Four Essential Aspects of SOH Battery Health Evaluation

Understanding the State of Health (SOH) of a battery involves assessing several key factors: capacity, internal resistance, voltage, and self-discharge. Here’s a detailed explanation of each:

1. Capacity

Battery capacity is the maximum amount of power a battery can provide during a complete charge and discharge cycle, typically measured in ampere-hours (Ah). As a battery ages, its capacity decreases due to the irreversible loss of active materials.

SOH Capacity Assessment Methods:

• Full Discharge Test: The most accurate but time-consuming method. The battery is fully discharged to the cut-off voltage, and the discharged power is measured.
• Model-Based Methods: These use mathematical models to estimate battery capacity without requiring a full discharge.

SOH Capacity Data Examples:

• New battery: 100%
• Battery after 500 cycles: 80%
• Battery after 1000 cycles: 60%

2. Internal Resistance

Internal resistance refers to the resistance within the battery that impedes the flow of current, typically measured in ohms (Ω). As the battery is used, its internal resistance increases, which can decrease efficiency and increase heat generation.

SOH Internal Resistance Evaluation Methods:

• AC Impedance Method: Uses an AC signal to measure impedance, distinguishing between active material impedance and ohmic impedance.
• DC Impedance Method: Uses a DC current to measure impedance. This method is simpler but less accurate.

SOH Internal Resistance Data Examples:

• New battery: 0.1 Ω
• Battery after 500 cycles: 0.2 Ω
• Battery after 1000 cycles: 0.3 Ω

3. Voltage

Battery voltage is the potential difference between the positive and negative electrodes, measured in volts (V). Voltage varies with the state of charge and is influenced by factors like temperature.

SOH Voltage Evaluation Methods:

• Open Circuit Voltage (OCV) Measurement: Measures the battery voltage in a static state to reflect its chemical state.
• Dynamic Voltage Measurement: Measures the battery voltage during charge and discharge to reflect internal impedance and polarization characteristics.

SOH Voltage Data Examples:

• Typical OCV range for lithium-ion batteries: 3.0 V to 4.2 V
• Discharge voltage: gradually decreases until the cutoff (around 2.5 V)
• Charge voltage: gradually increases until full charge (around 4.2 V)

4. Self-Discharge

Self-discharge is the slow loss of charge when a battery is not connected to a load. The self-discharge rate increases with battery use and temperature.

SOH Self-Discharge Evaluation Methods:

• Stationary Test: The battery is left idle for a period, and the voltage drop is measured.
• Constant Current Test: A constant current is applied, and the capacity drop is measured.

SOH Self-Discharge Data Examples:

• New battery: 1-2% per month
• Battery after 500 cycles: 2-3% per month
• Battery after 1000 cycles: 3-5% per month

Comprehensive Evaluation

By considering the indicators of battery capacity, internal resistance, voltage, and self-discharge, a comprehensive assessment of battery health can be achieved. Generally, a battery is considered severely aged and in need of replacement when its SOH falls below 80%.