# A Modular Cell Balancer Based on Multi-Winding Transformer and Switched-Capacitor Circuits for a Series-Connected Battery String in Electric Vehicles

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## Abstract

**:**

## 1. Introduction

## 2. Proposed Modular Cell Balancing Circuit

#### 2.1. Analysis of the Previous Cell Balancing Circuits

#### 2.1.1. Active Cell Balancing Circuit Using the MWT

_{k}(k = 1, …, 4). A diode (D) on the secondary side is used to reset the magnetizing inductance of the transformer. The MWTFC-based balancer equalizes the voltage of cells in the SCBS by selectively turning the switches S

_{k}on or off according to the constant duty cycle (D) simultaneously.

_{s}) of the circuit and the operation waveforms are shown in Figure 2b,c and Figure 3, respectively. In order to simply analyze the operating modes for the circuit, it is assumed that this circuit includes four series-connected cells; The relationship among the voltage of the battery cells is assumed as follows: V

_{cell1}> V

_{cell2}> V

_{aver}> V

_{cell3}> V

_{cell4}, the sum voltage is V

_{Total}= V

_{cell1}+ V

_{cell2}+ V

_{cell3}+ V

_{cell4}, and the average voltage is V

_{aver}where V

_{cell1}, V

_{cell2}, V

_{cell3}, and V

_{cell4}represent the voltage across Cell

_{1}, Cell

_{2}, Cell

_{3}, and Cell

_{4}, respectively. The operating modes of the circuit are presented as follows:

_{0}, t

_{1}] (see Figure 2b): At t

_{0}, four switches (S

_{1}, S

_{2}, S

_{3}, and S

_{4}) are turned on simultaneously as shown in Figure 3. In this mode, the energy is transferred from high voltage level cells (i.e., Cell

_{1}, Cell

_{2}) to the low one (i.e., Cell

_{3}, Cell

_{4}) through the transformer T.

_{m}, i

_{Lm}, and cell currents i

_{1}, i

_{2}, i

_{3}and i

_{4}, and voltage across L

_{m}, V

_{Lm}, can be expressed as Equations (2) and (3):

_{Lm}= i

_{1}+ i

_{2}+ i

_{3}+ i

_{4},

_{1}, i

_{2}, i

_{3}and i

_{4}are the currents of four switches of S

_{1}, S

_{2}, S

_{3}and S

_{4}, respectively.

_{TP}can be expressed as follows:

_{Cell1}and V

_{Cell2}are higher than V

_{aver}, V

_{Cell3}, and V

_{Cell4}. Therefore, currents i

_{1}, i

_{2}flow from the cells (i.e., Cell

_{1}and Cell

_{2}) to the transformer T, and i

_{3}and i

_{4}flow from T to the battery cells (i.e., Cell

_{3}and Cell

_{4}). This means that the electric charges in Cell

_{1}, Cell

_{2}are transferred to Cell

_{3}, Cell

_{4}through T.

_{1}, t

_{2}] (see Figure 2c): At t

_{1}, four switches are turned off simultaneously, as shown in Figure 3. The voltage across the L

_{m}becomes negative, V

_{Lm}. Therefore, the magnetizing inductance is reset through the diode, resulting in a decrease in the i

_{Lm}to zero level.

_{2}, t

_{3}]: Mode 3 starts when the i

_{Lm}is equal to zero. No current flows in the MWT from t

_{2}to t

_{3}. At t

_{3}, this mode is completed and changes to mode 1 occur in the next switching period.

#### 2.1.2. Cell Balancing Circuit Using SCC

_{Cell}

_{1}> V

_{Cell}

_{2}. When two of four switches (S

_{a1}and S

_{a2}or S

_{b1}and S

_{b2}) in the balancing circuit are turned on or off, the energy is transferred from a cell to another cell via a balancing capacitor (C

_{b}). The switches are controlled by pulse-width-modulated (PWM) signals with fixed duty cycle (50% T

_{s}), as shown in Figure 6. The analysis of the circuit is based on a proposition of voltage of Cell

_{1}and Cell

_{2}, as follows:

