The constant voltage (CV) phase is the later stage of lithium-ion battery charging where the charging system holds the battery at a fixed maximum voltage while the charging current gradually decreases. In EV charging, this phase is responsible for the noticeable slowdown near the end of charging and is the main reason why charging from ~80% to 100% takes much longer than charging from lower state of charge.
What Is the Constant Voltage Phase?
In the CV phase, the battery has reached a voltage limit set by the battery management system (BMS) to protect cells from overvoltage. The charger therefore:
– Maintains battery voltage at a controlled maximum level
– Reduces current over time to keep voltage from rising further
– Continues until a target state of charge or a minimum current threshold is reached
Because power equals voltage × current, power drops as current tapers down.
Why the Constant Voltage Phase Matters
The CV phase matters because it directly affects:
– Charging time at high state of charge (SoC)
– Station throughput and queueing at fast-charging sites
– Fleet planning for turnaround times and scheduling
– Battery health and safety, by limiting stress at high SoC
Understanding CV helps drivers and operators set realistic expectations for session duration and the value of stopping earlier.
How the CV Phase Works in EV Charging
A typical transition into CV looks like:
– The battery voltage rises during the constant current (CC) phase
– When the voltage approaches the maximum allowed limit, the BMS requests reduced current
– The charger holds voltage steady while current decreases
– Charging continues with decreasing power until the end condition is met
The exact behavior depends heavily on vehicle design, battery chemistry, and temperature.
Why Current Tapers During CV
Current tapering in CV happens because:
– High SoC increases internal resistance and heat risk
– Cell voltage limits must not be exceeded
– Lower current reduces stress, improving longevity and safety
This tapering is the core mechanism behind charging tapering during DC fast charging.
CV Phase in DC vs AC Charging
DC Fast Charging
– CV is often very visible: power drops significantly as SoC rises
– Charging from ~80% to 100% may take as long as charging from ~20% to ~80% depending on the vehicle
– Managing CV time is critical for station utilization and pricing design
AC Charging
– CV still exists, but onboard charger power is lower
– Tapering tends to be less dramatic until near full charge
– The vehicle manages CV internally as part of its battery charging algorithm
Operational Implications for Networks and Fleets
– High SoC charging increases session duration and reduces charger utilization rate
– Pricing models may include time-based fees or idle fees to reduce bay blocking
– Fleet depots may schedule vehicles to stop at a practical SoC and rotate chargers
– Analytics can identify how long vehicles spend in CV and optimize operational policies
Understanding CV behavior helps set better expectations for charging session analytics and throughput planning.
Common Pitfalls
– Assuming charging remains near peak power until 100%
– Planning quick turnarounds that require charging deep into the CV phase
– Encouraging public fast charging “full fills” that reduce site throughput
– Ignoring temperature effects that can push the vehicle into tapering earlier
Related Glossary Terms
Constant Current Phase
Charging Tapering
CC-CV Charging Profile
Battery Management System (BMS)
Peak Charging Power
Charger Utilization Rate
Charging Session Analytics
Direct Current (DC)