I. The Principle of SOH: What Does It Reflect?

The essence of SOH is to quantify the degree of battery aging. This aging is mainly reflected in two aspects, which are also the physical basis of SOH calculation:

1. Capacity Decay

This is the most core and intuitive manifestation of SOH. After long-term use, the total amount of charge that a battery can store and release will irreversibly decrease.

·Physical Principles:

o Active Lithium Loss: During cycling, the electrolyte undergoes side reactions with the electrode surface, forming a solid electrolyte interface film. This film continues to grow and consumes available free lithium ions, leading to a reduction in the "effective lithium" participating in the charge and discharge reactions.

o Electrode Material Structure Degradation: The crystal structure of the positive and negative electrode materials undergoes irreversible phase transitions, dissolution, or collapse during the repeated insertion and extraction of lithium ions, resulting in fewer "positions" for storing lithium ions or a decrease in their ability to do so.

Result: A new battery with a nominal capacity of 100Ah may, after several years of use, only release 80Ah after being fully charged and then discharged. Its capacity-type SOH is (80Ah / 100Ah) * 100% = 80%.

2. Decreased Power Performance (Increased Internal Resistance)

The battery's internal resistance determines its operating voltage and heat generation during charging and discharging. Increased internal resistance leads to poorer battery performance.

• Physical Principles:

* Increased Ohmic Internal Resistance: Corrosion of the current collector, increased contact resistance between the electrode material and the current collector, etc.

* Increased Electrochemical Polarization Internal Resistance: Due to the aforementioned thickening of the SEI film and decreased activity of the electrode material, it becomes more difficult for lithium ions to intercalate and deintercalate in the electrode, slowing down the reaction rate.

* Increased Concentration Polarization Internal Resistance: The transport speed of lithium ions in the electrolyte slows down.

Results: When discharging at the same high rate, the voltage drop of a new battery is very small, while the voltage of an aged battery drops sharply, causing the system to prematurely trigger low-voltage protection, failing to release all energy, and resulting in more severe battery heating. Its power-type SOH can be calculated by the increase in internal resistance.

II. The Role of SOH: Why is it so important? State of Harmony (SOH) is not an abstract academic indicator; it plays a crucial role throughout the battery's entire lifecycle, especially in terms of safety and economic efficiency.

1. For Users:

· Assessing Battery Remaining Value and Expected Lifespan:

* Electric Vehicles: SOH is a key indicator for measuring the value of a used car. It directly reflects the vehicle's remaining driving range. Users can use SOH to determine if the battery is still under warranty (manufacturers typically promise, for example, "SOH not lower than 70% within 8 years or 160,000 kilometers").

* Energy Storage Systems: Helps users understand the actual energy throughput capacity of the energy storage system and perform economic benefit calculations.

· Guiding Usage Habits: Understanding the downward trend of SOH allows users to use and maintain the battery more scientifically, delaying its aging.

2. For Battery Management Systems (BMS):

This is the core area of ​​SOH's role. The BMS uses SOH information to dynamically adjust its control strategies to achieve safety, efficiency, and longevity goals.

• Optimized Charge/Discharge Strategy:

o Power Limitation: As SOH decreases (internal resistance increases), the BMS gradually limits the battery's maximum charge/discharge power to prevent overcharging/over-discharging and excessive heat generation, ensuring safety.

o Capacity Calculation Baseline: The BMS calculates the remaining range (SOC) based on a "full capacity" baseline, which is the current maximum usable capacity adjusted in real-time according to SOH. A battery with an SOH of 80%, even if the SOC shows 100%, only has 80% of its original capacity.

• Ensuring Safe Operation:

o Aging batteries (usually with low SOH) have more unstable internal chemical systems and a higher risk of thermal runaway. The BMS can enter a more conservative management mode based on the SOH value, strengthening monitoring and protection.

• Achieving Balanced Management:

o SOH information helps the BMS more intelligently determine whether the inconsistencies in the cells within the battery pack are caused by reversible SOC inconsistencies or irreversible capacity decay (SOH inconsistencies), thus enabling more effective balancing measures.

3. For secondary use: When the State of Health (SOH) of a power battery drops to around 80%, it may no longer meet the vehicle's range and power requirements, but it still retains significant residual value.

· Screening for secondary use: SOH is a core criterion for determining whether retired batteries are suitable for secondary use scenarios such as energy storage, low-speed electric vehicles, and backup power supplies. Based on the energy density requirements of different application scenarios, retired batteries can be precisely graded and repurposed.

In summary, the SOH of lithium iron phosphate batteries is a comprehensive health indicator, stemming from irreversible chemical and physical aging within the battery. Its role is to provide crucial decision-making basis for battery safety management, state estimation, value assessment, and secondary use.