State of Charge (SoC) refers to the amount of energy stored within a EV battery at any given time. It is expressed in percentage. For electric vehicles, this metric is akin to the fuel gauge in a conventional internal combustion engine vehicle, indicating how much energy is available for propulsion.

Maximum Power Vs SoC

Why SoC Matters for Traction Batteries

The State of Charge plays a pivotal role in the performance, efficiency, and longevity of traction batteries in electric vehicles. This information is vital for drivers to plan their trips effectively, especially considering that electric vehicles have a limited range compared to conventional internal combustion engine vehicles. 

An accurate representation of SoC allows drivers to confidently reach their destinations without encountering unexpected battery depletion.

• Range Estimation: SoC is directly linked to the range an electric vehicle can travel on a single charge. A higher SoC implies more available energy, allowing for longer distances to be covered. Conversely, a lower SoC restricts the available range.
• Performance: Traction batteries tend to exhibit their best performance at certain levels of SoC. Operating the battery within an optimal SoC range ensures efficient power delivery and acceleration, providing a satisfying driving experience.
• Battery Health: Keeping a battery consistently at extremely low or high SoC levels can lead to degradation over time. Overcharging or deep discharging, particularly in extreme temperatures, can accelerate the aging process of the battery and reduce its capacity.
• Safety: Maintaining an awareness of SoC is essential for ensuring battery safety. Operating at very low SoC levels can lead to complete discharge, potentially damaging the battery cells and even causing permanent failure.

Measuring and Monitoring SoC

Several methods are used to measure and monitor the State of Charge of traction batteries:

Coulomb Counting: This method involves tracking the flow of electric current into and out of the battery. By integrating the current over time, it's possible to estimate the energy that has been added or removed, giving an approximation of the SoC.

SoC Estimation by Coulomb Counting

The SoC is represented by the following equation

where:

SoC(t) = estimated State of Charge at time, t

SoC(t-1) = previous State of Charge at time t-1

I(t) = charging or discharging current at time, t

Qn = battery cell capacity

Δt = time step between t-1 and t

Open Circuit Voltage (OCV): OCV is the voltage across the battery terminals when no current is flowing. This voltage is related to the SoC, allowing for estimation through voltage measurements.

Open-circuit voltage (OCV)-state of charge (SOC) relationship for different temperatures

Model-Based Methods: Complex mathematical models are employed that take into account various battery parameters, including internal resistance and capacity, to predict the SoC based on real-time measurements.

SoC Estimation Methods

• Kalman Filtering: Advanced algorithms like Kalman filters combine various sensor data, such as current, voltage, and temperature, to provide accurate SoC estimates even in dynamic operating conditions.

The structure of the extended Kalman filter (EKF) 

• Voltage-Based Method: One of the simplest methods involves measuring the battery voltage. Battery voltage tends to decrease linearly as the state of charge drops, making it a straightforward indicator. However, this method might not be highly accurate due to the influence of external factors like temperature and battery degradation.

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