A Modular Cell Balancer Based on Multi-Winding Transformer ... - MDPI

applied sciences Article

A Modular Cell Balancer Based on Multi-Winding Transformer and Switched-Capacitor Circuits for a Series-Connected Battery String in Electric Vehicles Thuc Minh Bui 1 1 2

*

ID

, Chang-Hwan Kim 1

ID

, Kyu-Ho Kim 2 and Sang Bong Rhee 1, *

Department of Electrical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Korea; [email protected] (T.M.B.); [email protected] (C.-H.K.) Department of Electrical Engineering, Hankyong National University, Anseong, Kyonggi 17579, Korea; [email protected] Correspondence: [email protected]; Tel.: +82-53-810-3097

Received: 30 June 2018; Accepted: 30 July 2018; Published: 1 August 2018







Abstract: In this paper, a cell balancing topology for a series-connected Lithium-Ion battery string (SCBS) in electric vehicles is proposed and experimentally verified. In particular, this balancing topology based on the modular balancer consists of an intra-module balancer based on a multi-winding transformer circuit and an outer-module balancer based on a switched capacitor converter, both offering the potential advantages and over conventional balancing methods, including short equalization time, simple control scheme, elimination of voltage sensors. In addition, a number of cells in the SCBS can be easily extended in this circuit. Furthermore, a system structure and an operating principle of the proposed topology are analyzed and experimentally verified for three different cases. The voltages of all cells in the SCBS reached the balanced state regardless of the various arrangement of the initial voltage, where the energy efficiency of the circuit reached 83.31%. Our experimental realization of the proposed balancing topology shows that such a technique could be employed in electric vehicles. Keywords: series-connected battery string; electric vehicles; modular cell balancing; multi-winding transformer; switched capacitor converter

1. Introduction For the past decades, the series-connected battery string (SCBS) has been widely used in several applications of energy storage systems for electric vehicles (EVs), hybrid electric vehicles (HEVs), subway and electric railroad [1–6] that require high voltage and capacity. The balance of the SCBS plays an important role in chemical and electrical characteristics of a battery string. In fact, the battery lifetime and the energy storage capacity are decreased due to an unbalance of the SCBS generated through the repeated charge and discharge operation. Particularly, this imbalance can lead to fire and/or explosion in the worst case [7]. Thus, the series-connected battery string in EVs needs to be balanced with the voltage [8]. In practice, internal and external sources are known as two main reasons that cause the unbalance of the battery cells [8,9]. For internal sources, the deviation in production results in variations of storage capacity value, total internal impedance, and rated self-discharge. In the case of the external sources, multi-rank rack protection Integrated circuits (ICs) primarily lead the unequal charge from the various series ranks in the pack [8]. Recently, the cell balancing methods have been introduced [10–20] and reviewed [7,8,21–23], as represented in Figure 1. There are two main ways to equalize a battery cell, which are passive and active methods [8,22]. Resistors are used in the passive cell balancing methods, which are connected

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Sci. 2018, 8, x FOR PEER REVIEW 2 of 22 in Appl. parallel with the battery cells to avoid overcharging through passive components so that one can equalize the low cells. Due to the power consumption of resistors, this method has low efficiency as equalize the low cells. Due to the power consumption of resistors, this method has low efficiency as reported in the literature [8]. The active cell balancing methods are based on inductors, capacitors and reported in the literature [8]. The active cell balancing methods are based on inductors, capacitors switched controllers or converters, the energy of the of cells transmitted from from a higha to a low and switched controllers or converters, the energy theiscells is transmitted high to avoltage low level [8,11,14]. Therefore, the active balancing method has higher efficiency than the passive balancing voltage level [8,11,14]. Therefore, the active balancing method has higher efficiency than the passive method as reported literature balancing method in as the reported in the[7,8]. literature [7,8].

Figure 1. The classifications of conventional battery cell balancer [21]. Figure 1. The classifications of conventional battery cell balancer [21].

The balancing method based on a multi-winding transformer (MWT) [18–20,24,25] has The balancing on a speed, multi-winding transformer (MWT)and [18–20,24,25] hasofadvantages advantages such method as fast based balancing simple control technique, repudiation voltage such as fast balancing this speed, simple control technique, and repudiation of voltage sensors. sensors. Nevertheless, circuit has the complexity of fabricating MWT, a limited number of windings due tocircuit the parameter matching forofthe turns ratio, and leakage a to Nevertheless, this has the complexity fabricating MWT, a limitedinductance, number ofespecially windingsfor due numbermatching of cells. The balancing circuit inductance, was improved by applying thelarge parameter for MWT-based the turns ratio, and leakage especially for aforward-flyback large number of conversion [23] to balance the cells. Thewas buck-boost converter-based cell balancing, which reduces cells. The MWT-based balancing circuit improved by applying forward-flyback conversion [23] switching loss to obtain high efficiency, is applied to EVs and HEVs [12,23,26]. However, this to balance the cells. The buck-boost converter-based cell balancing, which reduces switching loss technique requires an intelligent control a large However, number ofthis switches, resulting in to balancing obtain high efficiency, is applied to EVs and HEVsand [12,23,26]. balancing technique relativeancomplexity high and cost.aIn addition, a balancing method basedinonrelative a switched capacitor requires intelligentand control large number of switches, resulting complexity and circuit (SCC) [11,15,26,27] has been widely utilized in energy storage applications thanks to high cost. In addition, a balancing method based on a switched capacitor circuit (SCC) [11,15,26,27] such as simple design and control and elimination of a voltage sensor for each cell. hasadvantages been widely utilized in energy storage applications thanks to advantages such as simple design Importantly, the SCC can be employed in the modular method as well. However, the balancing time and control and elimination of a voltage sensor for each cell. Importantly, the SCC can be employed in is long, especially for a large SCBS since the energy is transferred between adjacent battery cells (i.e., the modular method as well. However, the balancing time is long, especially for a large SCBS since the cell to cell balancer). It is known that a higher number of cells and/or larger voltage difference energy is transferred between adjacent battery cells (i.e., cell to cell balancer). It is known that a higher between cells result in longer balancing time. number of cells and/or larger voltage difference between cells result in longer balancing time. There is a limited number of studies on battery balancing topology such as the requirements of Theresensors, is a limited number of studies battery balancing topology such theinrequirements voltage intelligent control, moreon importantly, the limited number of as cells the SCBS. of Therefore, voltage sensors, intelligent control, more importantly, the limited number of cells in the SCBS. a novel modular cell balancing circuit (MCBC) is proposed to solve these limitations. In Therefore, a novel modular cell balancing circuit (MCBC) is proposed to solve these limitations. In this approach, the SCBS, divided into several modules, is a balanced proposed MCBC, whichthis approach, SCBS, divided into modules, is a balanced proposed MCBC, which consists of an consists the of an intra-module andseveral outer-module balancers. The intra-module balancer based on the intra-module and outer-module balancers. intra-module balancer based on the circuit with MWT circuit with the forward converterThe structure (MWTFC) circuit is used to MWT transfer energy thebetween forwardthe converter structure circuit is transfer between the cells inside cells inside each (MWTFC) module, whereas theused SCCtobased on energy the outer-module balancer is each module, whereasthe the energy SCC based on the outer-module balancer is applied to transfertechnique the energy applied to transfer between the modules simultaneously. This balancing effectively deals with a small number of balancing cells whentechnique designingeffectively a balancing circuit. the between the modules simultaneously. This deals withTherefore, a small number of this balancing circuit becomes much easier. proposed circuit has of design cells when designing a balancing circuit. Therefore, theFurthermore, design of thisthe balancing circuit becomes numerous advantages of conventional circuits and overcomes that these much easier. Furthermore, thetwo proposed circuit has numerous advantages disadvantages of two conventional circuits conventional circuits were unable to eliminate. The structure and operational principles of the and overcomes disadvantages that these conventional circuits were unable to eliminate. The structure proposed MCBC are described in detail. The experimental results are presented to verify the efficacy and operational principles of the proposed MCBC are described in detail. The experimental results are of the proposed topology. presented to verifybalancing the efficacy of the proposed balancing topology.

