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Summary: Estimating the charging time of electric vehicles

1. The importance of short charging time
   • Reducing charging time is crucial for the adoption of electric vehicles (EVs), as users of cars with internal combustion engines (ICE) are accustomed to refueling their vehicles in minutes.
   • To understand the factors affecting charging time, it is necessary to be familiar with the basic equations related to this.
2. Charge/Discharge Rate (C-rate)
   • Definition: The charge/discharge rate describes the amount of current entering or exiting the battery relative to the battery's nominal capacity in ampere-hours (Qnom).
   • Explanation:
     • When a battery is discharged at a rate of 1C1C, it completely discharges its nominal capacity within one hour.
     • When discharged at a rate of 2C2C, the nominal capacity is discharged in half an hour.
   • Maximum rate: The maximum charge rates of currently available lithium-ion cells range between 3C3C and 6C6C, with high-power cells designed for continuous discharge at rates between 2C2C and 25C25C.
   • The equation: C-rate=IbattQnom
3. Power and current during charging
   • Equation: Charging capacity can be estimated using the following relationship:
   •  Pch=Vbatt⋅Ich
     • where:
       • Pch: Charging capacity (watts).
       • Vbatt: Battery voltage (volts).
       • IchI: Charging current (amps).
   • Explanation:
     • High currents lead to faster charging but may cause the battery or charger to overheat and reduce their lifespan.
     • The maximum charging capacity depends on the voltage and the maximum current that the battery can withstand or that the charger can provide.
4. Power and energy input during charging
   • Definition: The energy input to the battery is the integral of power over time. If the power is constant, the simplified equation can be used:
   •  Ech=Pch⋅tch
     • where:
       • Ech: energy (kilowatt-hours).
       • Pch: average power (kilowatts).
       • tch: charging time (hours).
   • Example: A 10-kilowatt charger for two hours adds 20 kilowatt-hours of energy to the battery.
5. Charging rate (C-rate)
   • Definition: The charging rate is the ratio of charging power to the nominal capacity of the battery (in kilowatt-hours):
   •  C-rate=Pch/EnomC
     • where:
       • Pch: charging power (kilowatts).
       • Enom: nominal capacity of the battery (kilowatt-hours).
   • Clarification: As the current increases, the CC-rate increases, but this increases battery losses and overheating, which reduces its lifespan.
6. Notes on units: power and energy
   • Explanation:
     • Energy: The time integral of power. Or simply, it is the product of power and time when power is constant: Energy=Power×Time
     • Units:
       • Energy: joules (J), watt-hours (Wh), or kilowatt-hours (kWh).
       • Power: watts (W), kilowatts (kW), or horsepower (HP).
     • Simple conversions:
       • 1 kilowatt (kW) = 1000 watts (W)
       • 1 horsepower (HP) = 745.7 watts
       • 1 watt-hour (Wh) = 3600 joules (J)
       • 11 kilowatt-hours (kWh) = 1000 watt-hours

Typical examples of electric vehicles and charging capacities

In the table, you can find a list of various electric vehicles, their maximum charging power in kilowatts, single or three-phase charging, battery capacity in kilowatt-hours, range in kilometers, and the option for charging outside the vehicle. Not all cars can do charging. It is important to take this into account when building the charging infrastructure. The list is not exhaustive but provides an overview.

Number of electric vehicles and charging infrastructure

As shown in the figure, the deployment of electric vehicle charging infrastructure begins with slow charging infrastructure at several locations. At some point, the first fast charging stations are added to the charging infrastructure, and the number of slow charging points peaks. The widespread deployment of electric vehicles can begin when there is a reliable network of fast and slow charging infrastructure.