Week 7 State of charge estimation

AIM :-

• To simulate the Simulink model of a lithium ion battery management system for 3 test cases and to explain the results obtained.
• To explain coulomb counting and and it's use in estimating the state of charge of a battery pack.

1. SIMULATION OF A LITHIUM ION BATTERY MANAGEMENT SYSTEM :-

Battery management system (BMS) is an electronic control circuit that monitors and regulates the charging and discharging of batteries. The battery characteristics to be monitored include the detection of battery type, voltages, temperature, capacity, state of charge, power consumption, remaining operating time, charging cycles, and some more characteristics. The task of battery management system is to ensure the optimal use of the energy present in a battery. In order to avoid loading the batteries, the BMS system protects the batteries from deep discharging, from over-voltage, which are results of extreme fast charging and extreme high discharge current. In the case of multi-cell batteries, the battery management system also provides for cell balancing function, to manage that different battery cells have the same charging and discharging requirements.

The model studied is a BMS Simulink model by Mathworks and is shown below.

The BMS model is a combination of various subsystems. The model consists of 3 parts, the BMS ECU, BMS plant and the dashboard.

BMS ECU,

It is the brain of the BMS which calculates the charging and discharging current, the state of charge (SOC), cell balancing and the state machine which provides the control logic. The calculated values are passed through a bus which is then relayed to the BMS plant.

BMS State Machine,

This sub-system is the control logic of the BMS ECU. It consists of a flow chart which calculates the current and voltage during the different operating conditions being driving, charging and balancing. The flow chart of the state machine is given below.

BMS Plant,

This subsystem is the acutuator of the BMS. The battery pack subsystem is a combination of battery modules. There are two options, 1 module or 16 modules. 1 module is selected by default and it has 6 cells. The module subsystem provides details of parameters like cell voltage, cell temperature, pack voltage and pack current. The second subsystem is the current charging circuit which is activated during the charging operation. The output system is called charger_load where the battery charger and motor drive are the outputs and depending on driving or charging condition the required subsystem is selected.

BMS Dashboard,

It is the model that controls the input parameters of the BMS system. There are 3 operation conditions which are driving, charging and balancing. The LED lights are used to represent the output conditions where they turn green if all systems are good and turn red if there is a fault. The test sequence variant block has 3 test conditions. The BMS model is simulated according to these 3 cases.

Output:

Test Condition 1,

The simulation is run for 20000 seconds. For 3000 seconds the vehicle is in driving condition, from 3001 to 4000 seconds the BMS engages the balancing condition and finally from 4001 to 9000 seconds the BMS charges the battery and starts balancing again from 9001 to 20000 seconds. The dashboard and scope results are given below.

The LED lights indicate that there are no issues and the simulation ran without fault. The graphs plotted are cell voltages, pack current, cell temperature, state of charge, BMS state and balance command. The voltage and current plots shows the variation during the 3 conditions, while driving the current and voltage fluctuates according to the motor load and remains constant during balancing. During the charging phase the current and voltage increases and then stabilises during the balancing phase. The cell temperature rises during the charging and discharging phases and reduces during the balancing phase. The state of charge decreases during the driving phase, increases during the charging phase and remains constant during the balancing phase. Here 2 cells are combined to produce one plot line and hence there are only 3 plots for state of charge.

Test Condition 2,

In the second test condition there is only driving and charging phase where for 10000s the vehice is in driving phase and from 10001 to 20000s the vehicle is in charging phase. The dashboard and scope results are shown below.

The simulation is run successfully without any faults as shown by the LED lights. The current and voltage fluctuations indicates the driving phase and the increase in the current and voltages after the fluctuations indicate the charging phase. The cell temperatures increases during the driving and charging phases and the state of charge decreases during the two phases.

Test Condition 3,

This condition is only to study the cell balancing during the simulation. The dashboard and scope results are shown below.

The simulation is successfully run without faults. The current and voltage values decreases when the BMS is balancing the cells. The temperature also reduces in this phase and the state of charge increases to a 100% and continues to remain at 100% for the remainder of the duration.

2. WHAT IS COULOMB COUNTING? HOW IS IT USED TO DETERMINE THE ESTIMATION OF SOC :-

State of Charge (SOC) is a fundamental concept to understand the use of batteries, which is used to describe the remaining capacity in a battery. As the SOC is an important parameter which reflects the battery performance, accurate estimation of the SOC can not only protect the battery, prevent overdischarge, and improve the battery life but also allow the application to make rational control strategies to save energy. SOC is defined as the ratio of current capacity of a battery to it's normal capacity and represents the maximum amount of charge that can be stored. The equation of SOC is,

Where,

Qt = Current capacity.

Qn = Nominal capacity.

The Coulomb counting method measures the discharging current of a battery and integrates the discharging current over time in order to estimate SOC. Coulomb counting method is done to estimate the SOC which is estimated from the discharging current, and previously estimated SOC values. The equation to calculate the SOC using Coulomb Counting is given as,

Where,

SOC(t-1) = Previous SOC value.

It = Discharge current.

The sub-system that calculates the state of charge in the BMS model is given below,

The system uses current and temperature as the inputs where the current is passed through a gain block and the temperature is passed through a 1D look-up table. The current and temperature are then divided and integrated to obtain the state of charge.

CONCLUSION :-

• The Battery Management System Model by Simulink is simulated and the output is studied based on the given test conditions.
• Coulomb Counting method to estimate the state of charge is explained along with the simulink model in the BMS system.