A Brief Discussion on Battery Remaining Capacity - State of Charge (SoC)

Why is SoC So Important?

1. Safety

The working principle of a battery is based on chemical reactions, and each battery has its maximum capacity limit. When a battery approaches or exceeds this limit, continuing to charge it will lead to a series of adverse consequences, such as irreversible changes in the positive electrode material, decomposition of the electrolyte, and may even generate a large amount of gas, causing an increase in internal pressure, deformation or leakage of the battery, which is known as overcharging. Similarly, excessive discharging will also cause serious damage to the battery, resulting in problems such as an increase in internal pressure, damage to the reversibility of the positive and negative electrode materials, decomposition of the electrolyte, lithium deposition on the negative electrode, etc. Eventually, the battery resistance will increase, and even if it is recharged, it cannot fully recover, and its capacity will drop significantly. Therefore, accurately monitoring SoC is essential for avoiding overcharging and overdischarging and protecting battery safety.

2. Efficiency

In energy storage systems, batteries are usually composed of multiple single cells combined in series or in parallel. Due to the characteristics of series and parallel circuits, the capacity of the entire system actually depends on the battery with the worst performance. In order to maximize the overall efficiency of the system, we need to ensure that the SoC of all batteries is as consistent as possible. This requires us not only to accurately measure the SoC of each battery but also to adjust the differences among them through technical means such as active balancing and passive balancing. This is one of the key issues that Battery Management System (BMS) engineers need to solve.

How to Measure SoC?

Currently, the main methods for measuring SoC are as follows:

– Open Circuit Voltage Method: Estimate SoC by measuring the voltage of the battery when there is no load. This method is simple and easy to implement, but its accuracy is relatively low and is greatly affected by factors such as temperature and aging.

– Ampere-Hour Integration Method: Record the current and time during the charging and discharging process of the battery, and calculate the accumulated charging and discharging amounts through integration to estimate SoC. This method is applicable in dynamic environments, but it is prone to cumulative errors.

– Kalman Filtering Method: Combine data from multiple sensors and predict SoC through mathematical models, which has relatively high accuracy and robustness.

– Deep Learning Methods Based on Neural Networks: Utilize machine learning algorithms to train models and predict SoC based on historical data, with even higher accuracy but higher computational complexity.

The accuracy of commonly used measurement methods is approximately around ±5%. However, for high-precision applications, such as electric vehicles and energy storage systems, higher measurement accuracy is particularly important.

SoC is not only the core parameter of the battery management system but also the key to ensuring battery safety and improving system efficiency. Through precise SoC measurement and management, we can make better use of battery resources, extend battery life, and enhance the performance of the overall system. With the continuous progress of technology, the measurement accuracy and management strategies of SoC will be further improved in the future, bringing more convenience and safety to our lives.