State of health (SoH)

Definition (what it is)

State of health (SoH) is a dimensionless metric, typically expressed as a percentage, that describes the current condition of a rechargeable battery (cell, module, or pack) relative to a defined reference state, usually its specified performance when new. A value of 100% indicates the battery meets its beginning‑of‑life (BOL) specification; values below 100% reflect performance loss from aging. SoH generally declines over time due to calendar aging (time and temperature effects) and cycle aging (charge/discharge usage).

What it represents and how it is defined

  • Single- or multi-indicator metric: SoH can be defined in different ways depending on the application.
    • Capacity-based SoH: SoH (%) = (current maximum capacity ÷ initial rated capacity) × 100.
    • Power-based SoH: based on deliverable power at specified state of charge (SoC) and temperature.
    • Composite SoH: a weighted combination of indicators such as remaining capacity, internal resistance/impedance growth, power capability, self-discharge rate, and coulombic efficiency.
  • No universal standard: Manufacturers and test labs may use different definitions, reference conditions, and weighting schemes, so SoH values from different sources are not always directly comparable.
  • Reference conditions and normalization: To be meaningful, SoH assessments specify or normalize for temperature, SoC window, C-rate, rest times, and other conditions that strongly affect measured capacity and power.

How it is measured or estimated

  • Not directly measurable: SoH is estimated from observed behavior. On-board estimates are typically produced by a battery management system (BMS); off-board estimates come from controlled diagnostic tests.
  • Common estimation methods
    • Capacity inference from full or partial charge–discharge tests under standardized conditions.
    • Open-circuit voltage (OCV) versus SoC mapping and curve shifts over life.
    • Incremental capacity analysis (ICA) and differential voltage analysis (DVA).
    • Model-based observers using equivalent circuit or physics-based electrochemical models, often with Kalman filters, particle filters, or other state observers.
    • Data-driven and machine-learning approaches trained on voltage–current–temperature (V–I–T) histories.
  • Update cadence and uncertainty: SoH estimates improve as more data are collected across relevant operating regimes. Uncertainty arises from sensor noise, limited observation of the full SoC/temperature space, and deviations from standardized test conditions.
  • Displayed versus true SoH: Some systems smooth or buffer reported values for user experience or warranty purposes; third-party tests under controlled conditions may yield different numbers.

Thresholds, end of life, and use in decisions

  • Application-specific thresholds define when a battery remains fit for purpose.
    • Electric vehicles (traction batteries): end of life is often defined near 70–80% capacity SoH or when power capability falls below a specified threshold.
    • Stationary energy storage: usable at lower SoH (for example 60–70%), depending on service requirements.
    • Consumer electronics: replacement thresholds vary widely and are typically capacity-based.
  • Remaining useful life (RUL): RUL predictions combine current SoH with expected duty cycles and environmental conditions to estimate time or cycles to end of life.

Why SoH matters (relevance and applications)

  • Range, performance, and charging: In EVs, SoH directly impacts usable energy, power delivery, fast‑charge acceptance, and thus driving range and acceleration over the product life.
  • Safety and reliability: Degradation mechanisms that drive SoH decline (for example impedance rise, lithium plating, loss of active material) affect heat generation, balancing, and thermal runaway risk. SoH-aware control can mitigate abusive conditions.
  • Warranty, residual value, and second life: SoH underpins warranty adjudication, resale value for used batteries and vehicles, and screening for second‑life redeployment into stationary systems.
  • Fleet and lifecycle optimization: Operators use SoH tracking for predictive maintenance, charging strategy optimization, asset scheduling, and total cost of ownership reduction.
  • Design validation: Engineers evaluate SoH trajectories under representative cycles to validate chemistries, cell designs, pack architectures, and thermal management systems against lifetime targets.

Examples

  • Capacity example: A battery pack rated at 65 kWh when new that now delivers 60 kWh under standardized test conditions has an SoH of approximately 92%, indicating modest capacity fade with likely continued suitability for normal operation.
  • Application example: A retired EV pack at 75% SoH may be repurposed for stationary applications where reduced energy is acceptable but power and reliability remain adequate.

Factors that influence SoH over life

  • Operating conditions and usage patterns
    • Temperature exposure (both high and low), including storage and operating temperatures.
    • Average SoC and time spent at high SoC; deep cycles and high C‑rate charging/discharging.
    • Fast charging frequency, low-temperature charging (risk of lithium plating), and aggressive acceleration/regeneration profiles.
    • Thermal management effectiveness and cell-to-cell temperature uniformity.
  • Cell chemistry and materials
    • Cathodes: NMC, NCA, LFP, high‑Mn layered oxides exhibit different fade behaviors (for example transition‑metal dissolution, structural changes).
    • Anodes: graphite, silicon–graphite blends, or LTO influence swelling, SEI stability, and impedance growth.
    • Electrolytes and additives: Li-salt carbonate systems and film-forming additives (for example FEC, VC) affect SEI/CEI formation and stability.
    • Separators, binders, conductive networks, and current collectors contribute to mechanical integrity and resistance evolution.
  • Design and manufacturing
    • Electrode processing (coating uniformity, porosity, calendaring density) and formation protocols shape early-life stability and dispersion in SoH.
    • Electrode balancing (N/P ratio), tab placement, and mechanical constraints influence local current density, heat generation, and degradation uniformity.
    • Module/pack integration, including compression, electrical interconnects, and cooling strategies, drives cell-to-cell SoH consistency.
    • BMS hardware and software (limits, balancing, estimation algorithms) protect against conditions that accelerate SoH decline.

Degradation mechanisms that reduce SoH

  • Loss of lithium inventory via SEI growth and side reactions.
  • Loss of active material due to particle cracking, electrode delamination, and structural changes.
  • Impedance increase from SEI thickening, pore clogging, and current collector corrosion.
  • Transition‑metal dissolution from the cathode and deposition on the anode.
  • Electrolyte oxidation/reduction, gas generation, and separator degradation.
  • Lithium plating during high-rate or low-temperature charging.

Standardization and testing

  • Standards bodies (for example IEC, ISO, SAE, UL) publish procedures for capacity and power testing, specifying temperatures, C‑rates, rest periods, and SoC windows to improve SoH comparability.
  • Despite available procedures, SoH definitions and test methods are not yet fully harmonized across industries and manufacturers, complicating cross-platform comparisons and secondary-market certification.
  • Best practice is to report the SoH definition used, the reference test conditions, and uncertainty bounds or confidence intervals.

Related terms and synonyms

  • Battery health or battery state of health: common synonyms.
  • State of charge (SoC): instantaneous charge level relative to full charge; distinct from SoH, which reflects aging.
  • State of power (SoP) or state of function (SoF): available power or ability to meet a specific function under given conditions; often derived from impedance and related to SoH.
  • Remaining useful life (RUL): projected time or cycles remaining until an end‑of‑life criterion is reached.
  • Capacity retention or power retention: terms used when SoH is defined solely by capacity or power, respectively.