allows the battery pack of an EV to have higher reliability, more capacity and longer cycle life in an optimised cost. II. PROPOSED DESIGN
As indicated in fig. 2, each battery module is equipped with a module management unit responsible for data acquisition, balancing cells inside the module, communication with the master controller and controlling the two selection switches. The slave controller can decide which cells are to be balanced and then executes the balancing inside the module independently. Therefore, all modules can balance their cells simultaneously as module balancing is operating. This can effectively improve the balancing time especially when the SOC of all the cells are diverse with high variance. In this paper only the module balancing technique is considered and the details of the cell balancing circuit of the module are excluded. However the proposed design can cope with any cell balancing topologies such as those detailed in the literature [3] – [5]. B. Operation Principle – Module Balancing This section explains the operational principle of module balancing using a 4-module battery pack as an example. The major components involved are the bidirectional flyback converter and the N.O. selection switches. The primary winding of the flyback transformer is connected to the two terminals of the battery pack. The two secondary windings are for odd numbered modules and even numbered modules respectively. This selective transformer design avoids the drawback of difficult packaging of the multi-winding transformer in [6]. Assuming that the SOC of module 4 (M4) in fig. 3 is lower than the other modules, M4 needs additional energy so that its SOC can increase to the same level as the others. Fig. 3 also shows the directions of the primary (Ip) and secondary current (Is) flow when M4 is selected to be charged. The controllers, the CAN bus and the sensing circuits are omitted from the figure to ease the illustration. The master controller firstly closes the selection switches SNO3 and SNO4 so that M4 is connected to the converter. Then the controller sends a pulse width modulated (PWM) switching signal to Qp with fixed frequency and duty cycle.
A. Circuit Structure Fig. 1 shows the structure of the proposed module balancing and redundant design. The module balancing circuit comprises a master controller, a bidirectional flyback converter, two normally opened (N.O.) selection switches SNO0 and SNOm+1. Each module is made up of the series connected cells, one N.O. and one normally closed (N.C.) selection switch and a battery module management unit. Placing the N.O. selection switches inside the modules allows them to be connected in daisy chain manner to simplify the implementation. The master controller controls the selection switches by sending commands to the slave controllers of the module management units though a CAN bus. The slave controllers follow the commands to send the control signal to the selection switches accordingly. All the selection switches are in their default states in fig.1.
Fig. 1 Proposed integrated design for module balancing and battery redundancy
Fig. 2 Block diagram of the module balancing unit

Fig. 3 Directions of the balancing currents when module M4’s SOC is low
Fig. 4 Directions of discharge current when module M2 is failed
Fig. 5 Directions of discharge current when module M1 and M2 are failed
On the secondary side, the mosfet Qe,ch will be set to remain on so that Is can flow through it and the body diode of Qe,dis. The net current flowing through the selected module and the remaining modules is ‘Is – Ip’ and ‘Ip’ respectively (Is > Ip). Thereby the SOC of M4 will increase after a certain time of balancing. On the contrary, if the SOC of M4 is higher than the other modules, the two current flows will be reverse to remove charge from M4 and to transfer it into M1 – M3. SNO3 and SNO4 are closed again while PWM signal is sent to mosfet Qe,dis this time. The body diodes of Qp and Qe,ch will be forward biased and conduct Ip and Is respectively. In the case of an odd numbered module, the operation is identical except that the mosfets Qo,ch and Qo,dis are controlled, and the secondary current will flow through the upper secondary winding of the flyback transformer. C. Operation Principle – Battery Redundancy For a battery pack consisting of m modules connected in series, the kth module can be disconnected from the pack by opening the kth NC selection switch, SNCk. To maintain the power supplied to the load, a new path is provided for the discharge current by closing the N.O. selection switches SNOm+1, SNOk and SNOk-1. The example in fig. 4 shows how the module M2 is disconnected and the remaining modules are reconnected by changing the states of SNC2, SNO5, SNO2, and SNO1. This reconfiguration prevents further deterioration of the faulty cells in M2 and allows other healthy battery modules to continue to power the electric vehicle. If the module is disconnected simply due to individual cells having very low SOC, the associated cell balancing circuit can perform the charge equalization independently and the module can be back online after equalization by setting the selection switches to the default states. Disconnecting more adjacent modules is feasible when there are more modules with failed cells. Fig. 5 shows how the reconfiguration of the circuit when both M1 and M2 have