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2. Proposed Modular Cell Balancing Circuit 2.1. Analysis of 2.1. Analysis of the the Previous Previous Cell Cell Balancing Balancing Circuits Circuits 2.1.1. Active Cell 2.1.1. Active Cell Balancing Balancing Circuit Circuit Using Using the the MWT MWT Figure active cellcell balancing circuit usingusing the MWT thewith forward converterconverter structure Figure 22shows showsanan active balancing circuit the with MWT the forward and operating modes [17,18,21,25,28]. This circuit of N series-connected cells. Each in structure and operating modes [17,18,21,25,28]. Thisconsists circuit consists of N series-connected cells.cell Each the SCBS is associated with a primary winding of the MWT (T) and a power switch S (k = 1, . . . , 4). k cell in the SCBS is associated with a primary winding of the MWT (T) and a power switch Sk (k = 1, A diode (D) on the secondary side is used to reset the magnetizing inductance of the transformer. …, 4). A diode (D) on the secondary side is used to reset the magnetizing inductance of the The MWTFC-based balancer equalizes theequalizes voltage of in the bythe selectively the transformer. The MWTFC-based balancer thecells voltage ofSCBS cells in SCBS byturning selectively switches S on or off according to the constant duty cycle (D) simultaneously. turning thek switches Sk on or off according to the constant duty cycle (D) simultaneously.

(a)

(b)

(c)

Figure2. 2. The The MWTFC MWTFC based based on on the the balancing balancing circuit circuit [21]: [21]: (a) Figure (a) Circuit Circuit diagram; diagram; (b) (b) Mode Mode 1; 1; (c) (c) Mode Mode 2. 2.

Three operational modes in one switching period (Ts) of the circuit and the operation Three operational modes in one switching period (Ts ) of the circuit and the operation waveforms waveforms are shown in Figures 2b,c and 3, respectively. In order to simply analyze the operating are shown in Figures 2b,c and 3, respectively. In order to simply analyze the operating modes for modes for the circuit, it is assumed that this circuit includes four series-connected cells; The the circuit, it is assumed that this circuit includes four series-connected cells; The relationship among relationship among the voltage of the battery cells is assumed as follows: Vcell1 > Vcell2 > Vaver > Vcell3 > the voltage of the battery cells is assumed as follows: V cell1 > V cell2 > V aver > V cell3 > V cell4 , the sum Vcell4, the sum voltage is VTotal = Vcell1 + Vcell2 + Vcell3 + Vcell4, and the average voltage is Vaver where Vcell1, voltage is V Total = V cell1 + V cell2 + V cell3 + V cell4 , and the average voltage is V aver where V cell1 , V cell2 , Vcell2, Vcell3, and Vcell4 represent the voltage across Cell1, Cell2, Cell3, and Cell4, respectively. The V cell3 , and V cell4 represent the voltage across Cell1 , Cell2 , Cell3 , and Cell4 , respectively. The operating operating modes of the circuit are presented as follows: modes of the circuit are presented as follows: Mode 1 [t0, t1] (see Figure 2b): At t0, four switches (S1, S2, S3, and S4) are turned on Mode 1 [t0 , t1 ] (see Figure 2b): At t0 , four switches (S1 , S2 , S3 , and S4 ) are turned on simultaneously simultaneously as shown in Figure 3. In this mode, the energy is transferred from high voltage level as shown in Figure 3. In this mode, the energy is transferred from high voltage level cells (i.e., Cell1 , cells (i.e., Cell1, Cell2) to the low one (i.e., Cell3, Cell4) through the transformer T. Cell2 ) to the low one (i.e., Cell3 , Cell4 ) through the transformer T. In mode 1, there are four loops forming four equations, and each loop equation can be In mode 1, there are four loops forming four equations, and each loop equation can be expressed as expressed as Equation (1). Figure 4 shows an equivalent circuit. The relationship among the current Equation (1). Figure 4 shows an equivalent circuit. The relationship among the current of a magnetizing of a magnetizing inductance Lm, iLm, and cell currents i1, i2, i3 and i4, and voltage across Lm, VLm, can be inductance Lm , iLm , and cell currents i1 , i2 , i3 and i4 , and voltage across Lm , V Lm , can be expressed as expressed as Equations (2) and (3): Equations (2) and (3): di (1)(1) = Vaver = = VCelli − VTPi = − Llki i dt iLm = i1 + i2 + i3 + i4 , iLm = i1 + i2 + i3 + i4,

(2) (2)

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VLm = = Lm

di Lm dt

where are the the currents currentsof offour fourswitches switches ofSS1,1S ,S , 3Sand = 3 and 4 , respectively. and ii44 are of 2,2S S4,Srespectively wherei1i,1,i2i2,, ii33 and

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(3) (3) (3)

where i1, i2, i3 and i4 are the currents of four switches of S1, S2, S3 and S4, respectively

Figure 3. Operation waveforms of the cell balancing circuit. Figure 3. Operation waveforms of the cell balancing circuit. Figure 3. Operation waveforms of the cell balancing circuit.

Figure 4. Laplace transform circuit of mode 1. Figure 4. Laplace of mode In this mode, four switches turnedtransform on. Thecircuit value the1. Figure are 4. Laplace transform circuit of of mode 1.leakage inductance and the magnetizing inductance can be used to apply the average voltage. From Equation (1) to Equation (3), In this mode, four switches are turned on. The value of the leakage inductance and the the V can be expressed as follows:are turned on. The value of the leakage inductance and the InTP this mode, four can switches magnetizing inductance be used to apply the average voltage. From Equation (1) to Equation (3), magnetizing inductance be used=to apply=the average ( + voltage. ), Equation (1) to Equation + + From (4)(3), the VTP can be expressedcan as follows: the VTP can be expressed as follows: ( + + + ), = = (4)

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VTP = Lm

dim d = L m ( i1 + i2 + i3 + i4 ), dt dt

(4)

d V − VLm i = Cell1 dt 1 Llk1

(5)

V − VLm d i2 = Cell2 dt Llk2

(6)

V − VLm d i3 = Cell3 dt Llk3

(7)

d V − VLm i = Cell4 dt 4 Llk4

(8)

In this mode, the inrush currents likely to flow in Figure 4 need to be considered. In the case, if there is a high voltage difference among the cells, the inrush currents likely to flow are large. Therefore, NTC thermistor [29] in series with the primary is suggested to limit the current. The NTC thermistor offers high resistance at the beginning of switching and limits the inrush current. After a short time, the NTC thermistor resistance decreases to a low value due to self-heating and it does not affect normal operation. However, in this study, the different voltages among the cells are small. Therefore, the circuit does not need to limit current and protect circuits. V Cell1 and V Cell2 are higher than V aver , V Cell3 , and V Cell4 . Therefore, currents i1 , i2 flow from the cells (i.e., Cell1 and Cell2 ) to the transformer T, and i3 and i4 flow from T to the battery cells (i.e., Cell3 and Cell4 ). This means that the electric charges in Cell1 , Cell2 are transferred to Cell3 , Cell4 through T. Mode 2 [t1 , t2 ] (see Figure 2c): At t1 , four switches are turned off simultaneously, as shown in Figure 3. The voltage across the Lm becomes negative, V Lm . Therefore, the magnetizing inductance is reset through the diode, resulting in a decrease in the iLm to zero level. VLm = −(VCell1 + VCell2 + VCell3 + VCell4 )

