What’s the Difference between Slow and Fast EV Charging?

We know that a battery has anode and cathode, and is charged with direct current. When the grid only transmits alternating current, an on-board charger is necessary.

The on-board charger converts the alternating current from the grid into direct current to charge the battery and adjust the output voltage or current at the same time. If you find it difficult to understand the on-board charger, just look at your cellphone or computer charger which functions in the same way.

cellphone charger

Slow Charging
Due to space and cost limitations, the on-board charger in an electric car is relatively small and supports slow charging only. This process is called slow charging.

With an AC charging pile, only AC current is output to the electric car and the on-board charger has to step in to do the transmission. That’s why an AC charging pile is called a slow charging pile.

Fast Charging
If the on-board charger is placed in the charging pile, its size and cost won’t be much limited, and multiple charger modules can be stacked together for fast charging. This process is called fast charging.

In fast charging, the fast charging equipment provides DC power for the electric vehicle, so it is also called DC fast charging station.

fast charging

Not all electric vehicles support fast charging and slow charging. Only electric vehicles with high-capacity batteries support fast charging, and electric vehicles with small batteries can only be charged slowly. Generally speaking, hybrid cars can only be charged slowly, while pure electric vehicles can be charged in both ways. There are exceptions.

Charging Speed

Generally, charging power is a symbol of charging speed. The higher the power, the faster the charging speed. The power calculation formula is: Charging Power (P) = Voltage (U) × Current (I).

For example, for fast charging: voltage=350V, current=200A, then the charging power is: 350×200=70000W=70KW (kilowatt)

70KW means that it can generate 70 kwh per hour. With known power, you can calculate how much electricity the car is charged. The formula is as follows: Charged power (W) = power (P) × charging time (t).

For example: if the power of a household slow charging pile is 7KW (kilowatt), and the charging time is 8 hours, then the charging amount is 7×8=56KWh (kilowatt hour).

The time required for slow charging can be calculated in the same way:

For example: if the vehicle battery capacity is 70 kwh, and the household charging pile’s power is 7KW (kilowatt), then the time to charge from zero to full is: 70/7=10 hours.

Note: The charging power of fast charging is constantly changing, and the power gradually decreases as the power increases; while the power of slow charging is relatively small, which is basically fixed, as shown in the figure.

charging curve

Therefore, calculating the charged electricity and charging time by the formula is only suitable for slow charging, and it is difficult to accurately calculate the time of fast charging. We may have this covered in the next article.

At the same time, the charging power of the vehicle itself also has its own limitation. If the maximum charging power of the vehicle is 60KW, and the maximum power can only be 60KW even if a 120KW charging station is used.

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