State of charge estimation (SOC)

OBJECTIVE-1:

What is coulomb counting?

It is one of the methods in the estimation of SOC. The Coulomb counting method measures the charge & discharge current of a battery and integrates the charging & discharging current over time, to estimate SOC.

`SOC(t)=SOC(t0)-int((n(t)*I(t))/(Qn))*dt`

Qn - capacity of battery pack

SOC(t0) - previous value of SOC

n(t) - efficiency

I(t) - charge & discharge current

Coulomb counting method is not very accurate to estimate SOC of battery. There are several factors that affect the accuracy of the Coulomb counting method including temperature, battery history, discharge current, and cycle life.

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OBJECTIVE-2:

How BMS implements coulomb counting for SOC estimation?

Steps for coulomb counting in BMS implementation.

1. Develop the algorithm for the coulomb counting method
2. Transfer the algorithm to the BMS (microprocessor)

The BMS is the hardware in PCB consisting of a processor, many sensors, connectors, indication lights, etc. Actually, BMS has many responsibilities like battery balancing, SOC, SOH, DOD estimation, under & over-voltage protection, thermal protection, providing battery pack cooling, calculating duty cycle, etc.

Here our only aim is to implement the coulomb counting method in BMS. so develop the algorithm according to the battery voltage, capacity.

The initial value of SOC is mainly by the OCV method. Then based on the voltage level and charging mode the algorithm will execute.

So in our case of soc estimation, there will be a separate unit in the BMS for the estimation. Implement the algorithm in the hardware(processor), make the necessary connection to batteries and sensors, and analyze the result for charging and discharging.

reference journal paper

https://drive.google.com/file/d/1wVpZuxRfj1XcMAkdxQ9H9jF5asFidxJ1/view?usp=sharing

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