State of Health (SOH)
A measure of battery capability retained relative to new condition, often expressed as usable capacity divided by rated capacity.
Definition
State of health describes long-term battery performance retention. Capacity percentage is common, but resistance growth or power loss may also define it. Unlike SOC, SOH changes over months and years and helps decide replacement timing, remaining usefulness and residual value. Definitions differ between manufacturers, so confirm the basis before comparing numbers.
Why Capacity Fades
Growth of the solid electrolyte interphase consumes cyclable lithium. Active-material degradation and cracking reduce storage capability; electrolyte decomposition and collector corrosion add capacity loss and resistance. Temperature, charge level and current influence these processes. Usage and storage conditions matter alongside age.
Calendar and Cycle Aging
Cycling causes wear through repeated use, while calendar aging continues during storage. A lightly used spare battery stored hot and fully charged can deteriorate substantially. Follow the manufacturer's storage SOC range and periodic checking and charging instructions rather than leaving it fully charged indefinitely.
Estimating SOH
Controlled capacity tests are direct but interrupt work. Resistance tests are quicker but need temperature and SOC normalization. Incremental-capacity analysis provides detailed aging information but requires high-quality data. Fleet systems can fit long-term capacity trends from charge quantities and voltage windows to reduce individual-measurement noise.
Fleet Decisions
Replace when usable capacity no longer supports required duty economically. Assign older batteries to lighter work and reserve healthier packs for demanding shifts. Comparable SOH trends support procurement, resale and lease conditions. Unusually rapid decline within one batch warrants investigation of manufacturing or usage differences.
Slowing Deterioration
Control heat during operation, storage and charging; avoid prolonged extreme SOC and repeated deep discharge. Use moderate charging when feasible, especially in hot conditions. Follow cold-weather preheating requirements. Maintain battery records and trend SOH so abnormal decline is identified before range becomes inadequate.
Frequently Asked Questions
Why does a barely used battery lose capacity?
Calendar aging continues even at rest, especially at high temperature and high SOC. Recommended storage conditions and periodic checks remain necessary.
At what SOH should a battery be replaced?
There is no universal number. Work backwards from required shift capacity and the cost of interruptions, and confirm how that manufacturer's SOH is defined.
Related Terms
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This glossary covers general industry concepts for technical reference and knowledge sharing. It contains no proprietary information from any specific manufacturer. For equipment specifications and operating procedures, always refer to the OEM documentation.