Week 7 State of charge estimation

Objective:

• To explain the coulomb counting and to discuss the coulomb counting method for SOC estimation.

Procedure:

State of Charge Estimation:

SOC is one of the most important parameters for batteries, but its definition presents many different issues. In general, the SOC of a battery is defined as the ratio of its current capacity (Q(t)) to the nominal capacity (Qn). The nominal capacity is given by the manufacturer and represents the maximum amount of charge that can be stored in the battery. The SOC can be defined as follows:

SOC(t)=Q(t)/Qn

The various mathematical method of estimation are classified into the following four categories:

1. Direct measurement: This method uses physical battery properties, such as the voltage and impedance of the battery.
2. Book-keeping estimation: This method uses discharging current as of the input and integrates the discharging current overtime to calculate the SOC.
3. Adaptive systems: The adaptive systems are self-designing and can automatically adjust the SOC for different discharging conditions. Various new adaptive systems for SOC estimation have been developed.
4. Hybrid methods: The hybrid models benefit from the advantages of each SOC estimation method and allow a globally optimal estimation performance. The literature shows that hybrid methods generally produce a good estimation of SOC, compared to individual methods.

These 4 categories are further divided into various methods they are

Direct Measurement has 4 methods they are:

1. Open circuit voltage method
2. Terminal voltage method
3. Impedance method
4. Impedance spectroscopy method

The book-keeping estimation has 2 methods namely

1. Coulomb counting method
2. The modified Coulomb counting method

Adaptive Systems has 4 methods namely:

1. BP neural network
2. RBF neural network
3. Support vector machine
4. Fuzzy neural network
5. Kalman filter

Hybrid methods have 3 types namely:

1. Coulomb counting and EMF combination
2. Coulomb counting and Kalman filter combination
3. Per-unit system and EKF combination

The book-keeping estimation method uses battery discharging current data as input. This method permits to include some internal battery effects as self-discharge, capacity-loss and discharging efficiency. Two kinds of book-keeping estimation methods have been employed: The coulomb counting method and the modified Coulomb counting method.

Coulomb Counting Method:

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(t), which is estimated from the discharging current, I(t), and previously estimated SOC values SOC(t-1). SOC is calculated by the following equation:

`SOC(t)=SOC(t-1)+I(t)ndeltat`

where

SOC(t)=Battery state of charge at time instant 't'.

SOC(t-1)=Initial SOC=Battery state of charge at time instant 't-1'.

Qn= Nominal capacity of the battery(Ah).

Ic(t)= Battery current at the time instant 't' and

△t = time step.

But there are several factors that affect the accuracy of the Coulomb counting method including temperature, battery history, discharge current, and cycle life.

Modified Coulomb Counting Method:

To improve the Coulomb counting method, a new technique called the modified Coulomb counting method is proposed. The modified Coulomb counting method uses the corrected current to improve the accuracy of estimation.

The corrected current is the function of discharging current. There is a quadratic relationship between the corrected current and the discharging current of the battery. By the practice of experimental data, the corrected current is calculated by the following form:

IC(t)=K2I(t)2+K1I(t)+K0IC(t)=K2I(t)2+K1I(t)+K0

Where k2,k1andk0k2,k1andk0 are the constant values obtained from the practice experimental data.

In the modified Coulomb counting method, SOC is calculated by the following equation:

`SOC(t)=SOC(t-1)+Ic(t)*Qndeltat.`

The experimental results show that the accuracy of the modified Coulomb counting method is superior to the conventional Coulomb counting method.

Coulomb Counting method implementation inside a Battery management system Simulink model:

• From the BMS Closed-loop model, the above coulomb counting block can be viewed. The path for the coulomb counting block is: In the BMS CLosedloop model: BMS ECU(BMS_Software)>SOC_Estimation>Coulomb Counting).
• The two inputs to the system are Current and Temperature. The above blocks indicate the way Coulomb counting is implemented in a BMS.
• First the Current will be multiplied with time in seconds. The temperature is given as an input to the Lookup table having the value of capacity. The temperature is given so that we know the temperature of the battery in the corresponding capacity.
• Now as per the Formula the Current for the given time and Nominal capacity is divided. This result is given to the integration block where we integrate it and estimate the SOC.

Reference:

The BMS Closed Loop model is opened using this reference

https://skill-lync.com/knowledgebase/simulation-of-bms-model-in-simulink