An electric vehicle battery pack holds enough energy to injure an assembler or a service technician, and its wiring has to stay safe through years of vibration, thermal cycling and repair. The high-voltage harness inside and around the pack is designed around that risk. This guide sets out the main design points a specification should address, for two-wheelers, three-wheelers, cars and commercial vehicles alike.
Two harnesses, not one
A battery pack normally carries two separate wiring systems. The high-voltage harness connects modules, the main contactors, the fuse and the pack connectors to the inverter and charger. The low-voltage harness carries cell-voltage and temperature sensing to the battery management system, plus control and communication circuits. They are kept physically separate, and the sensing leads are protected so that a fault on a sense wire cannot draw high current from the cells.
High-voltage harness design points
| Design point | What it involves |
|---|---|
| Identification | High-voltage cables and conduits are coloured orange under SAE J1673, so anyone working on the vehicle can see them. See why EV high-voltage cables are orange. |
| Cable rating | Voltage rating, temperature class and flexibility suited to the pack voltage and the route. The ISO 19642 series covers road vehicle cables, including high-voltage types. |
| Shielding | Inverter switching generates electrical noise. Shielded high-voltage cables, with the shield terminated all round (360 degrees) at the connector, keep that noise off nearby circuits. |
| High-voltage interlock loop (HVIL) | A low-voltage loop runs through each high-voltage connector. Unplugging the connector breaks the loop first, and the system opens the contactors before the power contacts separate. |
| Touch-safe connectors | Contacts are recessed or shrouded so they cannot be touched with a finger when unmated. |
| Creepage and clearance | Spacing between high-voltage parts, and between them and the pack enclosure, suited to the working voltage and pollution conditions. |
| Mechanical protection | Conduit, clips and grommets where cables pass enclosure walls or structural edges; strain relief at every connector. |
| Service disconnect | A manual disconnect that splits the pack for safe servicing, located where technicians can reach it. |
Standards and regulations
ISO 6469-3 sets electrical safety requirements for electrically propelled road vehicles, including protection against electric shock. In India, AIS-038 (Rev. 2) applies to traction battery and high-voltage safety for M and N category vehicles, and AIS-156 to L category two- and three-wheelers. The vehicle maker’s own specification normally sets the detailed cable and connector requirements on top of these.
Thermal and vibration considerations
High-voltage cables carry large currents, so conductor size is set by current, ambient temperature inside the pack and the cable’s temperature class, not by voltage. Bend radius matters more than in low-voltage harnesses, because thick shielded cable resists bending and pulls on connectors if routed tight. Crimped terminations, clipped close to each connector, keep vibration out of the contacts. The harness testing guide covers continuity and insulation tests, which on a high-voltage harness are carried out at test voltages suited to the working voltage.
Specifying the harness
A supplier needs the pack drawing, cable and connector part numbers, routing with clip positions, HVIL wiring, shield termination details and the test plan. The harness RFQ checklist lists what to send. APAR builds harnesses for automotive and EV platforms to customer drawings, and manufactures automotive wires and cables to ISO 6722, SAE J1128 and JASO standards.