Battery State of Health (SOH) indicates how a battery’s current condition compares with a defined reference state, typically when new. It helps battery manufacturers, electric vehicle (EV) fleet operators, and energy storage providers assess degradation, monitor performance, and plan maintenance or replacement.
Battery SOH is commonly estimated using capacity retention, internal resistance, and other health indicators. The results help determine whether a battery can meet its application’s requirements, but SOH alone does not establish battery safety or predict its remaining life with certainty.
What Is Battery State of Health (SOH)?
Battery State of Health (SOH) is an estimate of a battery’s condition relative to a defined reference condition. For lithium-ion batteries, it commonly reflects changes in available capacity, internal resistance, and other indicators associated with aging.
A battery with 100% capacity-based SOH retains its reference capacity under the specified test conditions. If its measured capacity falls from 100 Ah to 80 Ah, its capacity-based SOH is 80%.
SOH is not a single universally standardised percentage across all battery systems. Its interpretation depends on the selected health indicator, battery chemistry, measurement conditions, and estimation method.
How Is Battery SOH Measured?
Battery SOH is assessed through controlled tests or estimated from operational data. Three common approaches are capacity testing, resistance or impedance measurement, and model-based estimation using a battery management system (BMS).
1. Capacity-Based Measurement
Capacity-based measurement compares a battery’s measured available capacity with its reference capacity. A controlled charge-discharge test measures how much charge the battery delivers under specified conditions.
This method directly evaluates capacity retention, making it useful for assessing battery performance and studying degradation. However, testing may require dedicated equipment, controlled conditions, and sufficient downtime.
2. Internal Resistance and Impedance Measurement
Battery aging can increase internal resistance and alter electrochemical impedance. These changes can indicate degradation that affects power delivery and heat generation.
Resistance measurements can support rapid diagnostics, while electrochemical impedance spectroscopy (EIS) evaluates the battery’s response across different frequencies. Neither method provides a universally interchangeable substitute for capacity-based SOH. Results depend on temperature, state of charge, measurement technique, and battery design.
3. Model-Based SOH Estimation
A BMS estimates battery health using operating data such as voltage, current, temperature, and charge-discharge history. Depending on its design, the system may use equivalent-circuit models, electrochemical models, or data-driven algorithms.
Model-based estimation supports ongoing monitoring without requiring a complete capacity test for every assessment. However, accuracy depends on sensor quality, model validation, calibration, battery chemistry, and operating conditions.
Battery SOH Measurement Methods Compared
| Method | Main advantage | Main limitation |
|---|---|---|
| Capacity testing | Measures capacity retention under defined conditions | Can require lengthy testing |
| Resistance or impedance testing | Provides information about electrical and electrochemical changes | Sensitive to test conditions; does not directly establish capacity |
| BMS-based estimation | Enables ongoing health monitoring | Depends on algorithms and operational data quality |
The appropriate method depends on whether the priority is capacity verification, rapid diagnostics, or continuous monitoring. Combining multiple indicators can provide a more comprehensive assessment than relying on one measurement alone.
How to Calculate Battery SOH
For a capacity-based estimate, use this battery SOH formula:
Battery SOH (%) = (Measured capacity ÷ Reference capacity) × 100
For example, if a battery’s measured capacity is 80 Ah and its reference capacity is 100 Ah:
SOH = (80 ÷ 100) × 100 = 80%
The battery retains 80% of its reference capacity under the specified test conditions.
This calculation applies to capacity-based SOH. Resistance-based or model-based methods may use different indicators and estimation procedures, so their reported health values may differ.
What Is a Good Battery SOH Percentage?
A good battery SOH depends on the battery chemistry, application, manufacturer specifications, and required performance.
- 100% SOH: The battery matches its defined reference condition for the selected health indicator.
- Below 100% SOH: The battery has degraded relative to that reference.
- Around 80% SOH: A commonly used capacity-retention benchmark in some applications, but not a universal replacement threshold.
- Below 80% SOH: May require closer evaluation against application requirements and manufacturer criteria.
For example, an EV battery may become unsuitable for a particular driving range before it becomes unsuitable for stationary energy storage. Battery replacement decisions should consider usable capacity, power capability, safety assessments, warranty requirements, and operating needs—not SOH alone.
What Factors Affect Battery SOH?
Battery degradation depends on operating conditions, chemistry, cell design, and aging history.
1. Temperature
High temperatures can accelerate degradation, while low temperatures can temporarily affect available performance and measurement results.
