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:
- Electrical Sizing
- Cross-sectional area
- Current carrying capacity
- Transient vs Continuous
- Electromagnetic forces under short-circuit conditions
- keeping busbar cross section sufficient for the energy released by the battery at short-circuit
- purity (IASC %)
- Cross-section shape
- Thermal impact on other components
- Heat conduction
- Joints between busbars
- Contact resistance
- surface treatment – plating
- Overlap area
- Relaxation in bolts
- thermal cycling
- vibration
- Contact resistance
- Electrical isolation
- Coatings
- Insulation related specifications like material details and test methods
- Creepage and clearance
- Coatings
- Mechanical
- heat treatment
- Expansion and contraction
- Forces imparted into other components
- changes in shape over lifetime
- Mechanical restraints
- Supports for busbars
- Thermal runaway
- HV arcing between exposed busbars
- Parasitic capacitance
- Optimization for stamping
- Manufacturing requirements, Weight, Cost
Aluminium Busbars
Electrical grade aluminum busbar material also known as ec grade aluminium 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.
Aluminium vs Copper Busbars
💰 Cost
Copper is 4–6× more expensive per kg, and 2–3× per unit volume. Aluminium is significantly cheaper, especially for large-format packs.
⚖️ Weight
Al is ~3.3× lighter per unit volume. Even oversized by 1.6× to match current capacity, an Al busbar is still ~half the weight of an equivalent Cu bar.
🔩 Joining
Copper is straightforward to solder, braze, or weld. Aluminium requires ultrasonic welding or laser welding; native oxide layer complicates all joint types.
🔧 Corrosion & contact
Copper tarnishes but is stable. Al forms a tenacious oxide that increases contact resistance — especially problematic at bolted joints without anti-oxidant compound or tin plating.
📐 Sizing rule of thumb
To carry the same current, an Al busbar must be ~1.6× the cross-section of copper. For space-constrained designs this is a real penalty.
🏭 Application fit
Copper dominates cell interconnects and compact packs. Aluminium is common in large EV modules and grid storage where weight and cost savings justify the engineering overhead.
Busbar to Cell Connectors
The busbar to cell connectors need to have:
- low electrical resistance
- mechanical integrity
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.
Can we build a quite simple cell busbar thermal model? First thoughts are whether we can make some simple approximations and hence create a spreadsheet model.
We have a current [I] flow through the busbar (orange). The busbar has a resistance defined by it’s resistivity, cross-sectional area and length. Hence we can calculate the heating power.
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. Plating and coating choices for managing this are covered in Busbar Surface Treatment.
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.
Copper surface corrosion is an important subject when considering copper busbar joints. When copper is exposed to air and moisture, it forms a thin layer of reddish-brown copper oxide and other corrosion products. These products are not as conductive as the copper, leading to increased resistance at the joint.
Cross-Sectional Geometry
Thermal radiation surface ratio vs cross-sectional geometry. A flat busbar will have more surface to cool down than a round or square busbar. Also since rectangular busbar provides more surface area relative to its cross-sectional area, it is also mitigating the effects of the skin effect and improving current distribution.
Insulation and Isolation
EN IEC 60529 gives the following definitions [3]:
- Equipment marked IPXXB is protected against access with the finger, or rather the jointed test finger of 12 mm diameter, 80 mm length has adequate clearance from hazardous parts.
- A marking of IP2X shows equipment is protected against solid foreign object with diameter ≥ 12,5 mm, or rather the object probe, sphere of 12,5 mm diameter does not fully penetrate. Moreover, the mark IP2X also ensures protection against access to danger parts. In particular an equipment marked IP2X is protected against access to hazardous parts with the finger, or rather the jointed test finger of 12 mm diameter, 80 mm length has adequate clearance from hazardous parts.
Joints
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.
Bolted Joints
Fastener selection and torque have a direct bearing on joint life — see Busbar Fasteners for the options, and Examples of Busbar Bolted Joint Design for worked designs.
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.
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.
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.
Short Circuit Sizing
The short circuit current for a battery pack will be much higher than the peak current. This can result in movement of the busbar, high temperatures resulting in insulation failure and even thermal failure of the busbar.
Skin Effect
In a direct current (DC) system, busbars do not experience significant skin effect. This is because DC currents are uniform throughout the conductor’s cross-section, unlike alternating current (AC) where they concentrate near the surface. The absence of skin effect in DC means the entire cross-sectional area of the busbar is effectively used for current conduction.
Thermal Impact
Busbars are good electrical and hence good thermal conductors. This means they can conduct heat away or to other components.
Thermal Runaway
During the thermal runaway of a battery pack the composition of the gas within the enclosure can become more conducive to arcing. Thus increasing the rate of thermal runaway and instigating other mechanisms of failure.
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
- Protection from direct contacts: IPXXB or IP2X?, GT Engineering
See also: Resistance Increase with Single Side Busbars, Tesla Model 3 Cell Busbar Failures, Module Design, Nickel