failed cells. SNC1 and SNC2 are opened and SNO0 and SNO2 are closed by the controller. If there are more modules to be disconnected, the remaining healthy modules may not be capable of powering the vehicle. However in this scenario, the proposed design is still better than the current nonredundant designs in terms of safety since the reconnected healthy modules can support the power consumption of safety related auxiliaries such as vehicle control system, thermal management system and hazard warning flashers. III. ANALYTICAL RESULTS This section analytically illustrates how module balancing can improve the discharge capacity of the battery cells. To simplify the analysis, it is assumed that all the cells (healthy cells) in the series connected battery modules have the rated capacity (Ch) except that the cells (defective cells) in one module have a lower capacity (Cd). The SOC of any ith healthy or defective cell after discharging a capacity of, to the load from time t0 to t can be expressed as:
1
Discharging at a constant current I for a period of ?t, the discharge capacity will be simply equal to:
? 2
In the first case, a balancing circuit employing the same selective flyback converter topology but at a cell level is used. Only one defective cell can be selected for balancing each time. During the same discharge event, one defective cell is balanced for a time interval of ?tcb, the SOC of this cell after balancing is:
1 · 3
,
,

Ip,cb and Is,cb are the average primary and secondary current of the cell balancing circuit and are assumed to be constant. When there is more than one defective cell, each defective cell will accept a charge of (?tcb· Ip,cb) when another defective cell is being balanced for the same length of interval ?tcb. If there are j defective cells in total, the SOC of any defective cell is:
1 · · 4
Fig. 6 Improvement of DOD by module balancing
,
,
The total balancing time for j defective cells is j · ?tcb Consider the efficiency of the flyback converter charging the unbalanced cell is ηch, then the relationship of the input and output power is:
·
,
·
·
,
5
Vcell is the terminal voltage of the cell being balanced and Vpack is the total voltage of all the cells. Assuming that there are n cells in total and Vpack ≈ n·Vcell, Ip,cb can then be expressed in terms of Is:
, ,
·
6
By substituting (2) and (6) into (4), SOCd(t) can be expressed as a function of the discharge current I, secondary balancing current Is,cb, discharge time ?t and total cell balancing time j · ?tcb. If the discharge current and the balancing current are represented in C-rate with respect to Ch, the SOC of the defective cells at different point of time or discharge capacity can be determined accordingly. In the second case considered, module balancing is operated with secondary current Ip and Is. If all the defective cells are in one module, the SOC of the defective cells and healthy cells will be:
1 · 7
that there are 8 defective cells in one module and the capacities of them are just 90% of the healthy cells in the other modules, the discharge capacity will be determined by the capacity of the defective cells i.e. 90% of Ch Using the equations (2), (4) and (6) – (8), the SOC of the defective cells against discharge capacity (with respect to Ch) under the two balancing schemes are simulated and plotted in fig.6. Cell balancing and module balancing started when 20% of the capacity has been discharged. In the two balancing cases, both the secondary current Is,cb and Is are equal to Ch/9 and the efficiency ηch of the converter is 80%. With cell balancing only, the discharge capacity can only be increased to 90.9% before the defective cells reach 0% SOC whereas module balancing improves the discharge capacity to 97.7%. The proposed balancing scheme is effective in allowing a more utilization of the capacity and that the influence of the defective cells to the overall performance of the battery can be minimized. IV. EXPERIMENTAL RESULTS
where ?tmb is the time interval for module balancing. Similarly the primary current for the m-module battery pack is:
· 8
Consider a battery pack consists of 12 modules and each module has 8 cells in series and all the cells are discharged at a rate of 1C from fully charged state until the first cell is fully discharged. If the capacities of all the cells equal the rated capacity, the discharge capacity will be equal to Ch. However, assuming
A. Prototype Development The prototypes of the module balancing circuit and battery module management units were built and used to demonstrate the functions of module balancing and redundancy of the proposed design for six serially connected battery modules. The experiment setup and the prototypes are shown in fig. 7 and fig. 8. Each battery module consists of four Li-polymer cells connected in series therefore the battery pack has twenty four cells in total. The Li-polymer cells with a rated capacity of 1250mAh and 1130mAh are selected to represent the healthy cells and the defective cells respectively. This emulates the situation that the defective cells are approximately 10% capacity lower than the healthy cells. Each battery module is equipped with a module management unit that is designed based on the architecture described in Section II. The major components of the module management unit include a microcontroller, a battery cell monitoring integrated circuit (LTC6803), a digital isolators, a CAN transceiver and two electromechanical relays as the selection switches. Connections of digital I/O pins and communication channel are available as well for the cell balancing circuits, which were not implemented in the experiment of this paper.

