The Electrical Contact Resistance at a metal to metal contact is primarily governed by:
- surface structure
- mechanical load
The surface structure can be further described by the surface flatness, surface oxidation and water adsorption.
These joints in a battery pack will either be welded or bolted. Initially the easiest to think about is the bolted busbar joint.
The bolt torque can be used to estimate a force and hence a contact pressure.
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.
In addition to a greatly increased current density, the constriction of the lines of current flow also causes a lengthening of these lines and thus causes what is known as spreading resistance between the contacts.
Increasing the contact pressure will mean that some of the peaks will deform, hence increasing the contact area.
If the contact surfaces are not perfectly clean, there is also an interface resistance, caused by oxides or films.
Hence, surface preparation plays a very important role in the performance of overlapping bolted joints.
this page has been produced with the sponsorship and support of h.e.l group and TAE Power Solutions
Surface Coatings
Plating the surfaces of busbars where they connect has a number of functions:
- improved surface flatness
- reduce oxidation
- create a malleable surface that conforms under pressure
Rachman et al [2] show the contact resistance of copper busbars versus contact pressure for different surface coatings.
Joint Geometry
Braunovic [3] showed that sectioning the joint improved the contact resistance. A drawing of the split joint and a table of measured values show this clearly as an improvement. In addition this table shows the impact of brushing (using a steel wire brush in one direction) the surfaces and using a joint lubricant (an oxide inhibiting grease was used – Burndy Penetrox A-13).
Joint Lubricants
Greases / lubricants are widely used in the HV electrical industry. The table above shows that they can significantly reduce the resistance across an electrical joint. The main purpose of these:
- environmental barrier
- protect the surfaces from oxidation
- lubricate and prevent oxidation in fretting of electrical joints
References
- B. N. J. Persson, On the Electric Contact Resistance, Tribology Letters volume 70, Article number: 88 (2022)
- Noviadi A. Rachman, Agus Risdiyanto, Umar Khayam, Suwarno, Analysis of Surface Roughness and Contact Pressure at Copper Connector Using Nickel and Silver Plating for EV Battery, 2013 Joint International Conference on Rural Information & Communication Technology and Electric-Vehicle Technology
- Milenko Braunovic, Effect of Connection Design on the Contact Resistance of High Power Overlapping Bolted Joints, IEEE Transactions on Components and Packaging Technologies,Vol. 25, No.4, Dec 2002
- Roland E. Ott, Thermal and Electrical Resistance of Metal Contacts, Portland State University, 1967