2. Charge and Discharge Rates
High rates can increase heat and electrochemical stress under certain operating conditions.
3. Depth of Discharge
Repeated deep discharge can accelerate degradation in some battery systems.
4. Cycling History
Repeated charge-discharge cycles gradually change battery capacity and internal characteristics.
5. Calendar Aging
Batteries can degrade over time even when they are not regularly used.
6. Storage and Charging Conditions
Prolonged exposure to unsuitable temperatures or states of charge can affect long-term health.
These factors do not produce identical degradation rates across all batteries. Their effects depend on chemistry, operating profile, and system design.
Why Is Battery SOH Important?
Battery SOH helps organisations make informed decisions about performance, maintenance, reliability, and asset lifecycle management.
1. Assessing Battery Performance
Capacity-based SOH helps operators determine whether a battery can still meet required energy demands. Declining capacity can reduce EV driving range, backup runtime, or the usable energy available from a storage system.
2. Planning Maintenance
Tracking SOH over time helps identify degradation trends and unexpected changes. Maintenance teams can investigate abnormal readings, schedule diagnostic tests, and plan interventions before battery performance falls below operational requirements.
3. Supporting Safety and Reliability
SOH trends can indicate changes that warrant further investigation. However, a high SOH reading does not guarantee that a battery is safe. Safety assessments may also require voltage and temperature monitoring, fault detection, and inspection for physical damage.
4. Making Replacement and Lifecycle Decisions
Battery manufacturers, fleet operators, and energy storage providers can use SOH data to support warranty assessments, replacement planning, asset valuation, and second-life evaluations.
A practical decision process should combine SOH trends with actual application requirements, diagnostic findings, manufacturer limits, and the cost of continued operation. This reduces reliance on a single percentage when evaluating battery usability.
What Is the Difference Between SOH, SOC, and SOL?
SOH, State of Charge (SOC), and State of Life (SOL) describe different aspects of battery status.
| Parameter | Full form | What it indicates |
|---|---|---|
| SOH | State of Health | Battery condition relative to a defined reference |
| SOC | State of Charge | Current charge level relative to usable capacity |
| SOL | State of Life | Life status relative to a defined end-of-life criterion, where used |
SOH and SOC measure different aspects of a battery. A battery can have 100% SOC but only 80% capacity-based SOH: it is fully charged relative to its available capacity, but its capacity is lower than the reference capacity.
SOL is less consistently defined across battery systems, so its meaning should be established by the relevant manufacturer or monitoring framework.
How Does a Battery Management System Monitor SOH?
A battery management system collects operating data, including voltage, current, and temperature, to estimate battery health. Depending on its capabilities, it may also analyse charge-discharge history and changes in electrical characteristics.
BMS-based SOH monitoring allows operators to track trends, detect unexpected changes, and support maintenance planning. For reliable results, the estimation algorithm must be appropriate for the battery chemistry and validated under representative operating conditions.
Research reviews identify capacity, internal resistance, and temperature-related indicators as important inputs to battery health estimation. They also highlight challenges associated with estimation accuracy, changing operating conditions, and transferring models between battery systems.
Conclusion
Battery State of Health (SOH) helps organisations assess battery degradation, monitor performance, and plan maintenance or replacement. Combining capacity measurements, electrical health indicators, and validated BMS estimates provides a stronger basis for battery lifecycle decisions than relying on a single SOH percentage.
Frequently Asked Questions (FAQs)
1. How do you check a battery’s State of Health?
Use a controlled capacity test, resistance or impedance measurements, or a BMS-based estimate. The appropriate method depends on the battery type, required accuracy, and available testing facilities.
2. Can a battery have high SOC but low SOH?
Yes. SOC measures current charge availability, while SOH reflects the battery’s condition relative to a reference. An aged battery can be fully charged despite having reduced capacity.
3. How accurate is BMS-based SOH estimation?
Accuracy depends on sensor quality, calibration, model design, battery chemistry, and operating conditions. A controlled capacity test can help validate an estimate when greater confidence is required.
4. Can battery SOH be improved?
Aging-related degradation generally cannot be reversed. Correcting operating conditions or recalibrating an estimation system may improve battery performance under certain circumstances or correct an inaccurate reported estimate, but it does not restore lost electrochemical capacity.
5. Does 80% SOH mean a battery needs replacement?
Not necessarily. Replacement depends on the application’s performance requirements, manufacturer specifications, warranty conditions, diagnostic findings, and safety assessment.