Aluminium vs Copper Busbars

Choosing between aluminium and copper is one of the first decisions in any busbar design. Both materials are capable conductors, but they make very different engineering trade-offs. This page sets them side by side across the properties that matter most — electrical, mechanical, thermal, and practical.

Material Properties at a Glance

The table below compares the two materials using grades typical for battery pack busbars: ETP copper (C11000) and electrical-grade aluminium (Al 1350 / 1060 series). All values are for annealed / soft-temper condition at 20 °C.

Electrical Conductivity and Sizing

Copper has around 64% higher conductivity than aluminium. In practice this means that to carry the same continuous current, an aluminium busbar must have roughly 1.6× the cross-sectional area of a copper equivalent — the commonly cited ~62% increase. This is the central packaging trade-off: aluminium saves weight and cost, but demands more volume.

For initial sizing of copper busbars, a rule of thumb of 5 A/mm² (conservative) to ~6 A/mm² is widely used for continuous current ratings in battery packs. For aluminium, apply the same approach to the larger cross-section after scaling by 1.62 (the ratio of conductivities).

The temperature coefficient of resistance is nearly identical between the two metals (both ~0.004 /°C), so the fractional increase in resistivity with temperature — and therefore the sensitivity to busbar temperature rise — is similar. This matters when sizing for worst-case temperature conditions.

See the cell busbar thermal model for a worked example of how resistivity, cross-section, and current interact to produce heating power in a cell-level busbar.

Weight

Aluminium is approximately 3.3× less dense than copper. Even after accounting for the required increase in cross-sectional area, an aluminium busbar carrying the same current as its copper equivalent is typically around half the mass. At cell-interconnect scale this saving is modest, but at module and pack level — where main busbars can be substantial — it adds up quickly.

This weight advantage is a key reason large-format EV modules and grid-scale BESS increasingly favour aluminium for their main power busbars, while copper tends to dominate at cell-tab level where space constraints outweigh weight.

Mechanical Properties

Copper is significantly stiffer and stronger in the annealed state: roughly 2.5× the yield strength and 1.7× the elastic modulus of comparable aluminium. This has practical consequences:

• Vibration and restraint: Aluminium busbars are more susceptible to fatigue at mechanical supports and bends under repeated vibration loading. Constraint spacing and support clip design needs to account for this.
• Thermal expansion: Aluminium expands 35% more per degree than copper (CTE 23 vs 17 µm/m·°C). In long busbars subject to a wide operating temperature range, the resulting forces imparted into connected components — cells, fuses, contactors — need to be accommodated in the joint design or busbar routing.
• Forming: Aluminium’s lower strength makes it easier to extrude into complex profiles, which is an advantage when integrating features like cooling channels or mounting features into the busbar itself.

Corrosion and Surface Behaviour

Both metals corrode, but in fundamentally different ways that demand different design responses.

Copper surface corrosion produces a reddish-brown oxide layer that is more resistive than the base metal and can increase contact resistance at joints, but the oxide is relatively slow-forming and manageable with surface treatments such as tin or silver plating.

Aluminium forms a much harder, more tenacious oxide (Al₂O₃) almost instantaneously on any freshly cut or machined surface. This oxide layer is electrically resistive and can significantly increase contact resistance at bolted joints if not addressed. Mitigation strategies include:

• Anti-oxidant joint compound applied at assembly
• Tin or silver plating of contact faces
• Controlled torque on bolted joints, with re-torque provisions for in-service inspection

Corrosion at busbar joints is also an important long-term durability issue: a corroded joint increases resistance, generating more heat, which accelerates further degradation — a compounding failure mode. The Toyota Prius Gen 2 battery is a well-documented example of this in service. See the main busbars page for more on corrosion and joint design.

Joining and Welding

The oxide layer that makes aluminium corrosion-resistant also makes it significantly harder to join reliably.

Copper can be soldered, brazed, resistance-welded, or laser-welded with relatively straightforward process control. It is the easier material to work with across all joining methods commonly used in battery pack assembly.

