State of Health (SoH) is a measure of an EV battery’s long-term condition compared to when it was new. It reflects how much the battery has degraded over time and how well it can still store and deliver energy. SoH is usually expressed as a percentage, where 100% SoH represents a battery at its original performance, and lower values indicate reduced usable capacity and/or reduced power capability.
Unlike State of Charge (SoC), which shows how full the battery is right now, SoH describes how the battery has aged.
Why SoH Matters in EV Charging
SoH affects charging performance, fleet planning, and the total cost of EV ownership:
– Batteries with lower SoH may charge differently, with earlier power tapering and reduced peak power
– Range estimates become less predictable as capacity fades
– Fleets use SoH to plan vehicle rotation, maintenance, and replacement schedules
– Used EV valuation often depends on reported SoH and battery history
– Charging operators use SoH indirectly when sizing infrastructure for mixed vehicle populations
SoH also matters for warranties, as many battery warranties specify capacity retention thresholds over time.
What SoH Represents
SoH is not a single physical measurement—it is an assessment derived from multiple indicators, typically including:
– Capacity fade: reduced usable energy storage compared to original
– Power fade: reduced ability to accept or deliver high power (affects acceleration and fast charging)
– Increased internal resistance and heat generation
– Cell imbalance and reduced efficiency
Some systems separate SoH into capacity SoH and power SoH depending on what is being evaluated.
How SoH Is Estimated
Battery management systems estimate SoH using data and models such as:
– Cumulative energy throughput (charge/discharge history)
– Voltage behavior and cell balancing patterns
– Internal resistance trends
– Temperature exposure and thermal stress history
– Diagnostic cycles and calibration events
Because SoH is model-based, the displayed number can differ between manufacturers and may change after recalibration.
Factors That Influence Battery SoH
Battery degradation is driven by time, usage, and conditions:
– High temperatures and frequent thermal stress
– Frequent high-power DC fast charging (especially at high SoC)
– Keeping the battery at very high SoC for long periods
– Deep discharges and high cycle depth in heavy-use fleets
– High mileage, high energy throughput, and aggressive driving patterns
Good charging practices and thermal management can slow degradation but cannot eliminate it.
SoH and Charging Strategy
SoH can shape how vehicles and fleets charge:
– Aged batteries may have lower peak acceptance and reduced effective charging speed
– Managed charging can reduce thermal stress by avoiding unnecessary high-power sessions
– Fleets may schedule charging to optimize uptime and preserve battery life
– Charging targets (not always 100%) can reduce time spent at high SoC
For AC sites, SoH typically affects overall energy and range more than it affects charger sizing, but it still influences session duration and predictability.
SoH in Reporting and Fleet Operations
SoH is often used for operational decision-making:
– Maintenance planning and early detection of underperforming vehicles
– Warranty claims and condition assessments
– Battery lifecycle tracking for second-life or recycling pathways
– Total cost of ownership calculations and replacement forecasting
Related Glossary Terms
State of Charge (SoC)
Battery Management System (BMS)
Charging Curve
DC Fast Charging
Managed Charging
Battery Degradation
Battery Warranty
Range Anxiety