_{0}, t

_{1}] (See Figure 5b): At t

_{0}, S

_{a1}and S

_{a2}are turned on, while S

_{b1}and S

_{b2}are turned off simultaneously. In this mode, the switches S

_{a1}and S

_{a2}conduct. Thus, Cell

_{1}is connected in parallel with C

_{b}, as shown in Figure 5b. C

_{b}starts to be charged by Cell

_{1}. The voltage across C

_{b}(${v}_{Cb}$) starts increasing, and current flows of C

_{b}(i

_{Cb}) starts decreasing, as shown in Figure 6. The instantaneous voltage and current of the balancing capacitor in this mode can be expressed [30] by Equations (10) and (11)

_{Ch}is the sum of the equivalent series resistance of balancing capacitor R

_{ESR}, the turn-on resistance of two switches S

_{a1}, S

_{a2}.

_{1}, t

_{2}] (See Figure 5c): At t

_{1}, S

_{a1}and S

_{a2}are turned off, and S

_{b1}and S

_{b2}are turned on simultaneously. In this mode, the switches S

_{b1}and S

_{b2}conduct. Thus, Cell

_{2}is connected in parallel with C

_{b}, as shown in Figure 5c. The energy of C

_{b}starts to be discharged to Cell

_{2}. The current of C

_{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).

_{dis}is the total of the equivalent series resistance of balancing capacitor R

_{ESR}, and the turn-on resistance of two switches S

_{b1}and S

_{b2}

#### 2.2. Proposed Modular Cell Balancing Circuit

#### 2.2.1. Modular Cell Balancing Concept

#### 2.2.2. Proposed Modular Cell Balancing Circuit

_{1}, M

_{2}) and each module contains N cells in series-connection in which each cell in the SCBS connects the switch S

_{k}and the primary side winding of the MWTs in intra-module balancers. Each MWT has a secondary side winding in connection with one diode to reset the magnetizing inductance of transformers (L

_{m}) 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 (C

_{m}) is to balance the module voltages.

_{m}is used to equalize the energy between the modules through the outer-module balancer. It is controlled by the switches S

_{a1}, S

_{a2}and S

_{b1}, S

_{b2}which are controlled by 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.

_{1}), respectively. In order to simply analyze operating modes for the circuit, the following assumptions are made:

- All the switches, capacitor, diodes and transformer are ideal.
- Each module contains four cells.
- The relationship among voltage of the battery cells is arranged in decreasing order from Cell
_{11}(V_{cell11}) to Cell_{24}(V_{cell24}), and V_{cell11}> V_{cell12}> V_{aver1}> V_{cell13}> V_{cell14}> V_{cell21}> V_{cell22}> V_{aver2}> V_{cell23}> V_{cell24}(the module 1 voltage: V_{M1}= V_{Cell11}+ V_{Cell12}+ V_{Cell13}+ V_{Cell14}; the average voltage of M_{1}: V_{aver1}= V_{M1}/4; the module 2 voltage: V_{M2}= V_{Cell21}+ V_{Cell22}+ V_{Cell23}+ V_{Cell24}; the average voltage of M_{2}: V_{aver2}= V_{M2}/4; V_{M1}> V_{M2}).

_{s}) of the proposed MCBC. These modes based on the switching states of the primary side switches (S

_{11}, S

_{12}, S

_{13}, S

_{14}, S

_{21}, S

_{22}, S

_{23}, S

_{11}, and S

_{24}) in the intra-module balancer and the switches (S

_{a1}, S

_{a2}, and S

_{b1}, S

_{b2}) in the outer-module balancer. The theoretical waveforms and operating modes of the proposed circuit are shown in Figure 9 and Figure 10, respectively.

_{0}, t

_{1}] (see Figure 10a): At t

_{1}, eight switches in intra-module balancers (module 1: S

_{11}, S

_{12}, S

_{13}, S

_{14}; module 2: S

_{21}, S

_{22}, S

_{23}, S

_{24}) and S

_{a1}, S

_{a2}are turned on, while S

_{b1}, S

_{b2}are turned off simultaneously, as shown in Figure 9. In this mode, the energy is transferred from a high voltage level cell to a low level one inside each module through two MWTs (T

_{1}) and (T

_{2}), respectively.

