Nickel Strip vs Copper Busbar for 18650 and 21700 Battery Packs

Selecting an 18650 or 21700 cell is only one part of battery pack engineering. The conductor connecting those cells can also limit pack current, increase voltage drop and create unwanted heat. A high-performance cell cannot deliver its full capability through an undersized interconnect.

Pure nickel strip and copper busbars are two common choices. Nickel is relatively easy to resistance-weld to cylindrical cells. Copper offers much higher electrical conductivity but usually requires more specialized joining processes. This guide compares their electrical, thermal, mechanical and manufacturing characteristics.

Nickel Strip vs Copper Busbar at a Glance

Electrical Resistance and Heat Generation

The resistance of an interconnect depends on its material resistivity, length and cross-sectional area:

Resistance R = Resistivity ρ × Length L ÷ Cross-sectional area A

The Copper Development Association gives a representative electrical resistivity of approximately 1.71µΩ·cm for fully dense copper at 20°C. A Nickel Institute reference gives approximately 7.8µΩ·cm for high-purity nickel. On this basis, a nickel conductor can have roughly 4.5 times the resistance of a copper conductor with the same dimensions.

Actual resistance will vary with purity, alloying, plating, work hardening and temperature. Weld resistance must also be added to the resistance of the strip or busbar itself.

Power loss and heat P = Current² I² × Resistance R

Because current is squared in this relationship, doubling the current produces four times the heat at the same resistance. An interconnect that performs acceptably in a low-current pack can therefore become a significant hot spot when used with high-drain cells.

Busbar material alone does not determine current capability. Width, thickness, path length, weld quality, cooling and allowable temperature rise must be evaluated together.

Why Pure Nickel Strip Is Common in Battery Packs

Pure nickel is widely used because it can be resistance spot-welded to the terminals of cylindrical cells. The process passes a controlled current through the materials while applying electrode force. Electrical resistance generates localized heat at the intended joint.

Advantages of pure nickel strip

• Suitable for resistance welding to many cylindrical cell terminals
• Available in thin, flexible strips and stamped patterns
• Convenient for common series-parallel layouts
• Can simplify tooling and production equipment
• Can be shaped into narrow cell-level fuse links

Limitations of nickel strip

Increasing nickel thickness can reduce conductor resistance, but it also makes resistance welding more challenging. Current can travel through the strip between the electrodes instead of producing sufficient heat at the strip-to-terminal interface. This is known as current shunting.

Slots, projections and locally narrowed features can help direct welding energy. However, welding parameters must be validated against the actual strip thickness and cell terminal construction. Excessive welding energy may deform or penetrate the terminal and transfer damaging heat into the cell.

Why Copper Busbars Suit High-Current Packs

Copper’s low resistivity makes it attractive for the main current path in power tools, mobility systems, robotics and energy-storage equipment. It can reduce voltage drop between cell groups, the BMS, fuse and output connector.

Advantages of copper busbars

• Lower resistance than nickel at the same dimensions
• Reduced conductor voltage drop
• Lower busbar heat generation at high current
• Potentially smaller conductor cross-section for a specified resistance
• Suitable for multi-parallel modules and long current paths

Challenges of copper busbars

Copper conducts electricity and heat so effectively that conventional resistance welding has difficulty concentrating energy at the joint. Increasing welding energy without adequate process control can damage the cell terminal.

Depending on the material thickness and joint geometry, manufacturers may use laser welding, micro-TIG welding, specialized resistance welding or stamped projections. The joining process should be selected before the busbar geometry is finalized.

Nickel-Plated Copper and Hybrid Busbar Designs

A hybrid interconnect can provide a practical balance between weldability and conductivity. One approach uses a short, thin nickel tab at each cell terminal and connects those tabs to a larger copper busbar. Only the individual cell current passes through the short nickel section, while combined module current travels through copper.

Nickel-plated copper is another option. The nickel surface can improve corrosion resistance and compatibility with certain joining processes, while the copper core provides lower resistance. Nevertheless, nickel-plated copper does not behave exactly like pure nickel during resistance welding. Tooling, energy, force and tab geometry still require dedicated validation.

Nickel or Copper for an 18650 Battery Pack?

18650 cells are used in compact electronics, medical equipment, lighting, robotics and industrial devices. PKCELL offers 18650 lithium-ion cells in several capacities and supports custom series-parallel configurations.

