The constant current (CC) phase is the first main stage of lithium-ion battery charging, where the charger delivers a steady current (amps) while the battery voltage gradually increases. In EV charging, this phase typically provides the fastest energy delivery and is the period when charging power is closest to the charger’s and vehicle’s maximum capability—especially during DC fast charging.
What Is the Constant Current Phase?
During the CC phase, the charging system targets a fixed current level. As the battery accepts energy, its voltage rises. Power (kW) during CC is driven by:
– Current (A) held relatively constant
– Voltage (V) increases as the state of charge rises
Because power equals voltage × current, charging power can ramp up as voltage increases, then remain near peak until limits are reached.
Why the Constant Current Phase Matters
The CC phase matters because it defines:
– The fastest part of most charging sessions
– How quickly a vehicle can gain usable range at a low-to-mid state of charge (SoC)
– Station throughput and queue performance at high-traffic DC sites
– Real-world charging time expectations for fleets and drivers
For many EVs, the best time efficiency is achieved by charging primarily within the CC-dominant portion (often up to a mid-range SoC window).
How the CC Phase Works in EV Charging
A simplified CC phase sequence looks like:
– Charger and vehicle complete handshake and safety checks
– The vehicle’s BMS requests a target current based on battery conditions
– The charger supplies that current while monitoring voltage, temperature, and limits
– Battery voltage rises as SoC increases
– The system maintains current until a voltage limit, thermal limit, or SoC threshold is reached
When the battery approaches its maximum allowable voltage, the charging system transitions to the constant-voltage (CV) phase, and current begins to decrease.
What Limits the Constant Current Phase
Even if the charger is capable of high power, the CC phase may be reduced by:
– Battery temperature (cold or hot batteries limit current)
– Battery internal resistance and aging
– Vehicle-specific charging strategy (protection and longevity rules)
– Charger capability and cable current rating
– Site power caps or load balancing constraints
This is why two vehicles on the same charger can show very different power curves.
CC Phase in DC vs AC Charging
DC Fast Charging
– The CC phase is highly visible because charging power is high
– The charger supplies DC directly and follows BMS current requests
– CC typically dominates early charging and then transitions into tapering (charging tapering) as CV begins
AC Charging
– CC/CV still applies internally, but the vehicle’s onboard charger manages it
– Power is usually capped by the onboard charger (e.g., 7.4 kW, 11 kW, 22 kW)
– CC behavior is less dramatic and tapering is often less noticeable until high SoC
Operational Implications
For site operators and fleets, CC phase behavior affects:
– Expected session duration and charger occupancy
– Peak load planning and realistic delivered power assumptions
– Pricing models (time-based components matter more as tapering increases)
– Customer guidance (e.g., fast charge to a practical SoC rather than 100%)
Using charging session analytics helps identify typical CC duration and delivered power by vehicle mix.
Common Pitfalls
– Assuming “150 kW charger” means 150 kW for the whole session
– Planning turnaround times without accounting for temperature-limited current
– Charging to very high SoC on DC, where CC ends and tapering dominates
– Overlooking site power caps that reduce current below vehicle limits
Related Glossary Terms
Constant Voltage Phase
Charging Tapering
CC-CV Charging Profile
Battery Management System (BMS)
Peak Charging Power
Charging Session Analytics
Direct Current (DC)
Load Balancing