_{Cell11}, V

_{Cell12}are higher than V

_{aver1}, V

_{Cell13}, V

_{Cell14}in M

_{1}, and V

_{Cell21}, V

_{Cell22}are higher than V

_{aver2}, V

_{Cell23}, V

_{Cell24}in M

_{2}. Therefore, i

_{11}, i

_{12}and i

_{21}, i

_{22}flow from cells to T

_{1}, T

_{2}, respectively, and i

_{13}, i

_{14}and i

_{23}, i

_{24}flow from T

_{1}, T

_{2}to the battery cells. This means that, in module 1, the electric charges in Cell

_{11}, Cell

_{12}are transmitted to Cell

_{13}, Cell

_{14}, and in module 2, the electric charges in Cell

_{21}, Cell

_{22}are transmitted to Cell

_{23}, Cell

_{23}.

_{a1}and S

_{a2}conduct, so the C

_{m}will be charged by the V

_{M1}through the S

_{a1}and S

_{a2}so that the V

_{Cm}increases gradually to a steady state period. The instantaneous voltage and current of the balancing capacitor in this mode can be given by Equations (14) and (15).

_{1}, t

_{2}] (see Figure 10b): At t

_{1}, eight switches in the intra-modules balances are turned off simultaneously, as shown in Figure 9. The voltages across L

_{m1}and L

_{m2}are negative as expressed by Equations (16) and (17). The diodes D

_{1}and D

_{2}conduct. Then, V

_{D1}, V

_{D2}become zero because D

_{1}, D

_{2}are ideal devices. The L

_{m1}and L

_{m2}are reset through the secondary diodes D

_{1}, D

_{2}resulting in a decrease of the magnetizing currents of transformers 1 and 2 (i

_{Lm1}and i

_{Lm2}).

_{2}, t

_{3}] (see Figure 10c): Mode 3 starts when the currents through the magnetizing inductance of two transformers (i

_{Lm1}) and (i

_{Lm2}) are equal to zero. No current flows in the MWTs from t

_{2}to t

_{3}.

_{3}, t

_{4}] (see Figure 10d): At t

_{3}, the S

_{a1}, S

_{a2}are turned off, and the S

_{b1}, S

_{b2}starts to be turned on as shown in Figure 9. The voltage V

_{Cm}is higher than the V

_{M}

_{2}such that the energy of C

_{m}is in a discharging condition. Module 2 receives stored energy in C

_{m}gradually. The instantaneous voltage and current of the module balancing capacitor in this mode are respectively given in Equations (18) and (19). At t

_{4}, this operating mode is finished and returns to mode 1 in the next switching period.

## 3. Experimental Setup

_{m}, 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 S

_{11}, S

_{24}, S

_{a1}, and S

_{b1}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 (D

_{1}) of the signal to switches S

_{a1}, S

_{b1}is 50% of one switching period where the S

_{a1}and S

_{b1}are complementary, and switching frequency of f and f

_{1}are set for 40 kHz.

_{11}(i.e., V

_{cell11}) to Cell

_{24}(i.e., V

_{cell24}); Case 2: V

_{cell11}> V

_{cell21}> V

_{cell12}> V

_{cell22}> V

_{cell13}> V

_{cell23}> V

_{cell14}> V

_{cell24}, and case 3: the initial voltage of the cells is set randomly, as shown in detail in Table 2.

## 4. Results and Discussion

_{cell}is the capacitance of the battery cell; V

_{celli_start}and V

_{celli_end}are the cell voltage of the ith cell before and after balancing, respectively; V

_{min}and N are the initial lowest voltage and number of cells, respectively.

_{lk13}of the transformer was larger than others; hence, the current i

_{13}was smaller. In other words, the balance of the Cell

_{13}was slower than others, as shown in Figure 13a.

_{1}and M

_{2}, like the SCC-based balancer theory in Section 2.1. In detail, the initial voltage differences between the M

_{1}and M

_{2}for case 1, case 2, and case 3 are 892 mV, 287 mV, and 381 mV, the balancing times are 300 min, 150 min, and 200 min, respectively. The total voltage and the initial maximum different voltage of the cells are approximate; the voltage difference of the modules is different due to the arrangement of all cells. On the order hand, the balancing time of the circuit depends on the arrangement of the initial voltage of all cells. In order to more clearly show the reduction in the difference of the initial and balanced state voltage, the initial and balanced state voltages of cases 1, 2 and 3 are shown in Figure 14a–c, respectively.

