SOH (State of Health) Calculator

What is State of Health (SOH)?

State of Health (SOH) is a metric that quantifies the current condition of a battery compared to its original specifications. Expressed as a percentage, SOH = 100% indicates a new battery, while lower values reflect degradation from aging, cycling, and environmental stress.

SOH is primarily determined by capacity fade — the gradual reduction in the amount of charge a battery can store. Contributing factors include solid electrolyte interphase (SEI) layer growth, lithium plating, electrode cracking, and electrolyte decomposition. Temperature extremes and deep cycling accelerate these processes.

In electric vehicles, a battery is typically considered end-of-life for automotive use at 70-80% SOH, though it may continue serving in less demanding second-life applications such as stationary storage.

SOH encompasses two distinct degradation modes: capacity fade and power fade. A battery may retain 90% of its capacity but suffer significant internal resistance growth, reducing peak power capability by 30% or more. Both dimensions should be tracked for a complete health assessment.

Formula: SOH (%) = (Measured Capacity / Rated Capacity) × 100 Capacity Loss (%) = 100 - SOH

Example Calculation

A battery pack originally rated at 75 Ah now delivers 63 Ah when fully charged and discharged. SOH = (63 / 75) × 100 = 84%. Capacity loss = 16%. The pack is still above the 80% threshold for automotive use but should be monitored closely.

When to Use This Calculator

• Evaluating warranty claims by comparing measured capacity against the manufacturer's rated specification
• Deciding whether an EV battery pack qualifies for continued automotive service or should transition to second-life storage
• Establishing periodic capacity check benchmarks for fleet battery management programs
• Verifying BMS-reported SOH against an independent capacity measurement from a full charge-discharge test

Common Mistakes to Avoid

• Measuring capacity at a different C-rate than the rated specification — capacity measured at 1C will be lower than at C/3 due to Peukert losses, yielding a falsely pessimistic SOH
• Performing capacity tests at extreme temperatures — cold temperatures reduce deliverable capacity temporarily, which is not the same as permanent degradation
• Using a single cell measurement to represent pack SOH — the weakest cell limits pack capacity, so pack SOH is determined by the worst cell, not the average
• Confusing SOH with SOC (State of Charge) — SOC is how full the battery is right now; SOH is how much total capacity remains compared to when it was new

How to Interpret Results

• SOH 90-100%: Healthy — normal for batteries within first 20-30% of expected cycle life; no action needed
• SOH 80-90%: Fair — moderate degradation; monitor more frequently and consider reducing depth of discharge to slow further fade
• SOH 70-80%: Degraded — approaching end-of-life for automotive and high-performance applications; plan for replacement or second-life repurposing
• SOH below 70%: Replace — capacity loss is severe; internal resistance is likely elevated, reducing power capability and increasing heat generation

Related Standards & References

• IEC 62660-1 — Secondary lithium-ion cells for EV application: performance and life testing procedures including capacity measurement methods
• SAE J2464 — EV and HEV rechargeable energy storage system abuse test procedures referencing SOH thresholds
• ISO 12405 — Electrically propelled road vehicles: test specification for lithium-ion traction battery packs, including capacity retention criteria

Frequently Asked Questions

How is SOH measured in practice?

SOH can be measured by full charge-discharge capacity tests (most accurate but time-consuming), impedance spectroscopy (EIS), pulse load testing, or estimated from BMS coulomb counting data. Modern BMS systems continuously estimate SOH using voltage curves and machine learning algorithms.

At what SOH should a battery be replaced?

For EVs, the industry standard end-of-life threshold is 70-80% SOH. For consumer electronics, replacement is often considered below 80%. For stationary storage, batteries can operate down to 60% SOH since weight and volume constraints are less critical.

Can SOH ever increase or be restored?

True capacity recovery is not possible for irreversible degradation (SEI growth, active material loss). However, apparent SOH can improve slightly after reconditioning cycles that rebalance cells or recalibrate the BMS estimation algorithm. These gains are typically small (1-3%) and temporary.