Because the electric car has high voltage, the suspension operation of the system voltage must first be performed in accordance with the high-voltage operation regulations before servicing the electric car. After the high pressure of the system is stopped, it can be ensured to a certain extent that there is no high pressure between various parts of the high-voltage system of the automobile, thereby ensuring the safety of maintenance personnel.
When repairing cars, it is necessary to treat them differently according to the form of high-voltage electricity. For example, the power battery of pure electric cars will always have high voltage, so whenever the power battery is repaired, it is necessary to wear personal safety protection equipment. When the system no longer has high-voltage electricity, the maintenance of these components can no longer prevent the danger of being injured by high-voltage electricity.
1. The existence of high voltage in electric cars
The high-voltage system of electric cars is concentrated in the car’s drive system, air conditioning and heating system, power supply system and charging system with plug-in function. According to the time of high voltage existence, there are three main forms of high voltage in the high voltage system of electric cars, namely, continuous existence, existence during operation, and existence during charging.
The power battery of an electric car continues to have high voltage. Even when the car stops running, since the power battery always stores electrical energy, the component will continue to discharge when the discharge conditions of the power battery are met.
(2) Exist during operation
A component with high voltage during operation refers to a component with high voltage when the ignition switch is ON, RUN or other operating states.
There are two types of systems or components that are subject to high pressure during operation.
① As long as the ignition switch is in the ON or RUN state, there will be high voltage, such components include inverters, DCDC converters and connected high-voltage wires.
②Although the ignition switch is in the ON position, because the function performed by the system is not turned on, the related components will still not have high voltage. Such as high-pressure compressors and PTC heaters in pure electric cars, when the driver is not running the car’s air conditioning or heating functions, there will be no high pressure on these components.
(3) Exist during charging
The presence of high voltage during charging is mainly for plug-in hybrid and pure electric cars. The on-board charger of such cars and the connected wires will only have high voltage during charging when the car is connected to an external 220V (or 380V) grid.
It should be noted that the on-board charger and power battery of some cars are designed with an independent air-conditioning cooling system. During the charging of the car, since the power battery may generate high heat, the on-board air conditioner will operate to reduce the temperature of the power battery. At this time, the high-voltage compressor of the car is also running during charging, and there is also a high voltage.
2. Turning on and off of high voltage
In an electric car, except for the power battery, other components are controlled by the car control unit or the hybrid control unit through the contactor to control the on and off of the high-voltage electricity.
The contactor is a high-power relay, which is used to control the connection and disconnection between the positive and negative wires of the high-voltage wire. The contactor is usually arranged inside the power battery pack assembly or independently in a BDU (distribution box). ), if the contactor is disconnected, only the power battery of the car will have high-voltage electricity, and the high-voltage system components located downstream of the contactor will have no high-voltage electricity.
(1) Conditions for the contactor to be turned on
①The ignition switch is set to ON.
② The self-check of the high-voltage system did not find any faults such as leakage.
(2) Conditions for contactor disconnection
① Turn the ignition switch to OFF.
②The high-voltage system detected a safety accident.
The system self-checks that there is a security accident, mainly based on the inspection procedures set by itself. In the following cases, the high voltage will be automatically cut off due to abnormal conditions to avoid electric shock.
a. The high-voltage system self-checks that the interlock switch of the component is disconnected.
b. The high-voltage system self-checks that the components or the high-voltage cable has too low insulation resistance to the car
c. The car has collided and the airbag has been deployed.
3. Manually cut off the high voltage of the power battery
On the power battery, according to the national new energy car safety standards, a series manual maintenance switch will be designed to manually cut off the circuit of the entire power battery.
When the switch is turned off, the high-voltage components of the whole car will no longer have high voltage, and the positive and negative terminals of the total output of the power battery will no longer have high voltage.
It should be noted that even if the manual switch is turned off, the battery in the power battery and its connection circuit still have high voltage in series.
In addition, because the manual maintenance switch can physically cut off the high-voltage circuit of the power battery directly, car manufacturers will design a special locking structure for the switch to avoid accidental triggering or disconnection due to vibration and other factors during driving. The disconnection method of the manual maintenance switch is generally marked on the switch or in the owner’s user manual. Figure 1 shows the disconnection method of the manual maintenance switch for a typical pure electric car.
4. Suspension and inspection of high-voltage systems
Before repairing an electric car with high voltage, be sure to perform the suspension and inspection of the high voltage to avoid electric shock caused by accidental high voltage.
The operation steps of the suspension and inspection of the high-voltage system are mainly divided into two parts: the suspension of the high-voltage and the inspection of the high-pressure.
(1) Suspension of high pressure
High-voltage abort is mainly to shut down the car’s high-voltage system through correct operating procedures. Under normal circumstances, after the high-voltage abort is executed, the other parts of the car should not have high-voltage except the power battery.
The basic steps for high pressure abort are as follows.
①Turn off the ignition switch. After turning off the ignition, put the key in a safe area, usually away from the car being serviced.
② Disconnect the negative terminal of auxiliary battery. Locate the 12V auxiliary battery, disconnect the negative terminal of the battery, and secure the ground wire to prevent moving the negative battery terminal.
③ Remove the manual maintenance switch. Locate the service switch and disconnect it.
Be sure to wear insulating rubber gloves when handling orange high voltage components and wiring. Put the removed service switch in a pocket to prevent someone from fitting it back into the car, and cover the exposed service switch slot with insulating tape.
④Wait for 5 minutes. After removing the maintenance switch, you must wait for 5mn to fully discharge the capacitors in the high-voltage components before continuing the high-voltage inspection of the car.
(2) Inspection of high pressure
The high-voltage inspection is to use a digital multimeter to confirm that the high-voltage is stopped again, and then check whether the specific repaired components no longer have high-voltage. This step meets the high-voltage inspection operation standard.
Use a multimeter to measure each high-voltage terminal of the high-voltage component connector. After the high-voltage termination is performed, the voltage of each terminal to the body should be less than 3V, and the voltage between the positive and negative terminals of the terminal should also be less than 3V.
If any of the measured voltages exceeds 3V, it means that there is high-voltage bonding inside the system, which needs to be handled by specially trained engineers.