Relaxation in Bolted Busbar Joints

Relaxation in bolted busbar joints can be a significant battery durability issue. As joints relax the resistance of that joint increases, resulting in larger voltage drops and excess heat generation in the joint. The relaxation of the joint can be the result of a number of mechanisms:

• stress relaxation of the busbar material
• thermal cycling of the joint
• thermal cycling of the busbars and resultant movement
• bolt torque relaxation
• vibration loosening

Stress Relaxation

Stress relaxation is a function of time, temperature, and initial stress level. The alloy and temper used, as well as the material orientation play a role. 

Thermal Cycling

In bolted joints thermal cycling of the busbar can lead to relaxation of the bolt force. Dagur Ólafsson et al [1] measured the bolted force relaxation for a copper and aluminium busbar that started with a pre-load of 40kN.

The clamping force relaxes at a higher rate for AA6101-T4 than for Cu-OF-04. From the original 40 kN, the clamping force in the Al lowers to 25.2 kN after 881 cycles while the force on the Cu lowers to 32.9 kN after 1236 cycles.

The image below shows this in more detail.

One option is to use copper ends on the aluminium busbar.

This post has been built based on the support and sponsorship from: Thermo Fisher Scientific, Eatron Technologies, About:Energy and Quarto Technical Services.

Elongation of Busbars due to Thermal Cycling

The busbars will thermally cycle and depending on the size, current, cooling and cycle this could be over each charge/discharge cycle or many times during each discharge cycle. The busbars will expand as they get hot and contract as they cool.

This will result in a force being exerted on the joints. The orientation of the busbars and length will impact the force that is generated at the joint.

Bolt Torque Relaxation

This is the loss of preload or clamping force of any bolted joint over time.

John Van Parys [4] describes the assembly, clamping and relaxation in a bolted joint:

During Assembly the bolt is tightened to a preset value.

Just after the initial preload, a certain “loss” called “relaxation” occurs. All bolted assemblies show relaxation, but not all at the same level!  The largest part of relaxation happens in the first 10 to 50 milliseconds after assembly before a certain stabilisation takes place; this zone is called “the static zone”. 

There are a number of ways of reducing the initial relaxation [4]:

1. re-tighten after the setting has stabilized
2. tighten to a higher torque, then untightened and re-tightened to the required torque.
3. tighten, untighten and re-tighten, all in one step
4. assemble in two (or more) steps e.g. pre-tighten, pause and tighten, to the target torque
5. use pulsing power tools

Vibration Loosening

Vibration of the whole assembly can result in loosening of the busbar joints. Locknuts, washers and thread lock are just some of the ways of reducing the chances of a bolt vibrating loose.

It is important to analyse the design and to reduce the vibration in busbars by supporting them within the assembly.

References:

1. Ólafsson, D., Vilaça, P. & Vesanko, J. Multiphysical characterization of FSW of aluminum electrical busbars with copper ends. Weld World 64, 59–71 (2020)
2. Peter DINEFF, Valentin DIKOV, Raina TZENEVA, Stress Relaxation in Copper Bolted Busbar Assemblies with New Design Concept of the Bolt Holes, Technical University of Sofia
3. Overview of Stress Relaxation, The Copper Development Association
4. Relaxation of bolted joints: what is it and how to overcome, John Van Parys, LinkedIn