Electronic load
Host computer
Six battery modules
Modu management ules circuit Fig. 7 Experiment setup
Fig. 8 Li-Polymer cells (left), module manage ement unit (middle), module balancing circuit (right)
A module balancing circuit is also built for acquiring data t from the module management units, contro olling the selection switches and the bi-directional flyback, a communicating and with the host computer. The magnetizing inductance of the g transformer is 0.54mH and the turn rat of the primary tio winding to each secondary winding is 28:4 A graphical user 4. interface (GUI) is developed with LabVIE for display and EW logging the data such as the module and c cells voltages. The GUI also allows user to manually control the function of ost module balancing and redundancy on the ho computer. B. Experiment Results of Module Balancing Six serial connected battery modules were discharge from fully charged state with a constant current of 1.25A, which is ree equivalent to 1C of the healthy cells, in thr scenarios: i) all cells in the six modules are healthy; ii) fiv modules (M1 to ve M5) consist of healthy cells and one modu (M6) consists of ule defective cells. No module balancing is pe erformed; iii) same condition as scenario ii except that mod dule balancing was applied. cuit In scenario iii, the module balancing circ was operated to charge module M6 with a secondary ba alancing current Is varying between 0.13A and 0.14A, which is approximately a f C-rate of C/10 to C/9. The efficiency of module balancing based on the measured voltage of the entir battery pack, the re voltage of M6, the primary Ip and secondary balancing current y Is was within the range of 76% to 81% in th test. he Fig. 9 shows the voltage of each mo odule with module balancing in the discharge test scenario ii The voltage of ii. module M6 in the ‘No balancing’ scenario is also plotted in o the figure for the purpose of comparison. It shows that when the discharge capacity reached 250mAh, module balancing
M6 started and the voltage of M rose because of the net discharge current (discharge cu urrent less balancing current) was lower than that in scenario ii. Since M6 was discharged o in a lower rate than the ‘No ba alancing’ case, the cells in this module can be discharged for a longer period and a higher capacity can be yielded. fference between the voltage of Fig. 9 also shows that the diff M6 and the other five healthy m modules’ in scenario iii became smaller after module balancin was activated while the ng voltages were not equal. One reason is that no module e balancing was operated from 0 to 250mAh discharge capacity, d a SOC deviation was developed between the healthy modules (M1 to M5) and the defective m module (M6). Another reason is due to the differences of the characteristics such as open al circuit voltage and the interna impedance of the two LiPolymer cell models used in the experiment. e In all three scenarios, the di ischarge ended when the cell voltage of any cell reached the cut-off voltage, 3V. Fig. 10 e compares the measured disch harge capacity in the three scenarios and the percentages are calculated with respect to rio capacity determined in scenar i. It can be shown that module balancing improved the discharge capacity by 5.4% existed in the battery pack. even one defective module e Module balancing was activated continuously after the battery d had been discharged a capacity of 250mAh (12mins) until the end of the discharge (55mins). The four defective cells in the same time for 43 minutes to in M6 were balanced at the s achieve 5.4% capacity improvement. This shows the lancing over the balancing effectiveness of module bal schemes at cell level only.
Fig. 9 Module voltages in module balancing test n
Fig. 10 Comparison of discharg capacity in scenario i, ii, iii ge

C. Experiment Results of Battery Redundancy The function of battery redundancy was tested with all six modules consisting 1250mAh cells. The voltage the battery pack was approximately 96V and one of the modules was disconnected during constant power discharge (120W). Fig.11 shows the variation of the battery pack voltage (top blue trace) and the discharge current (bottom purple trace). The time span for the selection switches to disconnect one module and reconnect the remaining modules to the load was 96ms. The battery voltage decreased to 74.6V and the discharge current increased from 1.28A to 1.57A. The circuit demonstrated the function of redundancy to disconnect a faulty module whilst maintaining power to the load. A shorter response time can be achieved if devices such as solid state relays. However, considering the high current rating for electric vehicle application, the costs of employing solid state relays would be substantially higher. Thus electromechanical relays would be more preferable for the design proposed in this paper.
associated with immediate power cut-off. Half of the selection switches serve both the functions of module balancing and redundancy to reduce the cost. The experimental results demonstrate the feasibility and the benefits of the proposed integrated design. Further investigations of how the module balancing cooperating with cell balancing circuits to achieve the optimum battery balancing performance and the reliability of the battery redundancy under varying load are necessary. REFERENCE
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[2] [3]
[4] [5]
Voltage (20V/div)
[6]
Curremt (0.5A/div)
[7]
Time (20ms/div)
[8]
Fig. 11 Battery pack voltage and discharge current in redundancy test [9]
V.
CONCLUSION
[10]
This paper proposed an integrated design providing the functions of active balancing and battery redundancy at module level in order to maximize the performance and reliability of the battery packs of Electric Vehicles (EVs) economically. The analytical result shows that without an effective balancing scheme, the discharge capacity is limited by the weak cells. To verify the functions of module balancing and battery redundancy of the proposed design, a prototype was developed and tested a Li-Polymer battery pack with six serially connected modules. In the scenario that one module regarded as defective module is 10% capacity less than the other modules, the module balancing circuit transferred charge form the battery pack to the defective module through the use of a bidirectional flyback converter and the selection switches. This improves the discharge capacity by 5.4% comparing to the case without any balancing. In the constant power discharge test, the prototype circuit successfully disconnected one module and reconnected the other modules to maintain the power delivery to the load. This improves the system reliability by avoiding risks
[11]
[12] [13]
[14]
[15]
[16]