Pure nickel is often adequate for a small 18650 pack operating at low or moderate current. It supports compact layouts and established resistance-welding processes. Copper becomes more attractive as parallel count, continuous current or conductor length increases.

When using a high-rate 18650 cell, confirm that the interconnect, welds, BMS and connector can all carry the required current. Installing a higher-power cell does not improve pack output if another component remains the bottleneck.

Nickel or Copper for a 21700 Battery Pack?

21700 cells can provide greater capacity per cell and are available in high-rate configurations. PKCELL lists 21700 cells from 4,000 to 5,500mAh, as well as a 1S2P 21700 battery pack using parallel cells.

Nickel strip may still be suitable when high-capacity 21700 cells operate at modest current. For motor-driven equipment or other high-power applications, the interconnect should be checked carefully because a thin nickel strip may limit a high-rate cell.

The evaluation should include current per cell, total group current, current distribution between parallel cells, busbar length, voltage drop and temperature at every connection.

How to Size a Battery Interconnect

Avoid choosing strip thickness from a generic online ampacity chart. A useful design process includes the following steps:

1. Define the load profile: Record continuous, peak and pulse currents, including duration and frequency.
2. Set a voltage-drop target: Calculate the maximum acceptable loss between the cell groups and output connector.
3. Select the material: Compare pure nickel, copper and hybrid options using verified material specifications.
4. Calculate conductor resistance: Include material resistivity, dimensions and temperature.
5. Estimate heat generation: Apply the worst-case continuous current to each current path.
6. Include joint resistance: Model welds, BMS terminals, fuses, cables and connectors.
7. Check current sharing: Keep parallel paths electrically balanced to prevent individual cells from carrying excessive current.
8. Build and test prototypes: Confirm voltage drop and temperature rise under real operating conditions.

Busbar Layout and Safety Considerations

Balanced current paths

Cells in the same parallel group should have similar electrical paths. An asymmetric busbar can cause cells closest to the pack terminals to carry more current than cells farther away. Balanced pickup points and symmetrical conductor geometry can improve current sharing.

Cell-level fault protection

In larger parallel groups, a narrow fusible link may help isolate a cell that develops a severe fault. A thick continuous copper plate can carry very high fault current, so protection strategy must be considered when selecting the interconnect.

Mechanical support

Busbars should not transfer excessive vibration or assembly stress to cell terminals. Cell holders, insulation barriers and strain relief should keep the conductors stable during transport and operation.

Corrosion and dissimilar materials

Nickel, copper, plated steel and other metals can behave differently in humid or contaminated environments. Material compatibility, plating quality and enclosure sealing should be reviewed for the intended service conditions.

Common Design Mistakes

• Using nickel-plated steel while assuming it is pure nickel
• Specifying material without defining width, thickness and path length
• Calculating conductor resistance but ignoring weld resistance
• Checking peak current without testing continuous temperature rise
• Welding a thicker strip with unverified legacy parameters
• Creating unequal current paths within a parallel cell group
• Soldering directly to unprepared cylindrical cell terminals
• Using a thick copper plate without considering cell-level fault isolation
• Validating a hand-built sample without checking mass-production capability

Prototype and Production Validation

Electrical calculations provide a starting point, but the completed pack must be tested with the actual cells, conductor material and joining process. Recommended checks include:

• Four-wire resistance measurement of conductors and joints
• Temperature-rise testing at maximum continuous load
• Voltage-drop measurement during peak current
• Weld peel-strength and cross-section inspection
• Current-sharing measurement between parallel cells
• Vibration, shock and temperature-cycle testing
• Inspection after environmental and lifecycle testing
• Verification of BMS, fuse and overcurrent protection

PKCELL supports cell selection, series-parallel architecture, PCM/BMS, wiring, connectors and structural requirements for custom lithium-ion battery packs. Related cell and pack options are also available in the cylindrical Li-ion battery range.

Frequently Asked Questions

Conclusion

Pure nickel strip offers practical weldability and production simplicity for many 18650 and 21700 battery packs. Copper busbars provide lower resistance and better high-current performance but require more demanding joining and process control.

For compact, moderate-current packs, pure nickel may be sufficient. For high-output packs, copper or a nickel-copper hybrid can reduce voltage drop and heat. The final decision should be based on electrical calculations, thermal testing, weld validation, fault protection and repeatable production results.