Types of chargepoint - AC or DC charging and smart management

When charging an electric vehicle (EV), the type of charger used affects both the charging process and the time taken to recharge the battery.

EV batteries are charged using either alternating current (AC) or direct current (DC) electricity. While electricity from the grid is supplied as AC, EV batteries store energy as DC.

AC chargers are the simplest type of charging system. They rely on the vehicle’s onboard charger to convert AC electricity into DC electricity and manage the charging process to protect the battery.

DC charging systems are typically more advanced and deliver power directly to the vehicle’s battery, bypassing the onboard charger. To do this safely, the charge point must communicate with the vehicle and monitor factors such as battery capacity and state of charge (SoC).

The charging process 

You need to consider the non-linear nature of electric vehicle (EV) charging.

For example, if a vehicle returns to a 7.4 kW charger and requires 74 kWh of energy to recharge its battery, it may take longer than 10 hours to reach a full charge.

The charging rate decreases as the battery approaches full capacity. When the battery has a low state of charge (SoC), internal resistance is relatively low, allowing energy to be transferred more quickly. As the battery reaches around 80% to 90% SoC, internal resistance increases, reducing the current flowing into the battery.

To maintain the charging rate, the charging system increases the voltage. However, there is a maximum voltage limit that cannot be exceeded without risking damage to the battery. Once this limit is reached, the voltage remains constant while the current continues to decrease as resistance increases.

A constant voltage combined with a falling current results in a reduction in power output (kW), causing the charging rate to slow down. This is why a vehicle that requires 74 kWh of energy may take around 12 hours to fully charge using a 7.4 kW charger.

An illustration of a car with battery charging