Design and Research of Inner Harness of Power Battery Pack for New Energy Vehicles

In recent years, with the growing development of new energy vehicles, they have actively responded to the national energy conservation and emission reduction, promote the use of new energy, and promote the development of circular economy. The automakers have launched their own new energy automotive products, including pure electric vehicles and hybrid vehicles. Further, with the gradual improvement of technology, it has tended to replace conventional fuels with electricity as a source of power for automobiles.

For the design and research of the harness of power battery packs for new energy vehicles, there are various design problems and new design concepts. The battery pack inside the battery pack is used as the signal transmission of the power battery to achieve the effective output of power, and the power inside the battery pack. Battery power, cruising range, etc. are effectively monitored. In the design process, it also faces the design tests, design layout, EMC protection and other design aspects.

First, the analysis of Wire Harness design

At present, the new energy vehicle power battery pack determines the quantity and structure of the battery pack module and the unit according to the previous design goals. The battery pack cooling mode is divided into an air-cooled and a water-cooled battery pack. For the inside bundle design, different from the traditional gasoline vehicle complete Wiring Harness, there are high-voltage harness and low-voltage harness. Different types of battery pack inner harness design adopt different forms of design and layout scheme.

1.1 High voltage harness design scheme in battery pack

The high-voltage wire harness mainly provides high-voltage and high-voltage power supply for new energy vehicles. Therefore, it is particularly important for the design and layout of the wire harness. The following principles are mainly followed:

1) Harness Design: The high-voltage harness design adopts a dual-track system. Since the high voltage has exceeded the safety voltage of the human body, the vehicle body cannot be used as the cadastral point of the vehicle. Therefore, in the design of the high-voltage harness in the package, the DC high-voltage electric circuit must strictly implement the dual-track system. The high-voltage harness inside the package can be divided into high-pressure total positive and high-pressure total negative.

2) Selection of high-voltage Connector: High-voltage connector is mainly responsible for high-voltage and high-current connection and transmission, and is responsible for the human-machine safety of the high-voltage loop. Therefore, high-voltage harness connectors currently use high pressure resistance, high waterproof rating, loop interlock, and shield connection.

3) Shield design: Shielded high-voltage wire is used, and the shielded mesh is wrapped inside the high-voltage wire. Shield connection is achieved when the connector is connected. Taking into account the electromagnetic interference factors, the entire high-voltage wiring harness system is completely covered by the shielding layer.

4) Arrangement of high-voltage lines: In consideration of safety and electromagnetic interference, high-voltage lines are separated from low-voltage harnesses.

1.2 Low Voltage Wire Harness Design Scheme in Battery Pack

1.2.1 According to the working principle and design structure of the battery, the internal wiring harness is divided into:

1) BMU (BMS motherboard) harness: The main function is responsible for battery state estimation (SOC SOP SOH, etc.), actuator control, thermal management strategy, high voltage safety, fault diagnosis and other BMS main control functions.

2) LMU (BMS from the board) harness: The main function is responsible for the unit voltage, battery temperature acquisition monitoring.

3) HCU (BMS high voltage board) information acquisition harness: high voltage acquisition, insulation monitoring.

4) High Voltage Relay Coil Control Harness: Responsible for controlling high voltage circuit on/off.

5) Current sensor wiring harness: Hall sensor or shunt, mainly responsible for collecting current information.

6) PTC controller: control PTC heating

7) Solenoid valve: Controls the continuity of the battery pack air conditioning circuit.

8) Interlocking plugs for each high voltage connector: High voltage loop interlock signal transmission.

The above harness arrangement is separated from the high voltage harness in the battery pack to effectively avoid EMC interference.

1.2.2 Low-voltage wire harness fixing and buckle selection in battery pack:

Due to the environmental and structural limitations of the battery pack, the harness fixing method is mainly based on miniaturization, easy assembly, and simple fixed structure.

1.2.3 Shield Design:

The low voltage wiring harness is responsible for the function realization of the strong electric control unit module and the transmission of relevant signals. The low-voltage wire harness design and layout plan considers the interference protection caused by the high-voltage wire harness, and different signal sources use different low-voltage shielded wires.

High-frequency signals: The twisted pair is used for the wiring harness and the shielding layer is used for the foil layer shielding.

Low-frequency signals: twisted pairs are used for the wiring harness and shield layers are braided.

1.2.4 Grounding of shielded conductors:

Single-point grounding: Single-point grounding is used for low-frequency signals.

Multi-point grounding: Multi-point grounding is used for high-frequency signals.

Second, high and low voltage wiring harness layout program

In order to avoid electromagnetic interference when the high-voltage wire harness transmits strong current, which results in the risk of the low-voltage wire harness supplying power to the control unit and electromagnetic interference to the signal transmission, the power battery pack of our pure-electric vehicle adopts high-voltage wire harnesses and low-voltage wire harnesses for layering and juxtaposition. With the design, this design solution effectively avoids the interference caused by strong electrical work. 