(9)

Mode 3 [t2 , t3 ]: Mode 3 starts when the iLm is equal to zero. No current flows in the MWT from t2 to t3 . At t3 , this mode is completed and changes to mode 1 occur in the next switching period. This circuit has several advantages [17] such as short equalization time, simple control scheme, repudiation of the voltage sensor. However, if the SCBS is extended, the MWT requires a large number of windings. Meanwhile, this MWT is difficult to manufacture due to the parameter matching for the turns ratio and the leakage inductances. Equations (5)–(8) show that larger leakage inductances result in a smaller cell current. In other words, the balancing time is longer. In particular, the leakage inductances of the transformer are not uniform. Therefore, this causes a charge imbalance problem by itself. In order to reduce this problem, the number of windings of the transformer needs to be reduced [28]. This requirement motivates our proposed cell balancing circuit to solve these issues. 2.1.2. Cell Balancing Circuit Using SCC A cell balancing circuit based on an SCC and operation modes of the circuit are shown in Figure 5 [8,15,16,21]. In order to simplify the analysis for circuit operation mode, it is assumed that the circuit has two cells and VCell1 > VCell2 . When two of four switches (Sa1 and Sa2 or Sb1 and Sb2 ) in the balancing circuit are turned on or off, the energy is transferred from a cell to another cell via a balancing capacitor (Cb ). The switches are controlled by pulse-width-modulated (PWM) signals with fixed duty cycle (50% Ts ), as shown in Figure 6. The analysis of the circuit is based on a proposition of voltage of Cell1 and Cell2 , as follows: Mode 1 [t0 , t1 ] (See Figure 5b): At t0 , Sa1 and Sa2 are turned on, while Sb1 and Sb2 are turned off simultaneously. In this mode, the switches Sa1 and Sa2 conduct. Thus, Cell1 is connected in parallel with Cb , as shown in Figure 5b. Cb starts to be charged by Cell1 . The voltage across Cb (vCb ) starts increasing, and current flows of Cb (iCb ) starts decreasing, as shown in Figure 6. The instantaneous

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voltage and current of the balancing capacitor in this mode can be expressed [30] by Equations (10) and (11)   −t

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= (vCell1 − vCbmin ). 1 − e RCh .Cb

+ vCbmin

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v − vCbmin R −t.C (11) iCb (t)( =) =Cell1 e Ch. b (11) RCh . (11) ( )= where is the sum of the equivalent series resistance of balancing capacitor RESR , the turn-on Ch is the sum of the equivalent series resistance of balancing capacitor R ESR, the turn-on where RRCh Ch is the sum of the equivalent series resistance of balancing capacitor R ESR , the turn-on where R resistance , Sa2 . resistance of of two two switches switches SSa1 a1, Sa2 resistance of two switches Sa1, Sa2

(a) (a)

(b) (b)

(c) (c)

Figure 5. The switched capacitor circuit for two cells [21]: (a) (a) Circuitdescription; description; (b)Mode Mode 1; (c) Mode 2. Figure The switchedcapacitor capacitorcircuit circuitfor fortwo two cells cells [21]: [21]: 2. 2. Figure 5. 5. The switched (a) Circuit Circuit description;(b) (b) Mode1;1;(c)(c)Mode Mode

Figure 6. Operational waveforms of the switched-capacitor circuit. Figure 6. Operational waveforms of the switched-capacitor circuit. Figure 6. Operational waveforms of the switched-capacitor circuit.

Mode 2 [t1, t2] (See Figure 5c): At t1, Sa1 and Sa2 are turned off, and Sb1 and Sb2 are turned on Mode 2 [t1, t2] (See Figure 5c): At t1, Sa1 and Sa2 are turned off, and Sb1 and Sb2 are turned on simultaneously. this mode, the switches Sb1 and Sb2 conduct. Thus, Cell2 is connected in parallel Mode 2 [t1 , In tIn (Seemode, Figurethe 5c):switches At t1 , Sa1 turnedThus, off, and Sb2 are in turned on 2 ] this simultaneously. Sb1and andSa2 Sb2are conduct. CellS2b1isand connected parallel with C b, as shown in Figure 5c. The energy of Cb starts to be discharged to Cell2. The current of Cb simultaneously. In in this mode,5c. the switches Sb2 conduct. Thus, Cellto2 is connected in parallel b1 and with Cb, as shown Figure The energySof Cb starts to be discharged Cell 2. The current of Cb begins to boost in the opposite direction as represented in Figure 6. The instantaneous voltage across with C , as shown in Figure 5c. The energy of C starts to be discharged to Cell . The current of Cb 2 b b begins to boost in the opposite direction as represented in Figure 6. The instantaneous voltage across and the current of C b in this mode are respectively expressed in Equations (12) and (13). begins to boost in the opposite direction as represented in Figure 6. The instantaneous voltage across and the current of Cb in this mode are respectively expressed in Equations (12) and (13). and the current of Cb in this mode are respectively expressed in Equations (12) and (13). . (12) ). (1 − ( )= −( − ) . (12) ). (1 − ( )= −( − ) ( )=− ( )=−

. .

(13) (13)

where Rdis is the total of the equivalent series resistance of balancing capacitor RESR, and the turn-on where Rdis is the total of the equivalent series resistance of balancing capacitor RESR, and the turn-on resistance of two switches Sb1 and Sb2 resistance of two switches Sb1 and Sb2 This circuit has some advantages [8]; for example, it does not require voltage sensors or

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  −t vCb (t) = vCbmin − (vCbmax − vCell2 ). 1 − e Rdis .Cb iCb (t) = −

vCbmax − vCell2 R −t.C e dis b Rdis

(12) (13)

where Rdis is the total of the equivalent series resistance of balancing capacitor RESR , and the turn-on resistance of two switches Sb1 and Sb2 This circuit has PEER someREVIEW advantages [8]; for example, it does not require voltage sensors or closed-loop Appl. Sci. 2018, 8, x FOR 7 of 22 control. Therefore, this circuit is simple to implement with low loss. However, in the case of a large case of aof large numberthe of high cells unbalance and/or theof high unbalance between the cells, number cells and/or voltage betweenofthevoltage cells, the equalization timethe is equalization time is long [21]. Therefore, the novel MCBC needs to be considered carefully to reduce long [21]. Therefore, the novel MCBC needs to be considered carefully to reduce the mentioned the mentioned as disadvantages as shown disadvantages shown in Section 2.2. in Section 2.2. 2.2. Proposed Modular Cell Balancing Circuit 2.2.1. 2.2.1. Modular Modular Cell Cell Balancing Balancing Concept Concept Figure illustratesananSCBS SCBS structure based a conventional and modular balancing Figure 77 illustrates structure based on aon conventional and modular balancing circuit circuit [2,31–33]. The conventional cell balancer used the series-connected battery cell string, which [2,31–33]. The conventional cell balancer used the series-connected battery cell string, which consists consists N × Therein, M cells. each Therein, eacha cell has aenergy separate energy transmitting path in as Figure shown 7a, in of N × Mofcells. cell has separate transmitting path as shown Figure 7a,the whereas thecell modular balance is divided into of M the groups of each the SCBS; each module whereas modular balancecell is divided into M groups SCBS; module has N cells,has as N cells, as represented in Figure 7b,c [1,34,35]. For the modular cell balancer, M groups of the SCBS represented in Figure 7b,c [1,34,35]. For the modular cell balancer, M groups of the SCBS are are connected with M modules balance voltagesinineach eachmodule. module.The Thefeatured featuredenergy energy of of cells cells is connected with M modules to to balance thethe voltages is transferred transferred from from aa high high voltage voltage level level to to aa low low level level inside inside each each module module (i.e., (i.e., intra-module intra-module balancer) balancer) individually. These modules modules can can be individually. These be connected connected to to an an external external module module balancer balancer (i.e., (i.e., outer-module outer-module balancer) balancer) to to transmit transmit the the energy energy from from aa high high to to aa low low voltage voltage level level module module until until achieving achieving the the equalization of both energies between the modules. The modular cell balancing method is advanced equalization of both energies between the modules. The modular cell balancing method is advanced for for aa small small number number of of cells cells in in comparison comparison with with the the conventional conventional method. method. Therefore, Therefore, it it is is easier easier and and more more flexible flexible than than the the conventional conventional circuit circuit in in designing designing cell cell balancing balancing for forthe thelarge largeSCBS. SCBS.