_{1}and M

_{2}increases, resulting in the increased balancing time. However, the balancing time will slightly decrease as 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.

## 5. Comparison with Conventional Balancing Methods

_{eqi}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.

- 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.

## 6. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Nomenclature

Cell | battery cell |

C | capacitance |

D | duty cycle |

i | current |

L | inductance |

S | MOSFET |

V | Balancing capacitor voltage |

T_{s} | switching period |

Subscripts | |

aver | average |

cb | balancing capacitor |

cm | module capacitor |

ch | charge |

dis | discharge |

ds | drain-source |

eq | equivalent |

gs | gate-source |

m | magnetic |

min | minimum |

PP | transformer’s primary |

TS | transformer’s secondary |

Acronyms | |

C | Capacitor |

D | Diode |

DSP | Digital Signal Processor |

EV | electric vehicle |

HEV | Hybrid electric vehicle |

IC | Integrated circuit |

M | Module |

MCBC | Modular cell balancing circuit |

MWT | Multi-winding transformer |

MWTFC | MWT forward converter |

NTC | Negative Temperature Coefficient |

PWM | Pulse-Width-Modulated |

SCBS | Series-connected Lithium-Ion battery string |

SCC | Switched capacitor circuit |

T | Transformer |

## References

- Kim, C.H.; Kim, M.Y.; Moon, G.W. A modularized charge equalizer using a battery monitoring ic for series-connected li-ion battery strings in electric vehicles. IEEE Trans. Power Electron.
**2013**, 28, 3779–3787. [Google Scholar] [CrossRef] - Kim, C.; Member, S.; Kim, M.; Member, S.; Park, H.; Member, A.; Moon, G. A Modularized Two-Stage Charge Equalizer With Cell Selection Switches for Series-Connected Lithium-Ion Battery String in an HEV. IEEE Trans. Power Electron.
**2012**, 27, 3764–3774. [Google Scholar] [CrossRef] - Park, H.; Member, S.; Kim, C.; Kim, C.; Member, S.; Moon, G.; Lee, J. A Modularized Charge Equalizer for an HEV Lithium-Ion Battery String. IEEE Trans. Ind. Electron.
**2009**, 56, 1464–1476. [Google Scholar] [CrossRef] - Allègre, A.L.; Bouscayrol, A.; Delarue, P.; Barrade, P.; Chattot, E.; El-Fassi, S. Energy storage system with supercapacitor for an innovative subway. IEEE Trans. Ind. Electron.
**2010**, 57, 4001–4012. [Google Scholar] [CrossRef] - Panchal, S.; Mathew, M.; Dincer, I.; Agelin-Chaab, M.; Fraser, R.; Fowler, M. Thermal and electrical performance assessments of lithium-ion battery modules for an electric vehicle under actual drive cycles. Electr. Power Syst. Res.
**2018**, 163, 18–27. [Google Scholar] [CrossRef] - Panchal, S.; Dincer, I.; Agelin-Chaab, M.; Fowler, M.; Fraser, R. Uneven temperature and voltage distributions due to rapid discharge rates and different boundary conditions for series-connected LiFePO4batteries. Int. Commun. Heat Mass Transf.
**2017**, 81, 210–217. [Google Scholar] [CrossRef] - Lee, Y.; Jeon, S.; Lee, H.; Bae, S. Comparison on cell balancing methods for energy storage applications. Indian J. Sci. Technol.
**2016**, 9. [Google Scholar] [CrossRef] - Daowd, M.; Omar, N.; van den Bossche, P.; van Mierlo, J. A review of passive and active battery balancing based on MATLAB/Simulink. Int. Rev. Electr. Eng.
**2011**, 6, 2974–2989. [Google Scholar] [CrossRef] - Bentley, W.F. Cell balancing considerations for lithium-ion battery systems. Twelfth Annu. Batter. Conf. Appl. Adv.
**1997**, 223–226, 223–226. [Google Scholar] [CrossRef] - Bui, T.; Bae, S. Active Clamped Forward based Active Cell Balancing Converter. Indian J. Sci. Technol.
**2015**, 8, 1–6. [Google Scholar] [CrossRef] - Ye, Y.; Cheng, K.W.E. An automatic switched-capacitor cell balancing circuit for series-connected battery strings. Energies
**2016**, 9, 138. [Google Scholar] [CrossRef] - Wu, T.H.; Moo, C.S.; Hou, C.H. A battery power bank with series-connected buck-boost-type battery power modules. Energies
**2017**, 10, 650. [Google Scholar] [CrossRef] - Imtiaz, A.M.; Khan, F.H. “Time shared flyback converter” based regenerative cell balancing technique for series connected li-ion battery strings. IEEE Trans. Power Electron.
**2013**, 28, 5960–5975. [Google Scholar] [CrossRef] - Zeltser, I.; Kirshenboim, O.; Dahan, N.; Peretz, M.M. ZCS resonant converter based parallel balancing of serially connected batteries string. In Proceedings of the 2016 IEEE Applied Power Electronics Conference and Exposition (APEC), Long Beach, CA, USA, 20–24 March 2016. [Google Scholar]
- Pascual, C.; Krein, P.T. Switched capacitor system for automatic series battery equalization. In Proceedings of the APEC 97—Applied Power Electronics Conference, Atlanta, GA, USA, 27 February 1997. [Google Scholar]
- Kim, M.Y.; Kim, C.H.; Kim, J.H.; Moon, G.W. A chain structure of switched capacitor for improved cell balancing speed of lithium-ion batteries. IEEE Trans. Ind. Electron.