2.1 Layered Arrangement: High-voltage wire harnesses and low-voltage wire harnesses are divided into upper and lower levels.

2.1.1 Hierarchical cabling

The pre-arrangement of the battery pack considers the high-voltage power supply and low-voltage signal acquisition of the battery module for layered wiring. The modules are connected in series to ensure that the high-voltage connection line (red in the figure) is under the module, and the low-voltage signal acquisition and related low-voltage control wiring ( Blue in the picture above the module. However, the layered wiring effectively protects against EMC interference caused by the operation of high-voltage lines. Ensure the power supply and signal transmission stability in the battery pack.

2.2 Parallel layout:

2.2.1 Toward Dependent Battery Pack Internal Structure Arranged in Parallel

The front-end module high-voltage wiring and the BMS main board low-voltage wiring are arranged side-by-side, ensuring that the high-low voltage wiring harness does not cross. Effectively protect the controller against EMC interference when working with high-voltage wiring harnesses.

Third, the high and low voltage harness fixed design

3.1 Harness within the battery pack buckle selection: 

3.1.1 The tie is mainly used in the high and low voltage wiring harnesses. By connecting the front and rear plastic air ducts, the structure inside the air duct is used for snap-fit cable tie buckles to fasten the harness. The air duct body is provided with mounting holes for the wire harnesses to be fixed and assembled.

3.1.2 The stud-style clasps are mainly used under the front and rear battery modules. The studs are fixed at the bottom of the studs to fix the wire harness.

3.1.3 The cable tie fixing harness is mainly used on the back cover of the back cover for fixing the LMU slave board and the HCU signal acquisition harness.

IV. Design and analysis of high and low voltage harnesses in battery packs

The high-voltage wire harness adopts a dual-track design, connecting the front and back modules of the battery pack in series, the PTC inside the battery pack, the air-cooled fan, the strong power maintenance switch, and the charging pre-charging circuit to the principle circuit. And through the battery pack front high voltage connector to provide the vehicle with strong power supply. The high-voltage connector adopts plug-in body shielding and increases the high voltage interlock function to effectively protect EMC interference caused by high-voltage currents.

The principle of the low-voltage harness in the battery pack is the same as that for the selection of the conductors and conductors used in the exterior vehicle wiring harness for conventional vehicles. The difference is that the inner bundle of the battery pack mainly performs signal acquisition and the relevant sensor components are monitored within the battery pack. At present, the use of high-temperature grade wire, shielded wire, twisted pair and so on. All the collected information is exchanged to the BMU for power supply, thermal management in the battery pack, heat dissipation in the package, and battery charge and discharge control.

4.1 Battery Pack Harness EMC Protection Power Distribution Scheme

In the entire vehicle range, the EMC of the components is firstly ensured to meet the standard requirements, and the control units are connected together through the harness connection, and the protective measures adopted in the power distribution are adopted.

The formula is that the power supply circuit and the ground point circuit are arranged in the same way in the same connector.

4.2 EMC-protected harness design in battery pack

In order to effectively prevent the electromagnetic interference caused by too large current of the harness, the selection of the harness material generally adopts a twisted pair in the selection of the harness material and arranges the twisted pair loop to the outermost side of other harnesses, in terms of high frequency signals. Shielded twisted pair can be used.

90% of the conducted emission in the vehicle wiring harness is related to the power line. Therefore, the online beam evaluation and design need to pay attention to the following aspects:

1) The switching power supply is partially processed, and the loop control is considered in design.

2) Sensitive signals are transmitted using shielded cables, and the shielding layer is handled with a 360-degree overlap.

3) Keep signal cables away from high-voltage networks and strong sources of interference, and use tightly coupled ground cables.

4) Do a good job of "grounding" the filter and reduce lead inductance.

5) Ensure adequate letter-to-earth ratio in the cable, and need to make reasonable arrangements and configurations.

4.3 Design analysis of power line conduction transient immunity protection

Power line conduction transient immunity should be considered at the same time in the design of new energy vehicles, high voltage, low voltage surge protection during operation, pulse protection.

4.4 Impulse interference protection

In the process of turning on or off the switch relay and fuse in the battery pack, due to the interference pulse generated by the arc, the protection considered at the initial stage of the harness design is also required.

To sum up

The rational design and layout of the harness in the early stage of the design and development of the new energy battery pack harness, and the EMC considerations in the high and low voltage principle design phase of the battery pack, effectively avoids the interference generated by the strong wiring harness during operation and passes through the mounting table. Frame, real vehicle certification, continuous optimization of wiring harness layout and EMC design. The wire harness arrangement adopted at present, as well as various EMC protection measures and measures adopted, have been fully verified and approved in batch production projects.