Current and voltage are the two fundamental electrical quantities that determine how power is delivered in EV charging systems. Amperage is simply another word for current, measured in amperes (A). Understanding these terms is essential for sizing cables, protection devices, and chargers, as well as for interpreting charger power ratings (kW).
What Is Voltage?
Voltage (V) is the electrical “pressure” that pushes electric charge through a circuit. In EV charging, voltage determines:
– Whether a system is single-phase or three-phase
– The maximum potential power that can be delivered for a given current
– Compatibility with local grid standards and equipment ratings
Voltage is measured in volts (V).
What Is Current / Amperage?
Current is the rate at which electric charge flows through a conductor. Amperage refers to the amount of current and is measured in amperes (A). In EV charging, current is important because it:
– Drives heating in cables and contacts (higher current = more heat)
– Determines breaker sizing and conductor cross-section (mm²)
– Defines how much power an AC charger can deliver at a given voltage
– Is often the limiting factor in AC charging (e.g., 16 A vs 32 A)
High current increases losses and heat through I²R (current squared times resistance).
How Voltage and Current Relate to Charging Power (kW)
Charging power depends on both voltage and current:
Single-Phase AC (Typical Home)
– Power (kW) ≈ Voltage (V) × Current (A) / 1000
Example idea: higher current or higher voltage increases kW.
Three-Phase AC (Common in Commercial EU Sites)
– Power (kW) ≈ √3 × Voltage (V) × Current (A) / 1000
Three-phase systems deliver more power per phase at the same current, which is why 11 kW and 22 kW AC charging are common in commercial installations.
DC Charging
In DC fast charging, the charger delivers DC directly to the battery. Power is still:
– Power (kW) ≈ Voltage (V) × Current (A) / 1000
But voltage varies with battery state of charge and pack design, and current is controlled by the charger based on vehicle requests and limits (CC/CV phases).
Why These Terms Matter in EV Charging Projects
Understanding current and voltage helps with:
– Selecting the right charger rating (7.4 kW, 11 kW, 22 kW, etc.)
– Correctly sizing cables and circuit breakers
– Avoiding overheating from poor connections (contact resistance)
– Designing safe load management and load balancing strategies
– Explaining why charging speeds differ between vehicles (onboard charger limits, BMS limits)
Common Misunderstandings
– “Amperage” is not a separate concept from current—it is current measured in amps
– A charger’s kW rating is not guaranteed if site power caps or vehicle limits reduce current
– Higher voltage does not automatically mean faster charging if the current is capped
– AC charging is often limited by the vehicle’s onboard charger, not just the station rating
Related Glossary Terms
Power (kW)
Single-Phase vs Three-Phase
Conductor Cross-Section (mm²)
Circuit Breakers
Contact Resistance
Load Balancing
AC Charging
Direct Current (DC)