(a)

(b)

(c)

Figure 7.7.Modular Modularcircuit circuit balancing concept: (a) Conventional cell balancer; (b) Modular cell Figure balancing concept: (a) Conventional cell balancer; (b) Modular cell balancer; balancer; (c) Thecell modular cellbased balancer based on outer-module. (c) The modular balancer on outer-module.

2.2.2. Proposed Modular Cell Cell Balancing Balancing Circuit Circuit 2.2.2. Proposed Modular 1. System Systemstructure structurefor forproposed proposedMCBC MCBC 1. The system system structure structure of of the the proposed proposed MCBC MCBC is is shown shown in in Figure Figure 8. 8. This This circuit circuit is is constructed constructed by by The dividing the SCBS into two modules (M 1, M2) and each module contains N cells in series-connection dividing the SCBS into two modules (M1 , M2 ) and each module contains N cells in series-connection in which which each k and the primary side winding of the MWTs in in each cell cell in in the the SCBS SCBSconnects connectsthe theswitch switchSS k and the primary side winding of the MWTs intra-module balancers. Each MWT has a secondary side winding in in connection with oneone diode to in intra-module balancers. Each MWT has a secondary side winding connection with diode reset the magnetizing inductance of transformers (Lm) to prevent saturation of the MWTs. Each module connects with the intra-module balance individually, and all modules are connected to the outer-module balancer. In the intra-module balancer, a number of the transformer’s primary windings are equal to a number of the cells inside each module. The magnetizing energy of the MWT is to balance the cell voltages in each module, and a charge/discharge process of a module

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to reset the magnetizing inductance of transformers (Lm ) to prevent saturation of the MWTs. Each module connects with the intra-module balance individually, and all modules are connected to the outer-module balancer. In the intra-module balancer, a number of the transformer’s primary windings are equal to a number of the cells inside each module. The magnetizing energy of the MWT is to balance the cell voltages in each module, and a charge/discharge process of a module balancing capacitor (Cm )2018, is to8, balance the module voltages. Appl. Sci. x FOR PEER REVIEW 8 of 22

Figure 8. The proposed modular cell balancing circuit.

Figure 8. The proposed modular cell balancing circuit. In this proposed circuit, the energy is transferred from a high voltage level to a low voltage level in both the cells inside each module and modules simultaneously by the MWTs and the modular In this proposed circuit, the energy is transferred from a high voltage level to a low voltage level balancing capacitor, respectively. The Cm is used to equalize the energy between the modules in both through the cellstheinside each module and the MWTs and the modular outer-module balancer. It ismodules controlledsimultaneously by the switches Sby a1, Sa2 and Sb1, Sb2 which are balancing capacitor, respectively. The C is used to equalize the energy between the modules m in Figure 9. Voltage sensors and intelligent control controlled by PWM signals, as shown circuit arethrough the outer-module is controlled by the switches Sa1 ,designed Sa2 and and Sb1 ,controlled. Sb2 which are controlled by not used in balancer. this MCBC.ItTherefore, the circuit has been simply

PWM signals, as shown in Figure 9. Voltage sensors and intelligent control circuit are not used in this MCBC. Therefore, the circuit has been simply designed and controlled.

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Figure 9. Operational waveforms of the proposed modular cell balancing circuit (MCBC). Figure 9. Operational waveforms of the proposed modular cell balancing circuit (MCBC).

2. Operational principle 2. Operational principle The operational principle of the proposed MCBC is similar to that of the MWTFC and the SCC The operational principle of the proposed MCBC isand similar that ofdriven the MWTFC theduty SCC individually. In particular, the switches in the MWTFC the to SCC are by twoand fixed individually. particular, the switches in the MWTFC and the SCC are driven by two fixed duty ratios ratios (D) andIn(D 1), respectively. In order to simply analyze operating modes for the circuit, the (D) and (D ), respectively. In order to simply analyze operating modes for the circuit, the following 1 following assumptions are made: assumptions are made: • All the switches, capacitor, diodes and transformer are ideal. All the switches, capacitor, diodes and transformer are ideal. •• Each module contains four cells. Eachrelationship module contains cells. of the battery cells is arranged in decreasing order from Cell11 •• The amongfour voltage (Vcell11) to Cell24 (Vcell24), and Vcell11 > Vcell12 > Vaver1 > Vcell13 > Vcell14 > Vcell21 > Vcell22 > Vaver2 > Vcell23 > Vcell24 (the module 1 voltage: VM1 = VCell11 + VCell12 + VCell13 + VCell14; the average voltage of M1: Vaver1 =

