A critical aspect around battery pack busbars are the joints. It is important to consider the fundamental requirements of the joint:
- contact resistance
- overlap
- corrosion
- surface treatment
- mixed materials compatibility
Electrical Contact Resistance
In reality the connecting surfaces are not perfectly flat. The surface roughness will effectively reduce the actual electrical contact area.
Thus, if two of these meta surfaces are brought together under very low pressure, isolated points on the surfaces will touch. An electrical current will encounter a higher resistance at these restricted points.
What is the optimal busbar joint overlap?
For a bolted joint an overlap of 5 to 10 times the busbar thickness.
Then we need to think about the fixing methods:
- bolted
- welded
- compression connections
All of these have many options and related pros and cons.
Bolted Joints
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.
Bolting pattern is an important aspect of the joint design if the overlap area is large and more than one fixing is required to reach the required contact force.
Examples of Bolted Busbar Joint Design
Another technique for reducing contact resistance is to cut a longitudinal slot in both busbars. This relieves / reduces the localized contact-pressure differences and instead increases its uniformity across the contact area.
The Copper Development Association state that this approach can reduce the contact resistance by 30 to 40%.
Welded Joints
Weld Mechanics and Electrical Properties
The basic conclusion is that low electrical resistances were found to correlate with high mechanical strengths of the welds.
See also: Continuous Current Rating