Understanding EV Charging Station Power Ratings

We know that batteries have fixed positive and negative terminals, and charging them requires the use of direct current (DC). However, the electrical grid transmits alternating current (AC). So, what’s the solution? Enter the charger – it converts AC from the grid into DC to charge the battery while adjusting the voltage and current output. Think of your phone or laptop charger; it’s essentially a mini charger.

Slow Charging Process

Most new energy vehicles come equipped with an onboard charger. Due to size and cost constraints, onboard chargers are relatively small and provide slow charging speeds, hence this process is called slow charging.

During the slow charging process, the charger in the vehicle provides alternating current (AC), which is why it’s also referred to as AC slow charging.

Fast Charging Process

When the charger is placed within the charging station, and isn’t constrained by size and cost. Multiple charger modules are used in combination, the charging speed increases significantly, and this process is known as fast charging.

During fast charging, the charging station provides direct current (DC) to the vehicle, and this is why it’s referred to as DC fast charging.

Not all new energy vehicles support both fast and slow charging. Vehicles with larger batteries typically support fast charging, while those with smaller batteries can only use slow charging. Generally, hybrid models can only use slow charging, while pure electric models can use both, with some exceptions.

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Miles Energy Product Range
Charging Speed

Charging speed is generally indicated by charging power – the higher the power, the faster the charging. Power is calculated using the following formula:

Charging Power (P) = Voltage (U) × Current (I)

For example, during fast charging: Voltage = 350V, Current = 200A, which results in a charging power of: 350 × 200 = 70,000W = 70KW (kilowatts)

70KW indicates that the vehicle can be charged at a rate of 70-kilowatt hours per hour. Using this power, you can calculate how much charge the vehicle receives, and the formula is as follows: Charging Capacity (W) = Power (P) × Charging Time (t)

For example: A typical home slow charger has a fixed power of 7KW, and if the charging time is 8 hours, the charging capacity will be: 7 × 8 = 56KWh (kilowatt-hours) = 56 kWh

You can use a similar method to estimate the time required for slow charging:

For example: If the vehicle’s battery capacity is 70 kWh, and the home charger has a power of 7KW, the time required to charge from zero to full will be: 70 kWh / 7 KW = 10 hours

Note: Fast charging power varies as charging progresses, gradually decreasing as the battery fills up. Slow charging power remains relatively constant. Detailed estimates for fast charging times will be discussed in the next article.

Various EV Charging Charging Stations
  • AC Slow EV Charger

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  • DC Fast EV Charger

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