The High Voltage Design in Battery Electric Vehicles

1. High Voltage Warning Sign

High-voltage warning signs are used to warn car owners not to touch them for safety reasons. Battery electric vehicles usually use two forms of high-voltage warning signs, including high-voltage marks/labels and high-voltage warning colors.
High voltage mark and label are installed on high-voltage components, with clear indication that they are not safe for people to touch. After-sales service personnel can intuitively see the possible dangers of high voltage through the mark. The marks used are based on the international standard of hazard sign warning of dangerous electrical voltage.
  The high-voltage warning color of new energy vehicles mainly refers to yellow, which has been widely used in many countries as an indicator for high voltage equipment. All HV electrical cables, wiring harness and high voltage connectors are colored as orange.
Orange is the color of danger and caution, a bright and easily recognizable color that can be seen from a distance. This makes it ideal for safety purposes as workers can spot any potential hazards before they get too close to them or become injured by them. It is used for traffic signs to warn motorists of hazards or dangerous conditions ahead. Orange also represents the color of fire, which relates to electrical shock hazards associated with high voltage electrical equipment. Extension reading: Why are high voltage cables orange?
The use of high-voltage warning signs and colors can effectively prevent accidental contact with high-voltage systems by maintenance personnel or car owners during maintenance work or after service.
 

2. High Voltage Safety Design for New Energy Vehicles

In our previous article, we know that potential safety hazards of new energy vehicles include high-voltage system short circuit, high-voltage system insulation failure, high-voltage system disconnection, high-voltage charging risk, etc. According to these potential safety hazards and actual driving conditions, the safety design of electric vehicles is mainly from the following aspects, as shown in the figure:

2.1 Maintenance Safety

Maintenance safety of hybrid and electric vehicles is mainly to prevent high voltage electric shock. Most electric cars are designed with a manual service disconnect (MSD) on the system, which can be easily operated by hand to isolate the battery from the power supply. The MSD is placed in the following positions: 1) The outer box of the battery pack; 2) Integrated in the high-voltage box (PDU). If multiple battery packs are involved in a commercial vehicle, multiple MSDs may be used.
When performing major overhaul, maintenance, or replacement of high voltage components, it is necessary to turn off the MSD switch to ensure the safety of both personnel and vehicle.
Tips: Disconnect the power transmission of EV’s battery pack. Wait for another 5 minutes before proceeding further or touching HV parts.
  It is also necessary to pay attention to whether there are any warning signs on the car body or dashboard before opening or disassembling any component part. If there are warning signs indicating danger, they must be carefully followed; otherwise there may be hidden dangers when working on them.
 

2.2 EV Crash Safety

When an electric vehicle collides, the vehicle should protect the passengers from damage to the extent possible during and after the collision.
In order to achieve this goal, the EV safety system must meet the following requirements:
The structure of the vehicle must be strong enough to withstand external impacts and distribute forces evenly throughout its body. If a collision occurs at high speed, it is preferable that there is no deformation or damage to the passenger compartment. The impact absorption mechanism must react quickly enough to ensure that passengers do not suffer from whiplash injuries or other serious injuries due to excessive acceleration or deceleration. The strength and stiffness of all components should be sufficient so that they can withstand collisions with other vehicles or objects without suffering any significant damage.
◆ The battery pack must be protected against fire, explosion and other hazards caused by high-speed collisions.
◆ During and after collision, there should be no electrical hazards caused by short circuits or static electricity discharge on sensitive components such as touch screens and displays; this refers mainly to power supply lines between high voltage components in electric vehicles.
For this reason, some EVs are designed with a circuit as shown in the figure: The inertia switch is connected in series to the power supply circuit of the high-voltage contactor. When a collision occurs, the inertia switch is disconnected, thereby cutting off the power supply of the contactor. At this time, the high-voltage output of the power battery will be disconnected, ensuring the high-voltage safety of passengers, pedestrians, maintenance and rescue personnel.
 

2.3 Electrical Safety

In order to ensure the electrical safety of battery electric vehicles, some EVs are designed with the following safety devices:
◆ Water-proof and anti-dust high voltage connectors
The high voltage connector is a waterproof connector for interconnection of high voltage battery and other EV components. They should be withstand water pressure from any direction without leakage. The high voltage connector is also dustproof so that it does not get damaged by dust during transportation or storage. In addition, this type of connector can pass through all kinds of tests, such as vibration test, heat test and so on.
  ◆ A high-voltage contactor is designed between the power battery and the external high-voltage circuit
The contactor can be opened at any time, and is controlled by an automatic control device. When it is opened, it will stop charging or discharging, ensuring that there is no danger of electric shock when charging or discharging.
  ◆ Pre-charging circuit
The pre-charging circuit prevents the charger from applying current to a vehicle that has not been fully discharged. It mainly consists of precharge resistor. The resistor limits current flow until the battery reaches a high enough voltage level to allow current to pass through the compensation capacitance.
PS: compensation capacitor is designed between the positive and negative poles of the high-voltage components

◆ Insulation resistance detection system
This system monitors the insulation degree of the automotive high voltage cable relative to the vehicle chassis, by dividing the voltage between the positive power cable & chassis, and the negative power cable & chassis. To simplify the structure, the insulation resistance monitoring module is usually set in the power battery system, and the insulation resistance monitoring function is integrated into the BMS. Necessity: The positive and negative poles of the power battery form a current loop through the insulating layer and the chassis. When the insulation of the vehicle decreases, the leakage current will increase. When the leakage current reaches a certain value, it will endanger the safety of passengers and the normal operation of the electrical system of the vehicle.

◆ Short circuit protector
It is used to protect against an accidental short circuit in the high voltage system. A short circuit occurs when a path from one pole of a battery to another is accidentally created by physical contact or by a broken wire. When such a path is created, excessive current flows through it and can cause damage to the battery and its components. The short circuit protector detects this condition and shuts down or limits current flow until the problem has been corrected.

◆ High voltage interlock loop design
The high-voltage interlock design can monitor the continuity of the high-voltage circuit, and send an alarm signal to the VCU before the high-voltage power failure, so that the VCU can take countermeasures for the entire vehicle system.
Another risk existing in electric vehicles is human misoperation during system operation and manual disconnection of high voltage automotive connectors. The high-voltage interlock design can disconnect the output of the power battery at the same time when the connector is disconnected to avoid electric shock.
 

2.4 Function Safety

◆ Torque Safety Management
In order to prevent the unexpected movement of the vehicle, it is necessary to add a torque safety control strategy in the VCU. The specific strategy is as follows:
① The VCU is responsible for calculating the torque demand. If the difference between the calculated torque demand is greater than a certain calibration value, it is considered that there is a safety risk in the torque output. At this time, the VCU will limit the vehicle speed to a safe range.
② If the difference between the required torque of the VCU and the actual torque of the motor is greater than a certain calibration value, it is considered that the torque control of the motor is at risk, and the VCU will limit the torque output of the electric motor. If the difference between the two is always too large, the power transmission of the power battery will be cut off.
  ◆ Charging Safety
When charging, it is necessary to prevent the vehicle from moving, and avoid conflicts between fast charging, slow charging, and driving modes. For this purpose, the following designs are made:
① Charging is allowed only when the gear is in the P position.
② During the charging process, both the torque demand and the actual torque output should be 0.
③ When the EV charging cable is plugged in, it is not allowed to close the contactor that controls the high voltage output.
④ When the insulation resistance of the charging circuit is lower than the resistance value required by the standard, the charging should be stopped and the high-voltage contactor should be disconnected.