Traction Battery Pack Fastening: Connection and Anti-Loosening from Cells to Modules
Executive Summary
The traction battery pack is one of the most integrated assemblies on an EV, with the highest safety-redundancy requirements. From cell to module and from busbar to cover housing, every connection layer poses a different fastening problem: the cell layer needs torque consistency and anti-loosening, the busbar must control contact resistance, and the housing needs sealing and corrosion protection. Following the pack's structural layers, this article breaks down the design points for each joint and gives engineering practice for anti-loosening, sealing and torque control.
Table of Contents
- Four Special Problems of Battery-Pack Fastening
- Cell and Module Layer: Torque Consistency and Anti-Loosening
- Busbars and Main Bars: Fastening Control of Conductive Joints
- Cover and Housing: Sealing, Insulation and Corrosion Protection
- Lifting and Bottom Structure: Sleeves and Long-Term Reliability
- Failure Protection and Acceptance
The interior of a traction battery pack is compact and densely wired with high voltage, and although fasteners are small, they link both structural safety and electrical safety. Under GB 38031-2020 and GB/T 31467.3, the pack must pass vibration, shock and temperature-humidity cycling tests; these tests essentially verify whether the entire connection system can remain fastened for the vehicle's whole life.
Four Special Problems of Battery-Pack Fastening
Battery-pack fastening differs from ordinary structural joints in that it must answer four questions at once: anti-loosening—the joint must not loosen under whole-vehicle vibration; insulation compatibility—fasteners must not break the high-voltage insulation design; sealing—the cover and housing must reach IP67 or higher protection; and torque consistency—preload scatter must be small in mass production. These four problems are coupled and must be addressed separately at each structural layer.
| Structural Layer | Typical Joints | Core Requirements | Common Measures |
|---|---|---|---|
| Cell / module | Module end plates, compression structure | Torque consistency, anti-loosening, vibration resistance | Torque-angle method, lock washers |
| Busbar / main bar | Copper/aluminum bars and terminal posts | Stable contact resistance, no crushing of flexible links | Specified torque window, witness marks |
| Cover / housing | Cover bolts, sealing faces | Sealing, corrosion protection, insulation | Sealed fasteners, zinc-nickel coating |
| Lifting / bottom | Lifting points, underbody shield | Long-term reliability, shock resistance | High-strength sleeves, anti-corrosion coating |
Cell and Module Layer: Torque Consistency and Anti-Loosening
A module is a structure compressed from dozens of cells, and the preload of module end plates and compression bolts directly affects dimensional stability. Insufficient preload causes loosening and abnormal noise under vibration; excessive preload may crush cells or tabs. Engineering practice tightens module bolts by the torque-angle method and applies torque-coefficient K management to the same bolt batch, ensuring every bolt's preload falls in the design band. Vibration-sensitive positions are paired with lock washers or prevailing-torque lock nuts, and torque decay is re-inspected after vibration testing.
Busbars and Main Bars: Fastening Control of Conductive Joints
Busbar joints carry large current, and their fastening quality directly determines contact resistance and heat. Insufficient torque reduces contact area, raises contact resistance and causes local overheating; excessive torque crushes copper bars, flexible shunts or terminal-post seals. Busbar bolt torque windows must therefore be fixed on the part drawing, tightened on the line with torque-adjustable or angle wrenches, and verified through temperature-rise and contact-resistance sampling. When aluminum busbars contact steel bolts, galvanic corrosion must also be considered, usually handled by coating or isolation.
Cover and Housing: Sealing, Insulation and Corrosion Protection
The connection between the battery-pack cover and housing must meet both protection-grade and corrosion requirements. Engineering practice commonly uses sealing fasteners with O-rings, or gaskets plus bolts, to bring the whole pack to IP67; housing bolts generally use zinc-nickel plating + passivation to meet the 15-year vehicle corrosion requirement. Near high-voltage busbars, creepage distance between fasteners and live parts must be considered, and insulating bolts or insulating bushings used where necessary to prevent metal parts from breaking the insulation design.
Lifting and Bottom Structure: Sleeves and Long-Term Reliability
Lifting points and underbody-shield joints are long-term load-bearing positions that must carry the whole pack weight while resisting chassis impacts and stone chipping. Such positions commonly use high-strength bolts paired with aluminum-alloy lifting sleeves (such as 6061 aluminum sleeves), reducing weight while preserving strength; cold-heading forming also significantly lowers manufacturing cost. Underbody-shield bolts must be re-inspected for appearance and torque after salt spray and stone chipping, ensuring no loosening or rust over the full life cycle.
Failure Protection and Acceptance
- Anti-loosening: use lock washers/lock nuts at vibrating positions; torque decay after vibration testing must stay within specified limits.
- Torque: critical bolts use the torque-angle method, retain curves, and apply K-factor batch management.
- Sealing: cover connections undergo water testing, and the whole pack reaches the design protection grade.
- Corrosion protection: exposed bolts use zinc-nickel plating + seal, re-inspected after salt spray and stone chipping.
- Validation: complete vibration, shock and temperature-humidity cycling per GB 38031 and GB/T 31467.3, then re-inspect joints.
The essence of battery-pack fastening is to simultaneously get three things right in a high-voltage, compact, strongly vibrating space: structure that does not loosen, electricity that is not interrupted, and a shell that does not leak. Torque runaway on any single bolt can become the starting point of a safety hazard.
Conclusion
Battery-pack fastening is ultimately a long-term reliability problem disguised as a tightening problem. Over an eight-to-fifteen-year service life, a pack sees thousands of thermal cycles, road vibration, and occasional crush or drop events; any single loose bolt can degrade a low-resistance busbar joint into a resistive hotspot, which in a high-voltage system is a safety issue rather than a performance issue. This is why busbar torque is specified with such tight bands and why torque–angle or yield-point control is preferred over plain torque. At the cell-to-module layer, the focus shifts to pressure uniformity—too little clamping allows cell swelling and uneven contact, while too much compresses the soft busbars and creeps the gasket seats. Cover sealing depends less on the bolts themselves and more on how evenly preload is distributed across the flange; uneven torque sequencing is a leading cause of water-entry failures. Lifting and bottom sleeves, which are often overlooked because they are not visible after assembly, must survive the highest static loads and should be validated with both pull-out and fatigue tests. Treating every layer as a separate engineering problem—not as one shared torque spec—is the mark of a mature pack design.
Traction battery-pack fastening is a layered system engineering problem: the cell-module layer pursues torque consistency and anti-loosening, the busbar layer controls contact resistance, the cover-housing layer balances sealing and corrosion protection, and the lifting-bottom layer pursues long-term reliability. The whole system takes GB 38031 and GB/T 31467.3 safety tests as the acceptance baseline, with torque-angle tightening, anti-loosening structures, zinc-nickel protection and sealing design as the implementation means. YF Zhichengjia serves NEV battery-pack connection scenarios and can provide selection and testing support for bolts, sleeves and seals. See the technical document library or contact our technical engineers.
