SOH (State of Health) is a key indicator used to evaluate the current performance of a battery compared with its original, brand-new state. It is usually expressed as a percentage:

• 100% SOH = brand-new battery

• 70–80% SOH = typical end-of-life threshold

For EverExceed LiFePO₄ batteries—widely used in UPS, telecom, data centers, marine systems, and energy-storage applications—accurate SOH estimation is essential for ensuring long-term reliability and safety.

However, due to the flat voltage curve and nonlinear OCV–SOC characteristics of LiFePO₄ chemistry, calculating SOH is more challenging than with NCM/NCA batteries. Below are the mainstream SOH estimation methods used in EverExceed battery systems and BMS algorithms.

1. Direct Method: Capacity-Based SOH Calculation

This is the most intuitive and accurate method. It measures the actual usable capacity of the battery.

Formula

SOH (Capacity) = (Current Actual Capacity / Rated Capacity) × 100%

Approach A: Standard charge–discharge test (laboratory)

A full discharge cycle is performed under controlled conditions.

Advantages: Highest accuracy
Limitations: Time-consuming, interrupts normal operation; not suitable for daily use.

Approach B: BMS integration during a full cycle (real-world application)

EverExceed BMS uses coulomb counting to record total charged/discharged energy during a full cycle.

Advantages: Practical and relatively accurate
Limitations: Requires a full cycle (e.g., 5% → 95%), difficult in fragmented daily operation

2. Indirect Methods: Parameter-Based SOH Estimation

Because real-life full cycles are rare, EverExceed BMS uses indirect models to estimate SOH in real time.

2.1 Internal Resistance Method (Rising impedance)

Internal resistance increases as batteries age.

Formula (common model)

SOH (Impedance) ≈ f(R_increase rate vs. capacity fade)

Measurement Methods

• DCIR (most common in EverExceed BMS)

• AC impedance (more accurate, lab usage)

Advantages: Real-time, online measurement
Limitations: Strongly affected by temperature & SOC

2.2 Electrochemical Impedance Spectroscopy (EIS)

Analyzes impedance across multiple frequencies to extract aging-related parameters.

Advantages: Extremely accurate
Limitations: High computational load; mainly used in laboratories or high-end EverExceed R&D platforms

2.3 Incremental Capacity Analysis (ICA) / Differential Voltage Analysis (DVA)

One of the most effective SOH estimation methods for LiFePO₄ batteries.

Principle:
During constant-current charging, the dV/dQ curve shows characteristic peaks that shift as the battery ages.

Advantages: Very accurate for LiFePO₄
Limitations: Requires precise voltage measurement and CC charging stability

2.4 Curve Fitting With Battery Models

The BMS continuously adjusts model parameters (capacity, internal resistance, etc.) to fit real-time voltage/current data.

Advantages: Continuous estimation
Limitations: Relies heavily on accurate electrochemical models

2.5 Coulomb Counting + OCV Calibration (Hybrid Method)

This is the primary SOH estimation algorithm used in EverExceed BMS.

Process

1. Coulomb counting: Tracks SOC changes via current integration

2. Model-based estimation: Predicts SOC with temperature and impedance compensation

3. OCV calibration: When the battery rests long enough, the OCV is matched to a stored OCV–SOC curve

4. SOH update: Differences between integrated SOC and OCV-based SOC are used to correct the battery’s maximum capacity parameter

Challenge for LiFePO₄

The OCV–SOC curve is very flat (20%–80% region), so calibration is usually done at high or low SOC.

Comparison of SOH Estimation Methods (EverExceed Adapted)

Practical Tips for EverExceed Battery Users

To maintain accurate SOH readings:

• Perform a full charge–discharge cycle occasionally (e.g., 100% → 10% → 100%)

• Avoid long-term storage at 0% or 100%

• Ensure proper temperature control

• Use official EverExceed chargers/BMS-compatible systems