Understanding SOH in Battery – Key Definition and Measurement Methods

Lithium-ion batteries have become the backbone of modern energy solutions, powering electric vehicles, renewable energy storage systems, and countless electronic devices. To ensure safe and reliable performance, monitoring the SOH in Battery—which stands for State of Health—has become an essential practice. Evaluating this parameter helps predict a battery’s lifespan, maintain safety, and optimize overall performance.

In this guide, we’ll explore what SOH in Battery really means, why it matters, and the main methods used to measure it effectively.

Why is detecting SOH in Battery important?

With the rapid rise of electric vehicles and renewable energy systems, ensuring battery reliability has become more important than ever. As global demand for power batteries continues to surge, the focus on safety and performance is also increasing. Such rapid growth highlights not only opportunities but also the need to address potential risks.

The SOH in Battery reflects the overall health condition of a cell or battery pack. By monitoring it, manufacturers and users can detect early signs of aging, performance loss, or hazards such as overheating and failure. Regular evaluation helps prevent unexpected breakdowns, supports proactive maintenance, and extends battery lifespan while reducing operational costs.

What are the main reasons for measuring SOH in Battery?

Monitoring the SOH in Battery is not just about knowing whether a pack still works—it's about preventing risks and optimizing battery performance. Some key reasons include:

• Thermal safety: As batteries age, battery internal resistance increases, generating excess heat and raising the risk of thermal runaway.
• Accurate maintenance: Knowing the SOH helps decide whether to repair, replace, or continue using a battery safely.
• Performance variation: Different batteries degrade at different rates, and SOH tracking provides accurate comparisons.
• EV range estimation: For electric vehicles, SOH directly influences mileage predictions and corrects battery SOC (State of Charge) accuracy.

By monitoring SOH closely, operators can reduce costs, avoid accidents, and ensure smoother performance of energy systems.

How is SOH in Battery defined?

Battery aging is a gradual process, but engineers and researchers need measurable indicators to evaluate it. SOH in Battery can be defined from multiple perspectives:

• Internal resistance-based definition:

If internal resistance increases to a point where the battery can no longer deliver sufficient power, it has reached its end of life (EOL).

• Capacity-based definition:

SOH can also be expressed as the ratio of current capacity to the original capacity of the battery. Once capacity falls below 80% of the initial value, the battery is usually considered degraded.

• Other practical indicators:

These include self-discharge rates, cycle counts, and inconsistencies within battery modules.

Different industries adopt different SOH definitions depending on their application, but all serve the purpose of quantifying degradation in a reliable way.

What methods are used to measure SOH in Battery?

In practice, measuring SOH in Battery involves analyzing parameters like voltage, current, and temperature during operation. IEEE standards recommend replacing a power battery once its SOH drops below 80%.

Broadly, measurement approaches fall into two categories:

Direct measurement methods – where physical characteristics such as resistance or stress are measured.

Indirect analysis methods – which rely on data modeling, algorithms, and predictive analysis.

The direct methods for measuring SOH in Battery

How to measuring SOH in battery with direct methods:

• Electrochemical Impedance Spectroscopy (EIS)

This widely used method applies a small AC signal to the battery and measures its impedance across a frequency range. The resulting data helps evaluate charge transfer resistance, ion diffusion, and electrode polarization. EIS is non-destructive and provides deep insights into battery kinetics, making it one of the most accurate methods for determining SOH in Battery.

• Stress detection in battery stacks

As lithium-ion cells age, internal gas generation and expansion lead to stress between cells. Monitoring this stress gives valuable information about the aging process and contributes to SOH estimation.

• Filtering algorithms

By applying mathematical filters to voltage and resistance data, algorithms can dynamically calculate SOH. These models consider temperature effects and provide real-time insights.

• Voltage curve reconstruction

This approach uses the open circuit voltage (OCV) versus state of charge (SOC) relationship. By reconstructing the OCV-SOC curve, researchers can refine SOC estimates and indirectly measure SOH with improved accuracy.

The indirect methods for measuring SOH in Battery

• Sample entropy of discharge voltage

Entropy analysis captures irregularities and degradation trends in voltage curves during discharge cycles. This method is effective in highlighting local changes and early signs of deterioration.

• DTV, DVA, and ICA curve analysis

Differential thermal voltammetry (DTV), differential voltage analysis (DVA), and incremental capacity analysis (ICA) are techniques that study peak positions in capacity increment curves. These curves provide insights into aging mechanisms, though their accuracy is influenced by sampling frequency and environmental factors.

• Hybrid Pulse Power Characterization (HPPC) test

This test, originally from the US FreedomCAR program, subjects batteries to pulse charge and discharge under different SOC conditions. By calculating internal resistance and power capability, HPPC gives a practical estimate of SOH across varying operating conditions.

What challenges exist in estimating SOH in Battery?

Although many methods are available for measuring SOH in Battery, each approach comes with its own limitations. Factors such as temperature changes and varying operating conditions can significantly affect accuracy, while the complex and inconsistent degradation patterns of different batteries make it difficult to establish universal models.

In addition, advanced algorithms used for estimation often demand large datasets and considerable computational power. To address these challenges, researchers typically combine multiple methods, allowing them to achieve more reliable and precise SOH evaluations.

How does SOH in Battery impact electric vehicles and energy storage systems?

In electric vehicles, SOH directly affects driving range, charging efficiency, and overall safety. For stationary energy storage, such as solar-plus-storage systems, accurate SOH monitoring ensures stable grid support and reduces replacement costs.

Without proper SOH tracking, users risk unexpected failures, costly downtime, and even hazardous incidents. With the rise of big data and AI, future SOH monitoring will become smarter, integrating predictive analytics for even better results.

Conclusion

The estimation of SOH in Battery is vital to ensuring safety, reliability, and cost-efficiency in energy applications. While direct and indirect measurement methods each have strengths and weaknesses, combining them provides the best results.

As technology advances, leveraging artificial intelligence, machine learning, and big data will allow more precise monitoring and prediction of battery health. Ultimately, mastering SOH measurement not only protects investments but also paves the way for the sustainable growth of electric mobility and renewable energy storage.

FAQ

1. What is a good battery SoH?

The unit of SoH is percent (100% = the battery's conditions match the battery's specifications). For example, when the capacity of a new battery is same as the nominal capacity as per the battery specification, it is said to be in optimal health (SoH = 100%).

2. How often should SOH in Battery be checked?

For critical applications like EVs and energy storage, SOH should be monitored regularly—either through built-in battery management systems or periodic diagnostic tests.

3. What happens if SOH in Battery is ignored?

Neglecting SOH can lead to unexpected breakdowns, reduced performance, higher operational costs, and even safety hazards such as overheating or thermal runaway.

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