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•

The relationship among voltage of the battery cells is arranged in decreasing order from Cell11 (V cell11 ) to Cell24 (V cell24 ), and V cell11 > V cell12 > V aver1 > V cell13 > V cell14 > V cell21 > V cell22 > V aver2 Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 22 > V cell23 > V cell24 (the module 1 voltage: V M1 = V Cell11 + V Cell12 + V Cell13 + V Cell14 ; the average voltage of M1 : V aver1 = V M1 /4; the module 2 voltage: V M2 = V Cell21 + V Cell22 + V Cell23 + V Cell24 ; VM1/4; the module 2 voltage: VM2 = VCell21 + VCell22 + VCell23 + VCell24; the average voltage of M2: Vaver2 = the average voltage of M2 : V aver2 = V M2 /4; V M1 > V M2 ). VM2/4; VM1 > VM2). There There are are four four operating operating modes modes during during one one switching switching period period (T (Tss)) of of the the proposed proposed MCBC. MCBC. These These modes based on the switching states of the primary side switches (S , S , S , S , S , S11 , modes based on the switching states of the primary side switches (S1111, S1212 , S13,13S14,14 S21, 21 S22, ,SS2223,, SS23 11, and and S24the ) inintra-module the intra-module balancer and the switches , Sb2 ) in the outer-module a1 , SS a2b1, ,and S24) in balancer and the switches (Sa1, Sa2(S , and Sb2) S inb1the outer-module balancer. balancer. The theoretical waveforms and operating modes of the proposed circuit shown in The theoretical waveforms and operating modes of the proposed circuit are shown in are Figures 9 and Figures 9 and 10, respectively. 10, respectively. Mode [t00,, tt11]](see (seeFigure Figure10a): 10a):AtAtt1,t1eight , eight switches intra-module balancers (module S 12,, Mode 11 [t switches in in intra-module balancers (module 1: S1: 11, S11 SS12 , S , S ; module 2: S , S , S , S ) and S , S are turned on, while S , S are turned 14 24 a2 turned on, while Sb1b1 13, S13 14; module 2: S21, S2122, S2223, S2324) and Sa1, a1 Sa2 are , Sb2b2are turned off off simultaneously, as shown in Figure 9. In this mode, the energy is transferred from a high voltage level simultaneously, as shown in Figure 9. In this mode, the energy is transferred from a high voltage cell a low each module through two MWTs (T1 ) and 2 ), respectively. leveltocell to alevel low one levelinside one inside each module through two MWTs (T1)(Tand (T2), respectively. V , V are higher than V , V , V in M , and V are higher aver1 1 VCell21, Cell21 Cell11 Cell12 Cell13 Cell22 VCell11 , VCell12 are higher than Vaver1 , VCell13 , VCell14Cell14 in M1, and VCell22, Vare higher than Vthan aver2, V , V , V in M . Therefore, i , i and i , i flow from cells to T , T , respectively, and , aver2 11 12 21 22 1 2 2 Cell23 Cell24 VCell23, VCell24 in M2. Therefore, i11, i12 and i21, i22 flow from cells to T1, T2, respectively, and i13, i14 and ii13 23, ii14 and i , i flow from T , T to the battery cells. This means that, in module 1, the electric charges in 23 24 1 2 24 flow from T1, T2 to the battery cells. This means that, in module 1, the electric charges in Cell11, Cell , Cell transmitted are transmitted to13Cell in module electriccharges chargesin in Cell are 13 ,14Cell 14 , and 21,, Cell Cell11 12 are 12 to Cell , Cell , and in module 2, 2, thetheelectric Cell21 Cell22 22 are transmitted , Cell23. . transmitted to to Cell Cell23 23, Cell23

(a)

(b) Figure 10. Cont.

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(c)

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(d)

Figure 10. Operating modes of the proposed MCBC: (a) Mode 1; (b) Mode 2; (c) Mode 3; (d) Mode 4. Figure 10. Operating modes of the proposed MCBC: (a) Mode 1; (b) Mode 2; (c) Mode 3; (d) Mode 4.

At the same time, Sa1 and Sa2 conduct, so the Cm will be charged by the VM1 through the Sa1 and same Sa1 and Sgradually Cm will charged by instantaneous the V M1 through the Sa1 and Sa2At sothe that the time, VCm increases tosoa the steady statebeperiod. The voltage and a2 conduct, Sa2current so thatof the V Cm increases gradually to amode steady period. instantaneous and current the balancing capacitor in this canstate be given by The Equations (14) and voltage (15). of the balancing capacitor in this mode can be given by Equations (14) and (15). . (14) ). 1 − ( )=( −  + −t vCm (t) = (VM1 − VCmmin ). 1 − e RCh .Cm + VCmmin (14) . (15) ( )=

VM1 − VCmmin R −.Ct bm iCm (15) At(tt)1,= eight switches in etheChintra-modules balances are turned off Mode 2 [t1, t2] (see Figure 10b): RCh simultaneously, as shown in Figure 9. The voltages across Lm1 and Lm2 are negative as expressed by Mode 2(16) [t1 , and t2 ] (see 10b):DAt t1 , D eight switches in the intra-modules balances are turned Equations (17).Figure The diodes 1 and 2 conduct. Then, VD1, VD2 become zero because D1, D2 are offideal simultaneously, as shown in Figure 9. The voltages across Ldiodes L1m2 negative expressed m1 andD devices. The Lm1 and Lm2 are reset through the secondary , Dare 2 resulting in as a decrease byof Equations (16) and (17). The diodes D and D conduct. Then, V , V become zero because D1 , the magnetizing currents of transformers 1 and2 2 (iLm1 and iLm2). D1 D2 1 D2 are ideal devices. The Lm1 and Lm2 are reset through the secondary diodes D1 , D2 resulting in a (16) =− decrease of the magnetizing currents of transformers 1 and 2 (iLm1 and iLm2 ). (17) =− VLm1 = −VM1 (16) Mode 3 [t2, t3] (see Figure 10c): Mode 3 starts when the currents through the magnetizing inductance of two transformers (iLm1) and (iLm2 ) are=equal to zero. No current flows in the MWTs from t(17) 2 VLm2 −VM2 to t3. Mode (seeFigure Figure10d): 10c): the magnetizing Mode3 4[t[t2 ,3, tt34]] (see At Mode t3, the S3a1,starts Sa2 arewhen turnedthe off,currents and the Sthrough b1, Sb2 starts to be turned inductance of two transformers (i ) and (i ) are equal to zero. No current flows in MWTs from Lm1 Lm2 on as shown in Figure 9. The voltage VCm is higher than the VM2 such that the energythe of C m is in a t2 to t . 3 discharging condition. Module 2 receives stored energy in Cm gradually. The instantaneous voltage Mode 4 [t3of, tthe Figure 10d): Atcapacitor t3 , the Sa1in , Sthis turned and the Sgiven to be turned 4 ] (see a2 are b1 , Sb2instarts and current module balancing mode are off, respectively Equations (18) onand as shown in Figure 9. The voltage V is higher than the V such that the energy of C m is in a Cm M2 1 in the next switching period. (19). At t4, this operating mode is finished and returns to mode

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discharging condition. Module 2 receives stored energy in Cm gradually. The instantaneous voltage and current of the module balancing capacitor in this mode are respectively given in Equations (18) and (19). At t4 , this operating mode is finished and returns to mode 1 in the next switching period.   −t vCm (t) = vCmmin − (vCmmax − v M2 ). 1 − e Rdis .Cm iCm (t) = −

(18)

vCmmax − v M2 R −.Ct m e dis Rdis

(19)

According to the analysis of the above operation modes, it can be seen that the MWTFC-based intra-module balancer and SCC-based outer-module balancer work separately. The energy is transferred from a high to a low level of both the cells in each module and the modules simultaneously. Thus, the short balancing time, simple control scheme and the low mismatch between the leakage inductances can be obtained due to a small number of cells. 3. Experimental Setup In this paper, the MCBC was experimentally verified to validate the theoretical operation of the proposed topology. The experimental setup consisting of two parts is illustrated in Figure 11, and circuit components are shown in more detail in Table 1. The first part is the designed MCBC, which consists of twelve switches, two diodes, one module balancing capacitor Cm , and two multi-winding transformers. An IM 3533 LCR meter was used to measure the magnetizing and leakage inductances of two transformers. The second part is the available equipment such as power supply, voltage recorder, oscilloscope and DSP board. The voltage recorder (i.e., YOKOGAWA-GP10) was used to measure and record the voltage balancing process of eight cells, and the battery cell used Lithium-ion cell LIR17335-PCM C5264RR (2/3 A 3.7 V 700 mAh). Gate driver voltage V gs of the switches such as S11 , S24 , Sa1 , and Sb1 is shown in Figure 12. Therein, the duty ratio (D) of signals to power switches in the intra-module balancers is 37.5%, and the duty ratio (D1 ) of the signal to switches Sa1 , Sb1 is 50% of one switching period where the Sa1 and Sb1 are complementary, and switching frequency of f and f 1 are set for 40 kHz. Table 1. The parameters of the proposed balancing circuit. Parameter

