State of Charge (SOC)
The estimated percentage of usable battery charge remaining, serving as an electric vehicle's fuel gauge.
Definition
State of charge expresses remaining charge as a percentage of usable battery capacity. Stored charge cannot be measured directly, so SOC is inferred from voltage, current and temperature. Aging, temperature, discharge rate and rest time all affect accuracy. Treat the displayed value as an estimate, not a direct measurement.
Why SOC Must Be Estimated
Coulomb counting integrates current and works well over short periods, but sensor offsets accumulate and require correction. Open-circuit voltage can indicate SOC after sufficient rest, which normal operation rarely allows. Lithium iron phosphate has a particularly flat midrange voltage curve. Systems combine methods, often using state estimators such as Kalman filters.
Factors Affecting Accuracy
Cold raises resistance and reduces deliverable energy. Heavy current temporarily depresses terminal voltage, producing apparent charge drops that can recover after unloading. Aging reduces capacity; an outdated capacity reference makes percentages misleading. Repeated shallow cycling may also limit opportunities to recalibrate the estimate.
SOC, SOH and DOD
SOC describes charge remaining now. SOH describes long-term capacity or power retained relative to new condition. Depth of discharge records the fraction used during a cycle. SOH determines the capacity reference, SOC the remaining fraction and DOD the cycle's usage. SOH is particularly important for replacement and residual-value decisions.
Fleet Operations
Fleet-wide curves reveal deep discharge, coincident charging peaks and unusually high consumption among comparable trucks. Excess consumption can prompt investigation of dragging brakes, low tyre pressure or driving habits. Combining charge and work data supports charging schedules and battery sizing for continuous shifts.
Battery-Life Practices
Avoid repeated deep discharge, excessive current and uncontrolled temperature. Follow manufacturer heating and current limits for cold charging because lithium plating can cause damage. Store at the recommended charge level and check periodically. Manufacturer-prescribed full charging can help recalibrate SOC; avoid unnecessary deep discharges.
Frequently Asked Questions
Why can a truck lose power while the gauge still shows charge?
Cold, reduced capacity and estimation error can all contribute. Aged capacity references or a flat lithium iron phosphate voltage curve can make the display optimistic. SOC alone does not establish available power.
How do SOC and SOH differ?
SOC changes with charging and use. SOH tracks longer-term deterioration and sets the usable-capacity reference, informing replacement timing and battery value.
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.