State of Charge (SoC) is the percentage measure of how much energy remains in an electric vehicle’s battery compared to its usable capacity. It is the EV equivalent of a fuel gauge, typically shown as 0–100%. SoC is a core value used by the vehicle’s battery management system (BMS) to control charging, protect the battery, and estimate remaining driving range.
SoC is different from battery “health” because it describes the battery’s current level, not long-term degradation.
Why SoC Matters in EV Charging
SoC influences charging speed, charging strategy, grid impact, and user experience:
– Determines how long a charging session will take to reach a target level
– Impacts charge power because most EVs reduce power as SoC rises
– Supports managed charging and load balancing decisions at sites with limited capacity
– Enables fleet operators to plan vehicle readiness and shift charging to off-peak windows
– Helps pricing and session policies (for example, charging targets or avoiding long bay blocking at high SoC)
For public sites, understanding SoC behavior is essential because high-SoC charging tends to be slower and can reduce throughput.
How SoC Is Calculated
SoC is estimated by the BMS using multiple signals and models, such as:
– Cell voltage and voltage curves
– Current in/out of the battery (coulomb counting)
– Battery temperature and internal resistance
– Historical data and battery calibration models
Because it is an estimate, SoC can drift over time and may be recalibrated by the vehicle under certain conditions.
SoC and Charging Power
Charging power is not constant across the session. Most EVs follow a curve:
– Low to mid SoC often supports higher power (faster charging)
– As SoC increases, the BMS gradually reduces charge power to protect battery life
– Near 100% SoC, charging becomes significantly slower
This behavior affects both AC charging and DC fast charging, but it is most visible at high-power DC sites where tapering can be dramatic.
Target SoC and Everyday Charging
Many drivers and fleets use target SoC ranges to balance convenience and battery longevity:
– Daily charging often targets a moderate SoC rather than 100%
– Full SoC may be reserved for long trips or operational needs
– Smart charging schedules can time the final SoC increase closer to departure
Targets are typically set in the vehicle or in fleet charging software, not on the charger itself.
SoC in Public Charging and Site Policies
Public networks may use SoC-related strategies to improve availability:
– Encourage drivers to stop at a practical SoC rather than “topping off”
– Apply idle fees after charging completes to free bays
– Use messaging in apps to guide drivers to the most time-efficient charge window
SoC also affects how operators predict utilization and queueing at charging hubs.
SoC Data and Interoperability
SoC can be shared across systems, but it depends on vehicle and platform capabilities:
– The charger measures delivered energy, not battery SoC directly
– SoC may be communicated through vehicle-to-charger signaling in advanced standards such as ISO 15118
– Backend platforms may display SoC when supported by the vehicle, protocol, and ecosystem integrations
In many deployments, SoC remains primarily a vehicle-side value rather than a guaranteed data point for the charging operator.
Related Glossary Terms
Battery Management System (BMS)
Charging Curve
AC Charging
DC Fast Charging
Managed Charging
Load Balancing
Idle Fees
ISO 15118