Value

Twelve MOSFETs

IPP023N10N5

Two diodes

DO-204AC (DO-15) Core: EER2828N N1 :N2 = 1:1

Cell balancing circuit

Battery string

Two Transformers (Four primary/One secondary windings)

Balancing capacitor

700 µF

Gate driver

UCC27519A-Q1

Eight Lithium-ion cells

Nominal capacity

700 mAh

Nominal voltage

3.7 V

Weight Cell voltage recorder Controller

Lm1 = 1.88 mH; Lm2 = 1.83 mH Llk11 = 1.87 µH; Llk12 = 1.95 µH; Llk13 = 3.39 µH; Llk14 = 1.94 µH; Llk21 = 2.30 µH; Llk22 = 1.99 µH; Llk23 = 2.00 µH; Llk24 = 1.98 µH;

Series-connected cell string recorder

18 g YOKOGAWA-GP10

Switching frequency

40 kHz

Digital controller

TMS320F28335

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Figure 11. A prototype of the modular cell balancing circuit and experimental setup. Figure 11. A prototype of the modular cell balancing circuit and experimental setup. Figure 11. A prototype of the modular cell balancing circuit and experimental setup.

Figure 12. PWM Signals of switches S11, S24, Sa1 and Sa2. Figure 12. PWM Signals of switches S11, S24, Sa1 and Sa2.

Figure 12. in PWM Signals S11 , S24 ,voltage Sa1 and arrangements Sa2. In this work, the experiment three cases of ofswitches initial different of the cells is performed. The voltages of the cells are set as follows. Case 1: the voltages of eight cells are arranged In this work, the experiment in three cases of initial different voltage arrangements of the cells is in decreasing order from Cell 11 (i.e., V cell11 ) to Cell 24 (i.e., V cell24 ); Case 2: V cell11 > V cell21 > V cell12 >the Vcell22 > is In this work, the experiment in three cases of initial different voltage arrangements of cells performed. The voltages of the cells are set as follows. Case 1: the voltages of eight cells are arranged V cell13 > V cell23 > V cell14 > V cell24 , and case 3: the initial voltage of the cells is set randomly, as shown in performed. The order voltages ofCell the11cells set) to as Cell follows. Case voltages eight arranged in decreasing from (i.e.,are Vcell11 24 (i.e., Vcell241:); the Case 2: Vcell11 of >V cell21 >cells Vcell12are >V cell22 > detail Table 2.cell14from in V decreasing order Cell (i.e., V ) to Cell (i.e., V ); Case 2: V > V > V cell13 >in Vcell23 >V > Vcell24 , and case 3: the initial voltage of the cells is set randomly, as shown in > 11 24 cell11 cell24 cell11 cell21 cell12 detail> in Table>2.V cell23 > V cell14 > V cell24 , and case 3: the initial voltage of the cells is set randomly, as V cell22 V cell13 Table 2. The initial voltages of the cells for the experiment. shown in detail in Table 2. Table cells2 for the experiment. Case 1 2. The initial voltages of the Case Case 3 Table 2. The initial voltages of the cells for the experiment. Battery Initial Cell Initial Cell Initial Cell Case 1 Module Case 2 Module Case 3 Module Cells Voltages [V] Voltage [V] Voltages [V] Voltage [V] Voltages [V] Voltage [V] Battery Initial Cell Module Initial Cell Module Initial Cell Case 1 Case 2 Case 3 Module 3.896 3.871 Cell 11 3.880 Cells Voltages [V] Voltage [V] Voltages [V] Voltage [V] Voltages [V] Voltage [V] Battery Initial Cell Module Initial Cell Module Initial Cell Module CellCells 12 3.857 [V] 3.773 [V] 3.585[V] 3.896 3.871 Cell 11 3.880 Voltages Voltage [V] Voltages [V] Voltages Voltage [V] = 15.183 VVoltage M1 = 14.862 VM1 = 14.906 VM1 Cell1213 3.745 3.666 3.661 Cell 3.857 3.773 3.585 Cell11 3.880 3.896 3.871 VM1 = 14.862 VM1 = 14.906 VM1 = 15.183 Cell 14 3.701 3.527 3.789 Cell 13 3.745 3.666 3.661 Cell 3.857 3.773 3.585 12 V = 15.183 V = 14.862 V M1 M1 M1 = 14.906 3.816 3.744 Cell 21 3.652 Cell 3.745 3.666 3.661 Cell 14 3.701 3.527 3.789 13 Cell 3.701 3.527 3.789 Cell 22 14 3.604 3.695 3.529 3.816 3.744 Cell 21 3.652 VM2 = 14.575 VM2 = 14.437 VM2 = 14.291 Cell 23 21 3.543 3.574 3.674 Cell 3.652 3.816 3.744 Cell 22 3.604 3.695 3.529 VM2 = 14.575 VM2 = 14.437 VM2 = 14.291 Cell 3.604 3.695 3.529 Cell 24 22 3.492 3.490 3.490 Cell 23 3.543 3.574 3.674 V M2 = 14.291 V M2 = 14.575 V M2 = 14.437 Cell 3.543 3.574 3.674 23 ∑Cell 24 [V] VCell 3.492 29.474 3.490 29.437 3.490 29.343 Cell24

V∑Cell [V]

V∑Cell [V]

3.492

29.474

29.474

3.490

29.437

29.437

3.490

29.343

29.343

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4. Results and Discussion 4. Results and Discussion Figure 13 shows experimental results of the proposed circuit for three cases. A higher and a lower Figure shows experimental of the each proposed highervoltage and a voltage of the13cells inside the module results tend towards other circuit and all for cellsthree tend cases. to the A average lower voltage of the cells inside the module tend towards each other and all cells tend to the average value of the cells in series. The proposed balancing circuit indeed demonstrates the balancing trend voltage of of thecells cells in series. Thea balanced proposedstate. balancing circuit the indeed demonstrates the since thevalue voltages gradually reach Furthermore, results on the different balancing trend since of the cellsbalanced gradually reach a balanced state. Furthermore, the results cases indicate that the the cellsvoltages achieved state regardless of the various arrangement of the on the different cases indicate that the cells achieved the balanced state regardless of the various initial voltage of all the cells. arrangement of the initial voltage of allcircuit the cells. Energy efficiency of the balancing can be used Equation (20) [16] as follows: Energy efficiency of the balancing circuit can be used [16] as follows:  Equation (20)  2 2 − ∑∑iN=1 0.5C 0.5 cell (VCelli_end − Vmin ) E _   η == residue == Eimbalance 0.5 ( 2 _ − V2 ) − ∑∑iN=1 0.5C cell VCelli_start min

