Understanding EV Charging: A Layman's Guide

2 September 2024

As the electric vehicle (EV) market in Malaysia grows, it's crucial for both current and potential EV users to understand the basics of EV charging. With the rise of electric cars, knowing how charging times are calculated can help users make informed decisions. In this article, we'll break down the mathematics behind EV charging.

To determine how long it will take to charge an EV, you need to understand the following:

• Onboard Charger (OBC): This is the maximum rate at which the vehicle can accept AC power from a charging station, measured in kilowatts (kW).
• Battery Capacity (BC): This is the total amount of energy the battery can store, measured in kilowatt-hours (kWh).
• Charger Power: The power output of a charging station is measured in kilowatts (kW). A 7 kW charger typically uses a single-phase connection at 32 amps, common in homes. 11 kW and 22 kW chargers require a three-phase connection, more often found in commercial settings, using 16 amps and 32 amps per phase, respectively.

Charging Speed and Onboard Charger

The speed of EV charging depends on the onboard charger inside the vehicle. This charger converts AC power from the charging station into DC power for the battery. For example, an EV with an 11 kW onboard charger will charge at 11 kW maximum, regardless of whether the charger is capable of delivering more power.

Charging Speed Limitations

If you connect an 11 kW EV to a 7 kW charger, it will charge at 7 kW, which is the charger’s limit. The charging speed is constrained by the charger's power output.

Average Battery Capacity

Measured in kilowatt-hours (kWh), battery capacity affects both driving range (km) and charging time. Examples include:

Please refer to the disclaimer below

Note: The average battery capacity and driving range of plug-in hybrid electric vehicles (PHEVs) can vary depending on the model.

Charging Time

• Where: Current Battery Charge (kWh) is calculated by multiplying the current battery percentage by the total battery capacity.

This formula helps estimate how long it will take to charge an EV based on the battery’s total capacity, the current charge level, and the onboard charger's power rating.

Example Calculation:

• If an EV has a 60 kWh battery and is currently at 20% charge, and the onboard charger is 7 kW:

Current Battery Charge: 60×0.2=12?kWh

Remaining Capacity to Charge: 60?12=48?kWh

Time taken to charge: 48 kWh/ 7 kW= 7 hours 26 minutes

Maximizing Charge Speed: The 80% Sweet Spot

During charging, EVs typically charge quickly up to about 80% of their battery capacity. In this phase, the battery is charged at a constant current, allowing for rapid charging. After reaching 80%, the charging speed slows down as the system shifts to a constant voltage mode to protect the battery and extend its lifespan. Charging up to 80% is usually the most time-efficient option because the process is quicker during the constant current phase.

Understanding Watt, Kilowatt (kW), and kWh

Watt (W): A rate at which a device generates, transfers or uses energy.

Kilowatt (kW): 1,000 watts.

Kilowatt-hour (kWh): A unit of energy, showing energy used over time. For instance, running a 1 kW device for one hour uses 1 kWh.

The charger's kW rating shows how fast it charges, while the battery's kWh rating indicates how much energy it stores or uses.        

Examples:

1. When an EV with a 22kW OBC charges at a 22kW Charger: Chargespeed: 22kW
2. When an EV with an 11kW OBC charges at a 22 kW Charger: Chargespeed: 11kW - limited by it's OBC max charge speed
3. When an EV with a 22 kW OBC charges at an 11 kW Charger: Chargespeed: 11kW - in this case, the limiting factor is the Charger's max charge speed.
4. When an EV with a 7kW OBC charges at an 11 kW Charger: Chargespeed: 3.7kW - This is because the 11 kW charger delivers 11 kW when all three phases are connected, but only 3.7 kW per phase. Since the 7 kW OBC can only accept a single phase, it will charge at a maximum of 3.7 kW.

Conclusion

Understanding EV charging helps optimize your driving and charging habits. Key points include the role of the onboard charger, the impact of different chargers, and basic electrical concepts that will enhance your understanding of EV charging. For more detailed information, visit resources like Tesla's Charging Guide, BYD Atto Charging Guide, and BMW Charging Solution Overview.

Disclaimer: The data presented in this table has been compiled from brochures and other reputable sources. While every effort has been made to ensure accuracy, please verify the information with the original sources for confirmation.