State of charge estimation

(1)

coulomb counting: -

To know about the current state of charge (SOC) of a battery, it is important to know about the initial charge and charge taken out of battery.

For that we use coulomb counting technique to measure the removing current and integrating together all the removed current from battery over a period.

To determine available capacity, coulomb counting relies on the integration of the current drawn from and supplied to a battery over time as given by

where I is the current and η is the efficiency during charge and discharge operations. Coulomb counting is advantageous in terms of a simple structure and its ease of implementation. However, some problems make it difficult to accurately estimate battery SOC by coulomb counting. But there are several factors that affect the accuracy of Coulomb counting method including temperature, battery history, discharge current, and cycle life. Sensor accuracy is also important because accumulated errors can lead to a drift in the estimated battery SOC. Furthermore, one of the main problems in coulomb counting is that it is necessary to calibrate the initial battery SOC.

(2)

There are various methods to calculating the SOC of battery. Here we are taking the Book-keeping estimation method, which 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:

1) Coulomb counting method, and

2) 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, which is estimated from the discharging current, I(t), and previously estimated SOC values, SOC(t-1).

SOC is calculated by the following equation:

But there are several factors that affect the accuracy of 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 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 discharging current of battery. By practice of experimental data, corrected current is calculated by the following from:

Where`k_2,k_1 ,and k_0` are constant values obtained from the practice experimental data.

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

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

Reference:-

https://www.hindawi.com/journals/isrn/2013/953792/