(20) (20)

where E and E are unbalanced energy before cell balancing and remaining energy where and Eresidue are unbalanced energy cell balancing remaining energyand of of E Eimbalance after cell balancing, respectively. Ccell isbefore the capacitance of theand battery cell; Vcelli_start E after cell balancing, respectively. Ccelland is the capacitance the battery cell; V celli_start Vimbalance celli_end are the cell voltage of the ith cell before after balancing,ofrespectively; Vmin and N areand the V are the cell voltage of the ith cell before and after balancing, respectively; V and N are the min celli_end initial lowest voltage and number of cells, respectively. initialInlowest and number of highest cells, respectively. case 1voltage (see Figure 13a), the voltage is 3.880 V, the lowest voltage is 3.492, and the In case 1 (see Figure 13a), the highest voltage is 3.880 the lowest voltage and the initial maximum voltage difference is 388 mV, and then the V, difference reduces to is 1153.492, mV after the initial maximum voltage difference is 388 mV, and then the difference reduces to 115 mV after the balancing circuit runs for 300 min, and from (20) and experimental data and results, the energy balancing for 300 min, and from2(20) experimental results, the13c), energy efficiency circuit is = runs 76.27%. Similarly, in case (seeand Figure 13b) and data case and 3 (see Figure the efficiency is η = 76.27%. Similarly, in case 2 (see Figure 13b) and case 3 (see Figure 13c), the maximum 1 maximum voltage difference among the cells reduces from 406 mV to 112 mV and from 381 mV to 86 voltage difference amongcircuit the cells reduces mV to min, 112 mV 381 mV after mV after the balancing runs for 150from min,406 and 200 andand from = 83.31% andto 86=mV 81.74%, the balancingThe circuit runsefficiency for 150 min, and cases 200 min, and η2 =due 83.31% η3 = 81.74%, respectively. respectively. energy in three is different to theand difference in balancing time. The energy efficiency in three cases is different due to the difference in balancing time. If the If the balancing time is short, the energy efficiency will increase. In reality, the initialbalancing different time is short, the energy efficiency increase. the initial different voltage of voltage arrangement of the cells iswill set as in casesIn2reality, or 3. These are the cases with the arrangement highest energy the cells is set as in cases 2 or 3. These are the cases with the highest energy efficiency of the circuit. efficiency of the circuit. From From the the experimental experimental results results in in Figure Figure 13, 13, we we can can see see that that there there are are minor minor residual residual voltage voltage mismatches because the leakage inductances of the transformer were not uniform, as shown in 1. mismatches because the leakage inductances of the transformer were not uniform, as shown inTable Table L1.lk13 of the transformer was larger than others; hence, the current i was smaller. In other words, Llk13 of the transformer was larger than others; hence, the current 13 i13 was smaller. In other words, the was slower than others, as shown in Figure 13a. the balance balance of of the the Cell Cell13 13 was slower than others, as shown in Figure 13a.

(a) Figure 13. Cont.

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(b)

(c) Figure 13. The experimental voltage results of eight cells: (a) Case 1; (b) Case 2; (c) Case 3. Figure 13. The experimental voltage results of eight cells: (a) Case 1; (b) Case 2; (c) Case 3.

According to Table 2, the initial maximum voltage difference of the cells in three cases is a According to of Table 2, the initial voltage of cell the voltages cells in three cases is a minor divergence approximately 25 maximum mV. In addition, thedifference total of the is approximate. minor divergence of approximately 25 mV. In addition, the total of the cell voltages is approximate. In particular, cases 1, 2 and 3 are 29.474 V, 29.437 V and 29.343 V, respectively. However, the In particular, cases 2 and 3 areis29.474 29.437 Vdue andto29.343 V, respectively. However, the balancing balancing time for 1,three cases more V, different dependence on voltage unbalance between time for three cases is more different due to dependence on voltage unbalance between the 1 and the M1 and M2, like the SCC-based balancer theory in Section 2.1. In detail, the initial M voltage M , like the SCC-based balancer theory in Section 2.1. In detail, the initial voltage differences between 2 differences between the M1 and M2 for case 1, case 2, and case 3 are 892 mV, 287 mV, and 381 mV, the the M1 andtimes M2 for case 150 2, and case 3 are mV,respectively. 287 mV, andThe 381 total mV, the balancing times are balancing arecase 3001,min, min, and 200892 min, voltage and the initial 300 min, 150 min, and 200 min, respectively. The total voltage and the initial maximum different voltage maximum different voltage of the cells are approximate; the voltage difference of the modules is of the cellsdue areto approximate; the voltage modules different due to the of arrangement different the arrangement of alldifference cells. On of thethe order hand,isthe balancing time the circuit of all cells. order hand,of thethe balancing time ofofthe arrangement of the depends onOn thethe arrangement initial voltage allcircuit cells. depends In order on to the more clearly show the initial voltage of all cells. In order to more clearly show the reduction in the difference of the initial reduction in the difference of the initial and balanced state voltage, the initial and balanced state and balanced state theshown initialin and balanced voltages of cases 1, 2 and 3 are shown in voltages of cases 1, 2voltage, and 3 are Figure 14a–c,state respectively. Figure 14a–c, respectively.

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(a)

(b)

(c) Figure 14. Initial and balanced state voltages: (a) Case 1; (b) Case 2; (c) Case 3.

Figure 14. Initial and balanced state voltages: (a) Case 1; (b) Case 2; (c) Case 3. In this work, three different cases, in which initial maximum voltage difference and the total voltage of the three cells are approximate, tested to consider the correlation the voltage In this work, different cases, are in which initial maximum voltagebetween difference and the total difference of modules and balancing time in estimating the balancing time of the circuit. In case 1, voltage of the cells are approximate, are tested to consider the correlation between the voltage difference the voltage difference of two modules is 892 mV, the balancing time is 300 min, and each 1 mV

of modules and balancing time in estimating the balancing time of the circuit. In case 1, the voltage difference of two modules is 892 mV, the balancing time is 300 min, and each 1 mV difference needs a time of approximately 0.336 min for balancing. Similarly, cases 2 and 3 are 0.523 and 0.426 min, respectively. It shows that the voltage difference between the M1 and M2 increases, resulting in the

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difference needs a time of approximately 0.336 min for balancing. Similarly, cases 2 and 3 are 0.523 and Appl. Sci. 2018, 8, x FOR PEER REVIEW 17 of 22 0.426 min, respectively. It shows that the voltage difference between the M1 and M2 increases, resulting inneeds the increased balancing time. However, the balancing timecases will2 slightly difference a time of approximately 0.336 min for balancing. Similarly, and 3 aredecrease 0.523 andas 15. increased balancing time. However, the balancing time will slightly decrease as shown in Figure shown in Figure 15. It can be estimated the balancing time between in the range segment AC for other 0.426 min, respectively. It shows that the voltage difference the M 1 and M2 increases, It can be estimated the balancing time in the range segment AC for other experiments with the initial experiments the initialbalancing maximum voltage difference and the total of the cells like this resulting in with the increased time. However, the balancing timevoltage will slightly decrease as maximum voltage difference and the total voltage of the cells like this experiment. experiment. shown in Figure 15. It can be estimated the balancing time in the range segment AC for other experiments with the initial maximum voltage difference and the total voltage of the cells like this experiment.

Figure 15. The balancing time vs the voltage difference between two modules. Figure 15. The balancing time vs the voltage difference between two modules.

