Busbars are the main electrical connections between cells, modules and connect all of the HV system to the outlet connector. Normally made from copper or aluminium. Careful consideration needs to be taken:
- Cross-sectional area
- Creepage and clearance
- Thermal impact on other components
- Joints between busbars
- Contact resistance
- Electrical isolation
- Mechanical restraints
- Expansion and contraction
- Thermal runaway
- Parasitic capacitance
- Optimization for stamping
Aluminium Busbars
Electrical grade aluminum busbar material also known as ec grade aluminum busbar. Compared to copper busbars aluminium offers a weight and cost save, but requires an increase in cross-sectional area of ~62%. Hence aluminium busbars need more volume for packaging.
Aluminium Busbar Products
The highest conductivity is achieved by high purity aluminium (purity of 99.9 wt% Al and higher) in soft temper. Nevertheless, high purity alloys are not commonly used in volume application due to cost and volume constraints. Instead of this, commercially pure aluminium products of 1xxx series, with minor levels of impurities and alloying elements (< 99.9 wt% Al), are rather typical for conductors as they balance cost, availability, and high electrical conductivity.
Busbar to Cell Connectors
The busbar to cell connectors need to have:
- low electrical resistance
- mechanical integrity
Copper Busbars
The red circles show data from 5 electric vehicle battery busbars. The current is an estimated continuous rating and plotted versus the cross-sectional area in mm2.
The gradient of the “straight line fit” shows that 5.9A/mm2 is a rough estimate for copper busbar size. However, to be on the safe side of this I would initially size at 5A/mm2 before doing the detailed electrothermal analysis.
Corrosion
An important aspect to consider in all busbar designs is to consider the environment and the materials. Corrosion in a busbar joint will increase the resistance and eventually stop the pack from working.
A regular repair on these battery packs is to strip out the bus bars and replace the connecting plates and nuts on each battery module. Corrosion can be caused because the air used to cool the battery comes from the cabin that can be hot, cold, moist etc. When rebuilding always use a drop of Stabilant 22A on the nut/stud.
Andy Latham, Salvage Wire
Image supplied and copyright of Salvage Wire.
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.
Manufacturers
Whether the busbars are for the cell to cell interconnects or the main busbars connecting the cells to the contactors, fuses and out to the connectors they all need careful design and manufacture.
Photo Etching
Whether prototyping or creating busbars for production runs we often want high quality parts and this is where photo etching comes into it’s own.
Advanced Chemical Etching take us through the process of photo etching and how this can be used to create high quality aluminium or copper parts.
We are concentrating on busbars, but let your imagination run with what this process could allow you to do.
Mechanical Restraints
Busbars can move around a lot with a number of possible inputs:
- vibration and shock inputs
- thermal expansion
- electromagnetic forces
- tightening and loosening bolted joints
These inputs need to be considered when designing the mechanical constraints, their placement and distance between constraints.
These should be a physical clip that is designed for the life of the pack and considers all of the above points and all environmental inputs.
Single Side 21700 Busbars
In the case of cylindrical cells it is possible to connect to both the positive and negative terminals of the cell on the top surface.
The result is a simplified busbar that gets repeated through the module/pack.
Thus leaving the bottom of the cell free for cooling.
References:
- Guidance on Busbar Design and Installation – this is perhaps the foremost reference manual on copper busbar design, written and updated by the Copper Development Association
- Busbars and distribution – distribution standards, sizing busbars, shapes of busbars, distribution blocks and choice of products, Legrand, 2009