The principle of electric vehicle charging
The essence of electric vehicle charging is storing electrical energy in the vehicle's battery. During the charging process, electrical energy is converted into chemical energy that the battery can store through charging equipment (such as a charging station). Battery capacity is usually measured in "kilowatt-hours (kWh)," which indicates the amount of electricity the battery can store.
Currently, there are two main types of electric vehicle charging interfaces:
- AC charging: Typically used in home charging stations or slow charging stations, with lower charging power (3.7kW-22kW), suitable for charging overnight or for use when parked for a long time.
- DC charging: Commonly found in public fast charging stations, it has high charging power (50kW-350kW) and can charge the battery with a large amount of power in a short time, making it suitable for long-distance travel or emergency charging.
Charging speed depends not only on the charging station's power, but also on the battery's condition, temperature, and the vehicle's charging management system. For example, battery charging efficiency decreases in low temperatures, while excessively high battery temperatures during fast charging may trigger protection mechanisms, reducing charging speed.
Charging is a Chinese word that refers to the process of replenishing the power of devices such as batteries. Its principle is to allow direct current to pass through in the opposite direction of discharge so that the active materials in the battery can be restored. It has extended meanings such as studying and working. In September 2022, scientists at Harvard University developed a new type of solid-state lithium metal battery for electric vehicles (EVs), which is expected to be fully charged in 3 minutes [7].
The charging process of an electric vehicle is essentially the transfer of electrical energy from the power grid to the vehicle's battery. The battery is the "heart" of the electric vehicle, responsible for storing electrical energy and driving the motor. During charging, current flows through the charging device into the battery, where internal chemical reactions convert electrical energy into chemical energy for storage. During discharging, the chemical energy is converted back into electrical energy, propelling the vehicle.
Electric vehicle batteries typically use lithium-ion batteries due to their high energy density, long lifespan, and high charging efficiency. During charging, the Battery Management System (BMS) monitors the battery's status to ensure a safe and efficient charging process. There are three main charging methods for electric vehicles: home charging, public charging, and fast charging. Each method has its own advantages and disadvantages and is suitable for different scenarios.
Home charging is the most common charging method, suitable for overnight charging or use when parking for extended periods. Home charging typically uses alternating current (AC), resulting in lower charging power but longer charging times.
- Charging equipment: Home charging station (wall-mounted or portable) or ordinary power outlet (adapter required).
- Charging time: It usually takes 8-12 hours to fully charge, depending on the battery capacity and charging power.
- Advantages: Low cost, convenient, and minimal impact on battery life.
- Disadvantages: Slow charging speed, not suitable for emergency charging.
Public charging stations are typically installed in public places such as shopping malls, parking lots, and gas stations, suitable for temporary charging or long-distance travel. Public charging stations have higher power outputs, resulting in faster charging speeds than home charging.
- Charging equipment: Public AC charging piles or DC charging piles.
- Charging time: AC charging takes 4-6 hours, DC charging takes 30-60 minutes (to 80%).
The time required to fully charge an electric vehicle varies depending on battery capacity, remaining charge, and charging power, but it typically takes more than 5 hours, approximately 5 to 8 hours in summer and 8 to 10 hours in winter. It's worth noting that after the charger switches to the green light, it still needs to perform a low-power float charge for 1 to 2 hours.
Electric bicycles generally use a three-stage charging mode: first, constant current and voltage-limited charging with a fixed current; then, constant voltage and current-limited charging with a fixed voltage; and finally, trickle charging, which is maintenance charging at low power until charging is complete.
Initially, the charger indicator light is red because the voltage has not reached the rated value. As charging progresses, the voltage gradually rises and reaches the rated value, at which point the indicator light turns green. During the charging process, the power gradually decreases, eventually completing float charging at a low power level in 2 to 3 hours.
If constant current charging is continuously used, the charger will increase the voltage to maintain a stable current, which may lead to overcharging and damage to the battery. Conversely, if constant voltage charging is used for a long period, the charging current will be too high, causing the battery to heat up, the electrolyte to boil, and the internal chemical reactions to become more intense, posing a risk of explosion.
In addition, overcharging is also a concern. Prolonged charging can cause the battery to overheat, swell, and even explode. Therefore, it is recommended to charge at designated charging sheds or outdoor charging stations, as modern charging stations typically have an automatic shut-off function when fully charged, effectively protecting the battery.
On the other hand, over-discharging can also damage the battery. Recharging a battery after it has been completely depleted will cause irreversible damage. Therefore, it is recommended to start charging when the battery level drops to approximately 20% to extend battery life.