**2014**, 61, 3989–3999. [Google Scholar] [CrossRef] - Jeon, S.; Kim, M.; Bae, S. Analysis of a symmetric active cell balancer with a multi-winding transformer. J. Electr. Eng. Technol.
**2017**, 12, 1812–1820. [Google Scholar] [CrossRef] - Kim, J.W.; Ha, J.I. Cell balancing method in flyback converter without cell selection switch of multi-winding transformer. J. Electr. Eng. Technol.
**2016**, 11, 367–376. [Google Scholar] [CrossRef] - Stuart, T.; Zhu, W. Fast equalization for large lithium ion batteries. IEEE Aerosp. Electron. Syst. Mag.
**2009**, 24, 27–31. [Google Scholar] [CrossRef] [Green Version] - Kim, J.; Ha, J. Cell Balancing Control of Single Switch Flyback Converter Using Generalized Filters. In Proceedings of the 29th Annual IEEE Applied Power Electronics Conference and Exposition–APEC 2014, Fort Worth, TX, USA, 16–20 March 2014. [Google Scholar]
- Hoque, M.M.; Hannan, M.A.; Mohamed, A.; Ayob, A. Battery charge equalization controller in electric vehicle applications: A review. Renew. Sustain. Energy Rev.
**2017**, 75, 1363–1385. [Google Scholar] [CrossRef] - Ling, R.; Wang, L.; Huang, X.; Dan, Q.; Zhang, J. A review of equalization topologies for lithium-ion battery packs. In Proceedings of the 34th Chinese Control Conference, Hanzhou, China, 28–30 July 2015. [Google Scholar]
- Caspar, M.; Eiler, T.; Hohmann, S. Comparison of active battery balancing systems. In Proceedings of the 2014 IEEE Vehicle Power and Propulsion Conference (VPPC), Coimbra, Portugal, 27–30 October 2014. [Google Scholar]
- Shang, Y.; Xia, B.; Zhang, C.; Cui, N.; Yang, J.; Mi, C.C. An Automatic Equalizer Based on Forward-Flyback Converter for Series-Connected Battery Strings. IEEE Trans. Ind. Electron.
**2017**, 64, 5380–5391. [Google Scholar] [CrossRef] - Li, S.; Mi, C.C.; Zhang, M. A high-efficiency active battery-balancing circuit using multiwinding transformer. IEEE Trans. Ind. Appl.
**2013**, 49, 198–207. [Google Scholar] [CrossRef] - Cao, J.; Schofield, N.; Emadi, A. Battery balancing methods: A comprehensive review. In Proceedings of the 2008 IEEE Vehicle Power and Propulsion Conference, Harbin, China, 3–5 September 2008. [Google Scholar]
- Hua, C.C.; Fang, Y.H. A switched capacitor charge equalizer with cancellation mechanism of alternating current. In Proceedings of the 2015 IEEE 2nd International Future Energy Electronics Conference (IFEEC), Taipei, Taiwan, 1–4 November 2015. [Google Scholar]
- Lim, C.S.; Lee, K.J.; Ku, N.J.; Hyun, D.S.; Kim, R.Y. A modularized equalization method based on magnetizing energy for a series-connected lithium-ion battery string. IEEE Trans. Power Electron.
**2014**, 29, 1791–1799. [Google Scholar] [CrossRef] - Kanade, S.A.; Puri, V. Electrical properties of thick-film NTC thermistor composed of Ni0.8Co0.2Mn2O4ceramic: Effect of inorganic oxide binder. Mater. Res. Bull.
**2008**, 43, 819–824. [Google Scholar] [CrossRef] - Daowd, M.; Antoine, M.; Omar, N.; van den Bossche, P.; van Mierlo, J. Single switched capacitor battery balancing system enhancements. Energies
**2013**, 6, 2149–2179. [Google Scholar] [CrossRef] - Ji, X.; Cui, N.; Shang, Y.; Zhang, C.; Sun, B. Modularized charge equalizer using multiwinding transformers for Lithium-ion battery system. In Proceedings of the 2014 IEEE Conference and Expo Transportation Electrification Asia-Pacific (ITEC Asia-Pacific), Beijing, China, 31 August–3 September 2014; pp. 1–5. [Google Scholar] [CrossRef]
- Chen, H.; Zhang, L.; Han, Y. System-theoretic analysis of a class of battery equalization systems: Mathematical modeling and performance evaluation. IEEE Trans. Veh. Technol.
**2015**, 64, 1445–1457. [Google Scholar] [CrossRef] - Zhang, Z.; Gui, H.; Gu, D.J.; Yang, Y.; Ren, X. A hierarchical active balancing architecture for lithium-ion batteries. IEEE Trans. Power Electron.
**2017**, 32, 2757–2768. [Google Scholar] [CrossRef] - Park, H.-S.; Kim, C.-H.; Park, K.-B.; Moon, G.-W.; Lee, J.-H. Design of a charge equalizer based on battery Modularization. IEEE Trans. Veh. Technol.
**2009**, 58, 3216–3223. [Google Scholar] [CrossRef] - Uno, M.; Kukita, A. Modular equalization architecture using inter-module and switchless intra-module equalizer for energy storage system. In Proceedings of the 17th European Conference on Power Electronics and Applications, Geneva, Switzerland, 8–10 September 2015. [Google Scholar]
- Ling, R.; Dan, Q.; Wang, L.; Li, D. Energy bus-based equalization scheme with bi-directional isolated Cuk equalizer for series connected battery strings. In Proceedings of the 2015 IEEE Applied Power Electronics Conference and Exposition (APEC), Charlotte, NC, USA, 15–19 March 2015. [Google Scholar]
- Park, S.H.; Kim, T.S.; Park, J.S.; Moon, G.W.; Yoon, M.J. A new battery equalizer based on buck-boost topology. In Proceedings of the 7th Internatonal Conference on Power Electronics, ICPE’07, Daegu, Korea, 22–26 October 2007; pp. 962–965. [Google Scholar]
- Lee, Y.S.; Cheng, G.T. Quasi-resonant zero-current-switching bidirectional converter for battery equalization applications. IEEE Trans. Power Electron.
**2006**, 21, 1213–1224. [Google Scholar] [CrossRef]

