Constant Current (CC) and Constant Voltage (CV) are the two primary phases used to charge lithium-ion batteries safely and efficiently. In EV charging, CC/CV behavior explains why charging is fast at lower battery state of charge and slows down near the end—often experienced as charging tapering during DC fast charging.
What Are CC and CV Charging Phases?
CC/CV is a standard charging strategy for lithium-based batteries:
Constant Current (CC) Phase
During the CC phase, the charger delivers a steady current (amps) while the battery voltage gradually rises.
– Charging power can be high because current stays strong
– Battery state of charge increases relatively quickly
– The phase typically continues until the battery reaches a target voltage limit
In EVs, this phase usually corresponds to the fastest part of DC fast charging.
Constant Voltage (CV) Phase
During the CV phase, the charger holds the battery at a fixed maximum voltage while the current gradually decreases.
– Current tapers down to protect the battery
– Charging becomes slower as the battery approaches full
– The final percentage points take disproportionally longer
This phase protects battery cells from overvoltage and reduces stress, improving safety and battery longevity.
Why CC/CV Phases Matter in EV Charging
Understanding CC/CV behavior matters for drivers, CPOs, and fleet planners because it affects:
– Real-world charging time, especially beyond ~70–80% SoC
– Station throughput and charger utilization rate at fast-charging sites
– Queueing and dwell time planning at public hubs
– Battery health and long-term performance
– Power and energy forecasting for fleet depots
For many use cases, stopping earlier (e.g., 80%) can improve overall efficiency and reduce congestion at fast chargers.
How CC/CV Shows Up in Real Charging Sessions
In a typical DC session:
– Power rises quickly and remains near peak during early charging (CC phase)
– As voltage approaches the limit, the charger transitions to CV phase
– Current decreases, causing charging tapering
– Near 100% SoC, charging power can drop significantly
The exact transition point depends on battery chemistry, temperature, and vehicle charging strategy.
What Determines the CC-to-CV Transition
Several factors influence when and how strongly tapering occurs:
– Battery chemistry and cell voltage limits
– Battery temperature (cold batteries may limit current early)
– Battery management system (BMS) rules and protection logic
– Charger power capability and cable current rating
– State of charge and battery internal resistance
EVs often manage charging aggressively to balance speed, safety, and long-term degradation.
CC/CV in AC vs DC Charging
– In DC fast charging, the charger directly controls battery current and voltage, so CC/CV behavior strongly shapes the power curve
– In AC charging, the vehicle’s onboard charger performs AC/DC conversion and manages the battery charging profile; CC/CV still applies internally, but charging power is usually lower and tapering is less noticeable until high SoC
Operational Implications for Charging Networks
For CPOs and site hosts, CC/CV behavior affects:
– Peak power demand versus average delivered power
– Session duration forecasting and pricing strategy (idle fees, time components)
– Station capacity planning and queue management
– Customer education (recommended SoC targets for fast charging)
Using charging session analytics to understand typical power curves by vehicle type helps optimize site planning and customer experience.
Common Pitfalls
– Assuming a charger’s rated kW equals delivered kW throughout the session
– Planning fleet turnaround times without accounting for tapering at high SoC
– Using DC fast charging for routine 90–100% fills, reducing throughput
– Pricing only per kWh without considering occupancy time at high SoC
– Ignoring temperature effects that can limit CC phase power even at low SoC
Related Glossary Terms
Charging Tapering
CC-CV Charging Profile
Battery Management System (BMS)
Peak Charging Power
Charger Utilization Rate
Charging Session Analytics
Direct Current (DC)
AC Charging