In order to evaluate a voltage drop of the cells after balancing, the total voltage of before and 15. The balancing timeof vs the the voltage difference between modules. In order toFigure evaluate a voltage drop cells after balancing, thetwo total voltage of before and after balancing can be evaluated by the percentage of the voltage drop, which be expressed expressedby by the after balancing can be evaluated by the percentage of the voltage drop, which can can be efficiency as follows. In order to follows. evaluate a voltage drop of the cells after balancing, the total voltage of before and the efficiency as after balancing can be evaluated by the percentage of the voltage drop, which can be expressed by ∑ N ∑ _ _ the efficiency as follows. ∑N j= =1 VCellj_start − ∑ j=1 VCellj_end × 100%, (21) ∑

ηV =

where ∑

V

and ∑

=

∑

× 100%,

_

∑ ∑N j=_ 1 VCellj_start

(21)

_

× 100%, are the total voltage of eight cells before and(21) after ∑

_ _ _ and ∑ N where ∑ N are the total voltage of j=the 1 VCellj_end j=1 VCellj_start balancing, respectively. N is number of the cells in the SCBS.

eight cells before and after balancing, where ∑N isV the number and ∑of the cells inare the total voltage of eight cells before and after respectively. the SCBS. _ _ From (21) and experimental data and results, the percentage of the voltage drop of three cases balancing, respectively. the number of the the SCBS. From and experimental data andThey results, theinpercentage therange voltage drop three can be(21) plotted as shownNinisFigure 16. arecells displayed in the of time from 0 to of 150 min cases (i.e., can From (21) and experimental data and results, the percentage of the voltage drop of three cases be plotted as shown inthe Figure They are displayed the time rangedrop from to 150 minis(i.e., balancing time of case 16. 2). The percentage of theinhighest voltage of0three cases 0.8%balancing and can be plotted as shown in Figure 16. They are displayed in the time range from 0 to 150 min (i.e., there a minor difference. Theof voltage drop isvoltage due to drop the transmission energy of the through time of the iscase 2). The percentage the highest of three cases is 0.8% andcells there is a minor balancing time of the case 2). The percentage of the highest voltage drop of three cases is 0.8% and the transformers, capacitor difference. The voltage drop isetc. due to the transmission energy of the cells through the transformers, there is a minor difference. The voltage drop is due to the transmission energy of the cells through capacitor etc. the transformers, capacitor etc.

Figure 16. The percentage of the voltage drop of three cases (0–2.5 h). Figure 16. The percentage of the voltage drop of three cases (0–2.5 h).

Figure 16. The percentage of the voltage drop of three cases (0–2.5 h).

5. Comparison with Conventional Balancing Methods Table 3 shows the comparison of the proposed topology and existing topologies in terms of a number of required components within the case of n cells connected in series. A buck-boost

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converter-based balancing topology [12,14,36–38] or switched capacitor converter-based balancing topology [11,15,22,26,27,30] can be constructed with a reasonable number of components. However, the energy transfer of these topologies is limited between two adjacent cells (i.e., cell to cell balancing) when a large number of cells in SCBS increases (i.e., approximately 80 or more cells) resulting in relatively slow balancing speed and low balancing efficiency. Moreover, the buck-boost converter-based balancers need intelligent control and a large number of the switches resulting in relative complex control scheme and high cost. In the MWT-based balancing topologies [10,17,18], the multi-winding transformer needs to be considered as the main drawback due to not only the stringent requirement for parameter matching for the turns ratio but also the requirement to change the number of secondary windings. These lead to a mismatch between the leakage inductance that caused a charge imbalance drawback by itself. The circuit used the voltage multiplier to balance the cells [34]. The voltage multiplier circuit used the diodes and the capacitors. Therefore, there is no leakage mismatch issue. However, this circuit depends on the impedance of the capacitors. If Reqi and V D are not unified, they will cause a residual voltage imbalance. Additionally, the larger capacity of the capacitor results in a longer balancing time. The balanced voltages of the cells indicate a reduction much lower than the average voltage of the cells. In this study, the proposed balancing topology is constructed with the existing transformer by increasing the number of the transformers. The proposed circuit consists of the MWTFC-based intra-module balances and SCC-based outer-module balancer in transferring the energy from a high to a low level of both the cells in each module and the modules simultaneously. In other words, the intraand outer-module balancers deal with N cells in each module and M modules, respectively. However, in the conventional methods, one MWTFC-based balancer or one SCC-based balancer should equalize N × M cells. Hence, the proposed circuit deals with a small number of the cells, so that a balancing time for one MWTFC-based balancer can be a significant reduction. When compared to conventional balancing topologies using the MWTFC or switched capacitor converter, the proposed balancing topology offers some advantages as follows.

•

• • •

Basically, the proposed balancing circuit has some advantages originally found in the MWTFC-based balancer and SCC-based balancer such as the repudiation of the voltage sensors for the feedback control loop, simple control scheme. The MWTFC-based balancer is applied to a small number of cells. Therefore, the problem of mismatched leakage inductance can be minimized. The voltage stress of switches is low by applying the SCC-based balancer to the outer-module balancer. The number of cells in series can be easily extended. Table 3. Comparison of the proposed and conventional topologies in a number of components. No. of the Components

Topology

Switch

D

C

L

Transformer

Switched capacitor converter

Basic SCC [22] Double-Tiered SCC [26] Single SCC [30] Quasi-Resonant SCC [38]

2n 2n n+5 2n

-

n−1 2n − 3 1 n−1

n−1

-

Buck-boost Converter

Basis topology [12] Cuk converter [36]

2(n − 1) 2(n − 1)

-

n−1

n−1 2(n − 1)

-

Multi-winding transformer

Flyback converter [18] Forward converter [17]

1 n

n 1

-

-

1 (n primary windings) 1 (n primary windings)

n + 2M

M

M−1

-

M (n/M primary windings) 1

Proposed topology 1

M: a number of the modules; M ≥ 2.

6. Conclusions A cell balancing topology in combination with MWTFC and SCC has been proposed and experimentally verified in the present paper. The balancing circuit based on the intra-module and

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outer-module balancers has several advantages and overcomes the disadvantages of two conventional circuits. In this balancing topology, the energy of cells in SCBS is transmitted from a high voltage level to a low voltage level in both the cells in each module and the modules by simultaneous intraand outer-module balancers. The main advantages of the proposed MCBC are the elimination of voltage sensors and the simple control scheme. Further, the proposed MCBC can easily extend the number of cells in SCBS. The experimental results for three different cases show that the voltages of all cells in SCBS achieved a balanced state. Furthermore, regardless of the various arrangements of the initial voltage, the voltages of the cells have achieved a balance state. In the balanced state, the energy efficiency of the proposed circuit can reach 83.31%. We have obtained satisfactory results proving that the proposed balancing circuit could be applied for SCBS in electric vehicles. Author Contributions: Conceptualization, T.M.B. and S.B.R.; Methodology, T.M.B. and S.B.R.; Software, T.M.B.; Validation, T.M.B. and S.B.R.; Formal Analysis, T.M.B. and S.B.R.; Investigation, T.M.B. and S.B.R.; Resources, T.M.B.; Data Curation, T.M.B.; Writing-Original Draft Preparation, T.M.B.; Writing-Review & Editing, T.M.B., C.-H.K., K.-H.K. and S.B.R.; Visualization, T.M.B., C.-H.K. and K.-H.K.; Supervision, S.B.R. Funding: This research received no external funding. Acknowledgments: The authors are extremely thankful to Smart Power System laboratory, Yeungnam University and Power & Energy System laboratory, Hanyang University for providing necessary information, materials, equipment, and guidance to pursue this research. We would like to express our gratitude to one and all who were directly or indirectly involved in the research results. Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature Cell C D i L S V Ts Subscripts aver cb cm ch dis ds eq gs m min PP TS Acronyms C D DSP EV HEV IC M

battery cell capacitance duty cycle current inductance MOSFET Balancing capacitor voltage switching period average balancing capacitor module capacitor charge discharge drain-source equivalent gate-source magnetic minimum transformer’s primary transformer’s secondary Capacitor Diode Digital Signal Processor electric vehicle Hybrid electric vehicle Integrated circuit Module

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MCBC MWT MWTFC NTC PWM SCBS SCC T

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Modular cell balancing circuit Multi-winding transformer MWT forward converter Negative Temperature Coefficient Pulse-Width-Modulated Series-connected Lithium-Ion battery string Switched capacitor circuit Transformer

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