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

**Figure 2.**The MWTFC based on the balancing circuit [21]: (

**a**) Circuit diagram; (

**b**) Mode 1; (

**c**) Mode 2.

**Figure 5.**The switched capacitor circuit for two cells [21]: (

**a**) Circuit description; (

**b**) Mode 1; (

**c**) Mode 2.

**Figure 7.**Modular circuit balancing concept: (

**a**) Conventional cell balancer; (

**b**) Modular cell balancer; (

**c**) The modular cell balancer based on outer-module.

Parameter | Value | ||
---|---|---|---|

Cell balancing circuit | Twelve MOSFETs | IPP023N10N5 | |

Two diodes | DO-204AC (DO-15) | ||

Two Transformers (Four primary/One secondary windings) | Core: EER2828N | ||

N_{1}:N_{2} = 1:1 | |||

L_{m1} = 1.88 mH; L_{m2} = 1.83 mH | |||

L_{lk}_{11} = 1.87 µH; L_{lk}_{12} = 1.95 µH;L _{lk}_{13} = 3.39 µH; L_{lk}_{14} = 1.94 µH; L _{lk}_{21} = 2.30 µH; L_{lk}_{22} = 1.99 µH; L _{lk}_{23} = 2.00 µH; L_{lk}_{24} = 1.98 µH; | |||