Aluminium requires processes that disrupt or penetrate the oxide layer. Laser welding and ultrasonic welding are the most common approaches at cell-tab level. Ultrasonic welding works particularly well for thin foil tab-to-busbar joints. For thicker main busbars, laser or friction-stir welding are more applicable. All aluminium welding processes require tighter process control than their copper equivalents.

The relationship between weld mechanical strength and electrical resistance applies to both metals: low-resistance joints correlate with high mechanical integrity, and this is a useful quality indicator for production process monitoring.

Dissimilar metal joints — aluminium busbar to copper cell tab, or aluminium to copper terminal — introduce galvanic corrosion risk. These transitions typically require bimetallic transition inserts or clad material, and careful sealing against moisture ingress.

Bolted Joint Relaxation

Aluminium is significantly more susceptible to relaxation in bolted busbar joints than copper. Creep at the contact interface — driven by temperature cycling, vibration, and the sustained clamping stress — causes the joint to lose clamping force over time, increasing electrical contact resistance. This is one of the most common long-term failure modes in aluminium busbar systems.

Mitigation approaches include:

• Belleville (disc spring) washers to maintain clamping force as the joint relaxes
• Higher bolt grades and controlled assembly torque
• Plated contact surfaces to reduce oxide contribution to resistance
• Joint resistance monitoring over lifetime testing as part of pack validation

Copper bolted joints also relax, but at a much lower rate under equivalent conditions.

Thermal Performance

Copper’s thermal conductivity (400 W/m·K) is nearly twice that of aluminium (230 W/m·K). In applications where the busbar is expected to conduct heat away from cell terminals — such as tab-cooling designs — copper has a clear advantage.

The lower melting point of aluminium (660 °C vs 1,085 °C for copper) is a consideration for thermal runaway scenarios. Under the elevated temperatures associated with a propagating thermal runaway event, aluminium busbars will reach and exceed their melting point at lower severity levels than copper. This has implications for arc propagation and pack-level failure mode analysis.

Grade Selection

Not all aluminium or copper is equal in electrical performance:

Copper grades: ETP copper (C11000, electrolytic tough pitch) is the standard busbar material at 99.9% purity and 100% IACS conductivity. Oxygen-free copper (C10200) offers marginally better conductivity and is preferred where hydrogen embrittlement is a concern (e.g. near welding or elevated temperature processing).

Aluminium grades: The 1xxx series alloys — principally Al 1350 and Al 1060 — are the standard electrical-grade choices, with conductivity in the 59–61% IACS range. Higher-purity alloys (99.9%+) achieve slightly higher conductivity but at increased cost and reduced availability. The 6xxx series alloys (e.g. 6101) offer higher mechanical strength at a modest conductivity penalty and are used where structural integrity is prioritised, such as large extrusions. See aluminium busbar products for more detail on alloy selection.

When to Use Each Material

Copper is typically preferred for:

• Cell-level interconnects (laser-welded tab busbars, flex busbars) where space is constrained and welding to cell tabs is required
• High-density module designs where the 1.6× cross-section penalty on aluminium cannot be accommodated
• Any design requiring soldering or conventional welding
• Applications requiring maximum thermal conductivity for heat spreading from joints

Aluminium is typically preferred for:

• Main pack-level busbars in large-format EV modules and stationary storage, where weight saving at scale is a primary design driver
• Cost-sensitive applications where the material saving justifies the additional engineering for joints and oxide management
• Designs where complex extruded profiles are advantageous (lower extrusion tooling cost and easier forming vs copper)
• Cylindrical cell pack designs using single-side busbars where the tab welding process is already set up for aluminium

In practice, many pack designs use both: copper for cell interconnects and aluminium for the main high-current runs between modules and to the output connector.

Summary

Copper has higher conductivity, better mechanical properties, easier joining, and superior thermal performance. Aluminium is lighter and cheaper, but demands more careful joint engineering — particularly around oxide management, bolted joint relaxation, and welding process control. The 1.6× cross-section rule of thumb is the starting point for any like-for-like comparison of the two.

For detailed design guidance on each material, see the dedicated pages for copper busbars and aluminium busbars, and the main busbars overview page for sizing, joints, insulation, and mechanical restraint design.

See also: Aluminium