Calculating the State of Charge of a Lithium Ion Battery System using a Battery Management System

Summary

In this Stifle Systems Insights video, Eric Staal discusses the concept of State of Charge (SOC) in battery packs, emphasizing its definition as the remaining capacity relative to the total capacity in amp-hours. He illustrates the difference between SOC based on capacity and SOC based on energy, highlighting the importance of understanding the energy available for applications like electric vehicles. Staal also explains the method of Coulomb counting for calculating SOC, the challenges of current sensor drift, and the role of open circuit voltage (OCV) lookup in ensuring accurate SOC estimation for reliable battery performance.

Takeaways

• 🔋 The state of charge (SOC) is defined as the remaining capacity in amp-hours or coulombs that can be discharged over the total capacity of the battery pack.
• 📉 SOC is measured in units of amp-hours, not energy, which is an important distinction because the voltage of a battery changes as it discharges.
• 📊 The discharge curve for lithium-ion batteries typically shows a varying downward slope, with higher voltage in the initial stages and lower voltage towards the end of discharge.
• 🚗 In electric vehicle applications, it's more useful to consider the energy available (SOC II) rather than just the capacity remaining, as it provides a more accurate fuel gauge algorithm.
• 🔍 The 50% SOC point on the discharge curve does not necessarily correspond to 50% of the energy remaining; it's where 50% of the amp-hours have been discharged.
• 🔄 Coulomb counting is the primary method for calculating SOC, which involves integrating the current over time to estimate the amp-hours discharged.
• 🛠 Current sensors used in Coulomb counting can have drift and integration errors, which is why open circuit voltage (OCV) lookup tables are used to correct and recalibrate the SOC estimation.
• 🔄 Depth of discharge (DOD) is the inverse of SOC and represents the percentage of the battery's capacity that has been used.
• 🔄 The BMS uses OCV lookup to determine the SOC when the battery is at rest, ensuring an accurate starting point for Coulomb counting and reliable SOC estimation.
• ⚙️ Accurate SOC and SOC II algorithms are crucial for the reliable and predictable operation of battery packs, preventing issues like sudden drops in estimated charge.

Q & A

What is the state of charge (SOC) of a battery pack?

The state of charge (SOC) is defined as the capacity remaining over the total capacity in amp-hours or coulombs that you can discharge over the total capacity of the battery pack.

How is the SOC percentage calculated?

The SOC percentage is calculated by dividing the remaining capacity by the total capacity of the battery pack. For example, if a battery pack has a total capacity of 100 amp-hours and 70 amp-hours are left, the SOC is 70%.

Why is it important to distinguish between SOC and energy units?

It's important because SOC is measured in amp-hours, which is a measure of capacity, whereas energy is measured in watt-hours. This distinction is crucial for applications like electric vehicles where the available energy, not just capacity, determines the range.

What is the significance of the discharge curve for lithium-ion batteries?

The discharge curve for lithium-ion batteries typically shows a varying downward slope in voltage as capacity is discharged. This curve is significant because it illustrates the relationship between voltage, capacity, and energy, which is essential for understanding battery performance.

What is the difference between SOC based on capacity (SOC I) and SOC based on energy (SOC II)?

SOC I is based on the remaining capacity in amp-hours, while SOC II is based on the available energy. SOC II provides a more accurate representation of the expected runtime or range, as it accounts for the varying energy output at different states of charge.

Why is Coulomb counting a primary method for calculating SOC?

Coulomb counting is a primary method for calculating SOC because it involves integrating the current over time to estimate the amp-hours discharged, providing a direct measure of the battery's state of charge.

What challenges does Coulomb counting face in accurately estimating SOC?

Coulomb counting faces challenges such as current sensor drift and integration error, which can lead to inaccuracies in the estimation of discharged amp-hours. To mitigate this, additional methods like open circuit voltage (OCV) lookup are often used.

What is open circuit voltage (OCV) lookup and how does it help in SOC estimation?

OCV lookup is a method where the BMS compares the actual voltage of the battery at rest with a lookup table to determine the corresponding state of charge or depth of discharge. This helps in recalibrating the SOC estimation, ensuring accuracy and reliability.

What is the relationship between depth of discharge (DOD) and SOC?

Depth of discharge (DOD) is the inverse of SOC. While SOC represents the remaining capacity, DOD represents the amount of capacity that has been used. For example, at 70% SOC, the DOD would be 30%.

Why is it crucial to have an accurate SOC estimation in applications like electric vehicles?

In electric vehicles, an accurate SOC estimation is crucial for predicting the remaining range and ensuring reliable operation. Inaccurate SOC estimation can lead to unexpected battery depletion, causing inconvenience and potential stranding of the vehicle.