Balancing capacitor | 700 µF | ||

Gate driver | UCC27519A-Q1 | ||

Battery string | Eight Lithium-ion cells | Nominal capacity | 700 mAh |

Nominal voltage | 3.7 V | ||

Weight | 18 g | ||

Cell voltage recorder | Series-connected cell string recorder | YOKOGAWA-GP10 | |

Controller | Switching frequency | 40 kHz | |

Digital controller | TMS320F28335 |

Case 1 | Case 2 | Case 3 | ||||
---|---|---|---|---|---|---|

Battery Cells | Initial Cell Voltages [V] | Module Voltage [V] | Initial Cell Voltages [V] | Module Voltage [V] | Initial Cell Voltages [V] | Module Voltage [V] |

Cell_{11} | 3.880 | V_{M1} = 15.183 | 3.896 | V_{M1} = 14.862 | 3.871 | V_{M1} = 14.906 |

Cell_{12} | 3.857 | 3.773 | 3.585 | |||

Cell_{13} | 3.745 | 3.666 | 3.661 | |||

Cell_{14} | 3.701 | 3.527 | 3.789 | |||

Cell_{21} | 3.652 | V_{M2} = 14.291 | 3.816 | V_{M2} = 14.575 | 3.744 | V_{M2} = 14.437 |

Cell_{22} | 3.604 | 3.695 | 3.529 | |||

Cell_{23} | 3.543 | 3.574 | 3.674 | |||

Cell_{24} | 3.492 | 3.490 | 3.490 | |||

V_{∑Cell} [V] | 29.474 | 29.437 | 29.343 |

Topology | No. of the Components | |||||
---|---|---|---|---|---|---|

Switch | D | C | L | Transformer | ||

Switched capacitor converter | Basic SCC [22] | 2n | - | n − 1 | - | - |

Double-Tiered SCC [26] | 2n | - | 2n − 3 | - | - | |

Single SCC [30] | n + 5 | - | 1 | - | - | |

Quasi-Resonant SCC [38] | 2n | - | n − 1 | n − 1 | - | |

Buck-boost Converter | Basis topology [12] | 2(n − 1) | - | - | n − 1 | - |

Cuk converter [36] | 2(n − 1) | - | n − 1 | 2(n − 1) | - | |

Multi-winding transformer | Flyback converter [18] | 1 | n | - | - | 1 (n primary windings) |

Forward converter [17] | n | 1 | - | - | 1 (n primary windings) | |

Proposed topology | n + 2M | M | M − 1 | - | M (n/M primary windings) ^{1} |

^{1}M: a number of the modules; M ≥ 2.

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## Share and Cite

**MDPI and ACS Style**

Bui, T.M.; Kim, C.-H.; Kim, K.-H.; Rhee, S.B.
A Modular Cell Balancer Based on Multi-Winding Transformer and Switched-Capacitor Circuits for a Series-Connected Battery String in Electric Vehicles. *Appl. Sci.* **2018**, *8*, 1278.
https://doi.org/10.3390/app8081278

**AMA Style**

Bui TM, Kim C-H, Kim K-H, Rhee SB.
A Modular Cell Balancer Based on Multi-Winding Transformer and Switched-Capacitor Circuits for a Series-Connected Battery String in Electric Vehicles. *Applied Sciences*. 2018; 8(8):1278.
https://doi.org/10.3390/app8081278

**Chicago/Turabian Style**

Bui, Thuc Minh, Chang-Hwan Kim, Kyu-Ho Kim, and Sang Bong Rhee.
2018. "A Modular Cell Balancer Based on Multi-Winding Transformer and Switched-Capacitor Circuits for a Series-Connected Battery String in Electric Vehicles" *Applied Sciences* 8, no. 8: 1278.
https://doi.org/10.3390/app8081278