# Energy Management of Parallel-Connected Cells in Electric Vehicles Based on Fuzzy Logic Control

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Simulink Model Parallel-Cell Pack

#### 2.1. First-Order RC Equivalent Circuit Model

_{m}represents the open circuit voltage; R

_{1}and C

_{1}represent the resistance and capacitance of polarization effect, respectively; and R

_{0}represents the ohmic resistance of battery. E

_{m}, R

_{1}, C

_{1}, and R

_{0}are functions of SOC and temperature. Specifically, these four elements can be obtained from the two-dimensional look-up tables:

_{0}= R

_{0}(SOC, T)

_{1}= R

_{1}(SOC, T)

_{1}= C

_{1}(SOC, T)

_{m}= E

_{m}(SOC, T)

_{0}) is calculated based on each measured open circuit voltage (OCV) reading using linear interpolation. SOC (t

_{0}) = f (OCV, T), and only when the dv/dt < 4 uV/s condition is satisfied are OCV readings taken; SOH is calculated using Formulas (6) and (7) according to [30]. In Formulas (6), SOC

_{1}and SOC

_{2}are the SOCs of the battery before and after discharging, respectively, and they are calculated only when the OCV is accurate enough.

#### 2.2. The Simulink Model for Parallel-Cell Pack

_{x}Mn

_{y}Co

_{z}O

_{2}(NMC) cells. Thermal influence was considered in the estimation process [21].

## 3. PCCEM Strategy Based on Fuzzy Control Logic

#### 3.1. Principle and Process of PCCEM Strategy

^{2}× R), and connecting all the cells into the circuit will reduce overall internal resistance. When the pack SOC (generally, the minimum value of cell SOC is considered as the pack SOC) is low, we should appropriately increase the number of cells in the circuit even in light load to reduce internal resistance and prevent voltage drop to reach the cut-off voltage early. Conversely, the ideal situation in the charging mode is to charge cells with lower SOC first, followed by cells with higher SOC, and finally, cells with the highest SOC. This order will increase the energy utilization rate. Thus, the cell’s status and load determines the accessibility of the cell to the circuit. The PCCEM system also can isolate poorly performing cells by turning off the corresponding switches. Then, the system will send a message to the owner.

#### 3.2. Fuzzy Logic Control Strategy

## 4. Discussion of Simulation Results

_{x}Mn

_{y}Co

_{z}O

_{2}chemistry) using the parameter identification method in [21].The initial distributions in SOH and capacity among cells in the PCCP are listed as Table 1.

_{v}, correction coefficient of rotating mass δ = 1.2, coefficient of air resistance C

_{D}= 0.3, and windward area A = 2.1 m

^{2}.

_{s}) can be calculated as:

_{s}= I

^{2}× R

_{0}+ I

_{1}

^{2}× R

_{1}

_{1}are the current through the resistors R and R

_{1}, respectively. The meaning of R

_{0}and R

_{1}are presented in Figure 3.

_{m}(t) is the voltage of the cell and i

_{m}(t) is the current flow through the cell.

_{s}. Then, adding the values yields dissipated energy.

^{−8}1/Ohm. The switching frequency was restricted to very small scales; thus, we calculated for the power consumed by the switches closed resistance and the current flow through it. The formula is:

_{s}= 0.597 kJ when the power consumed by the switches is taken into account. The PCCEM strategy reduces the dissipated energy from 2.145 to 1.168 kJ, which indicates that 45.5% is the cut off, rather than 73.3%. Therefore, the power consumed by the switches is not negligible in the experiment.

## 5. Conclusions and Prospects

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**Figure 2.**Operating principle of parallel-connected cell energy management (PCCEM): (

**a**) without PCCEM; and (

**b**) with PCCEM.

**Figure 7.**Membership functions of inputs. (

**a**) Membership of discharge current; and (

**b**) membership of battery SOC.

**Figure 8.**Decision surface of fuzzy logic controller. (

**a**) The membership of N; and (

**b**) the membership of SOC.

**Figure 9.**The profiles of speed and discharge current. (

**a**) Speed profile for the experiment; and (

**b**) discharge current profile of the packs.

**Figure 10.**Comparison of the SOCs. (

**a**) SOC of each cell without PCCEM; and (

**b**) SOC of each cell with PCCEM.

**Figure 11.**Comparison of loop currents. (

**a**) Current of each cell without PCCEM; and (

**b**) current of each cell with PCCEM.

**Figure 12.**Comparison of cell temperatures. (

**a**) Temperature of each cell without PCCEM; and (

**b**) temperature of each cell with PCCEM.

No. | Cell 1 | Cell 2 | Cell 3 | Cell 4 | Cell 5 | Cell 6 | Cell 7 | Cell 8 | |
---|---|---|---|---|---|---|---|---|---|

Parameters | |||||||||

Capacity/Ah | 28.0081 | 27.6250 | 27.6392 | 25.8622 | 26.8487 | 24.7735 | 25.0653 | 24.1322 | |

SOH/(%) | 90.34 | 89.11 | 89.15 | 83.42 | 86.60 | 79.91 | 80.85 | 77.84 | |

SOC/(%) | 100 | 95.57 | 88.33 | 84.71 | 70.91 | 73.85 | 71.04 | 66.61 |

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**MDPI and ACS Style**

Song, C.; Shao, Y.; Song, S.; Chang, C.; Zhou, F.; Peng, S.; Xiao, F.
Energy Management of Parallel-Connected Cells in Electric Vehicles Based on Fuzzy Logic Control. *Energies* **2017**, *10*, 404.
https://doi.org/10.3390/en10030404

**AMA Style**

Song C, Shao Y, Song S, Chang C, Zhou F, Peng S, Xiao F.
Energy Management of Parallel-Connected Cells in Electric Vehicles Based on Fuzzy Logic Control. *Energies*. 2017; 10(3):404.
https://doi.org/10.3390/en10030404

**Chicago/Turabian Style**

Song, Chuanxue, Yulong Shao, Shixin Song, Cheng Chang, Fang Zhou, Silun Peng, and Feng Xiao.
2017. "Energy Management of Parallel-Connected Cells in Electric Vehicles Based on Fuzzy Logic Control" *Energies* 10, no. 3: 404.
https://doi.org/10.3390/en10030404