Charge tapering is the gradual reduction of charging power (kW) as an EV battery approaches a higher state of charge (SoC). It is primarily controlled by the vehicle’s Battery Management System (BMS) to protect the battery from overvoltage and overheating, which is why charging slows significantly from around 80% to 100%.
What Is Charge Tapering?
Charge tapering happens when an EV reduces the charging current as the battery voltage rises. It is most commonly associated with the constant voltage (CV) stage of a CC-CV charging profile, where:
– The charger holds the voltage at a limit
– The current decreases over time
– Power drops as current tapers down
Tapering can occur in both AC and DC charging, but it is more visible in DC fast charging because power levels are higher.
Why Charge Tapering Matters in EV Charging
Charge tapering affects both user experience and charging network performance. It matters because it:
– Increases session time at high SoC, reducing charger throughput
– Impacts queueing and bay turnover at public fast-charging sites
– Affects fleet scheduling and depot readiness planning
– Changes the economics of time-based pricing and idle fee policies
– Explains why “charging speed” is not constant during a session
– Helps operators reduce misdiagnosed “slow charger” complaints
For CPOs, tapering is one of the main reasons why charging to 100% at a DC fast charger can be operationally inefficient.
What Causes Charge Tapering
The EV’s BMS reduces charging power based on:
– High SoC and cell voltage limits
– As cells approach their maximum voltage, current must drop to prevent overvoltage
– Battery temperature
– If the pack heats up, the BMS may reduce current to stay within battery thermal limits
– Battery condition and aging
– Higher internal resistance can increase heat generation, causing earlier tapering
– Cell balancing needs
– The BMS may taper to allow cell balancing near full charge
– Battery chemistry and pack design
– Different chemistries and thermal systems have different taper behavior
– Charger or site constraints
– Active power throttling, load sharing, or site caps can reduce power independently of vehicle tapering
How Charge Tapering Appears in Real Charging Sessions
Common patterns include:
– High power at low-to-mid SoC (fast energy delivery early)
– A transition point where power begins to drop (vehicle-dependent)
– A steep taper above a certain SoC (often around 70–80% for many EVs, but not universal)
– Extended time from high SoC to full charge, even though the kWh added is relatively small
Tapering behavior varies significantly across EV models, battery temperatures, preconditioning, and charger types.
Typical Use Cases
– Public DC charging hubs optimizing bay turnover and pricing policies
– Fleet depots planning when to stop charging vehicles (target SoC vs full)
– Customer support diagnosing “slow charging” reports
– Analytics distinguishing EV-limited sessions from charger-limited sessions
– Energy and capacity planning where peak power demand is influenced by taper behavior
Key Benefits of Charge Tapering
– Protects battery health by reducing high-voltage stress near full charge
– Improves safety by limiting overtemperature and overvoltage risk
– Enables predictable, controlled charging across battery chemistries
– Supports longer battery lifetime through reduced degradation of drivers
Limitations to Consider
– Extends charge time at high SoC, reducing operational efficiency at fast chargers
– Can frustrate users expecting constant “headline” kW performance
– Makes session duration harder to predict without vehicle-specific curves
– Can be compounded by cold weather and thermal constraints
– Site-level throttling can be mistaken for tapering without proper diagnostics
Related Glossary Terms
CC-CV Charging Profile
Charging Curve
State of Charge (SoC)
Battery Management System (BMS)
Cell Balancing
Battery Thermal Limits
Battery Aging
Charge Acceptance Rate
Active Power Throttling
DC Fast Charging