Week 7 State of charge estimation

Question:

What is coulomb counting? How BMS implements coulomb counting for SOC estimation?

Answer:

SOC:

The state of charge (SOC) of a cell denotes the capacity that is currently available as a function of the rated capacity. The value of the SOC varies between 0% and 100%. If the SOC is 100%, then the cell is said to be fully charged, whereas a SOC of 0% indicates that the cell is completely discharged. In practical applications, the SOC is not allowed to go beyond 50% and therefore the cell is recharged when the SOC reaches 50%. Similarly, as a cell starts aging, the maximum SOC starts decreasing. This means that for an aged cell, a 100% SOC would be equivalent to a 75%–80% SOC of a new cell.

(i)

Coulomb counting:

The Coulomb counting method measures the discharging current of a battery and integrates the discharging current over time in order to estimate SOC [20]. Coulomb counting method is done to estimate the , which is estimated from the discharging current, , and previously estimated SOC values, . 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 columb 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 form:

where k1,k2 and k0 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.

I was tasked with implementing Coulomb counting in Simulink. To do so, I created a charging and discharging circuit for a battery. Simulink has a battery block that has various parameters. I set its nominal ratings and created the circuit with various switches. The cycling was controlled by pulse generators that coordinated with each other based on time. This circuit can be seen below.

The lower left half of the model shows the Coulomb counting circuit. The current is sent through an integrator block, which outputs the number of Coulombs flowing into or out of the battery. The output is sent through a gain 100/3600, which simply normalizes the output to a percentage for SOC. A constant “100” is added such that the SOC begins at 100% and drops towards 0%. The SOC can be seen in the second plot.

(ii)

The current and voltage is been taken as the inout, the capacity vakue will be changing with respect to the temperature and a constant curent is been flowing in the circuit, then the value of the capacity is been into the divider end and that is been integrating and the SOC is been obtained.