Why are the charging power of some 7kW AC charging piles only 6.1kW?

This is a very common after-sales question. In fact, for some models and in some cases, the charging power of 6.1kW cannot be achieved.

 

This is a very common after-sales question. In fact, for some models and in some cases, the charging power of 6.1kW cannot be achieved.

 

The 7kW of the 7kW AC charging station (hereinafter referred to as the AC station) is a general term, and the charging power is not necessarily 7kW. The AC station sends the AC power of the grid to the on-board charger (OBC) in the vehicle through the AC relay. The AC station does not change the power size, and the power size is determined by the OBC. The output power of the OBC is equal to the input power of the OBC multiplied by the efficiency of the OBC. The input power of the OBC is equal to the grid voltage multiplied by the output current of the AC station. The output current of the AC station is also the input current of the OBC. The OBC limits the input current to 32A.

All factors that may affect the output power of the 7kW AC station are as follows. These influencing factors may not just cause the charging power to drop to 6.1kW, but may also drop to 5kW or 3.3kW.

 

1. The AC station itself is derated

 

Overheating inside the AC station will trigger derated. For example, if the connection at the charging station terminal connector is loose, the contact resistance will increase, which will cause greater loss. The heat generated by the loss is transferred to the temperature sensor inside the charging station through the copper bar; the ADC sampling port of the AC station main control chip samples the temperature value and makes a judgment, the PWM duty cycle of the CP signal will become smaller, and the corresponding OBC output power will decrease.

 

2. Problems with the on-board charger (OBC) in the vehicle

 

The impact of the power grid as an input source on the performance of the OBC

OBC has a hardware circuit to detect the input current, and the software of the OBC limits the input current to 32A. If the input voltage of the OBC is less than 220V, for example, only 200V, then the output power of the OBC is only 200V32A95% (efficiency) = 6.08kW.

In fact, if the distance from the distribution room to the charging station is 100 meters, the voltage drops by about 19V with a copper core cable; the voltage drops by about 30V with an aluminum core cable. Considering that the efficiency of the OBC is about 95%, the corresponding charging power is only 6.1kW and 5.7kW. If the wiring of the circuit breaker is loose, resulting in excessive contact resistance, the voltage will be significantly reduced even if the distance from the distribution room to the AC station is short, and the OBC power will also decrease.

The impact of vehicle battery as a load on OBC performance

OBC is a switching power supply, which belongs to the category of rectifiers and converts AC power into DC power. As a switching power supply, there is a "load effect".

As the charging time increases, the SOC gradually increases, and the corresponding battery voltage gradually climbs. Within a certain voltage range, the charging power is constant, the so-called constant power voltage range. Outside a certain voltage range, the charging power decreases. OBC has its own VI curve. Some OBCs have a constant power range of 300V-450V (400V battery platform vehicles). When the battery voltage is lower than 300V, charge according to the current corresponding to 300V, for example, charge at 22A, then when the battery voltage is 280V, the charging power is only 6.16kW (280Vx22A=6.16kW); when the battery voltage is higher than 450V, the power will also decrease.

Impact of vehicle air conditioning/PTC and other electrical equipment on charging power

In extremely cold weather, such as below -5℃, the battery pack needs to be heated before charging. When the battery temperature reaches a certain temperature, it is heated while charging. Part of the OBC output power is consumed by the PTC, and the charging power cannot reach full power. Generally, the battery heating power ranges from 1kW to 3kW.

Vehicle OBC is only 3.3kW

Some vehicles have a built-in OBC of only 3.3kW, but the user uses a 7kW AC station, so the charging power is of course limited to the 3.3kW of the OBC.

OBC high temperature protection power reduction

If the cooling system of the vehicle OBC is damaged, such as: air cooling system failure, water cooling system failure, etc., the OBC will choose to reduce power charging or stop charging. If the OBC ambient temperature is too high, or the temperature of the internal components of the OBC such as magnetic parts and power semiconductors is too high, the OBC will choose to reduce power charging until the temperature of the internal components meets the derating requirements.

 

3. Problems with the vehicle itself

 

Some vehicles will reduce the power of charging in the following conditions, such as: the car window is open, the car key is nearby and the vehicle is unlocked, the vehicle air conditioner is turned on, and someone is inside the vehicle.

Some vehicles will reduce the power when the gun is directly plugged in to charge when the car is locked. The vehicle must be unlocked first, the gun is plugged in to charge, and then the car is locked before full power charging.

The owner has set a charging current of less than 32A in the vehicle app or vehicle dashboard

The vehicle charging socket is overheated: the problem of the charging gun head causes heat, and the heat is transferred to the vehicle socket through the copper needle. The overtemperature signal is detected by the thermocouple on the vehicle socket, so the vehicle reduces the power of charging; the charging socket itself has structural process problems (the plug socket of the leaf spring process), long-term plugging and unplugging leads to poor contact and reduced diversion area. The charging heat is detected by the thermocouple on the vehicle socket, so the vehicle reduces the power of charging.

The electronic lock on the vehicle plug socket is damaged and cannot be locked normally or the interaction between the electronic lock and OBC is abnormal. For the sake of safe charging, some vehicles will reduce the power of charging by half.