Battery Busbar Design Failures: 10+ Deadly Mistakes Engineers Overlook and How to Prevent Them

Introduction to Battery Busbar Safety and Performance

In the rapidly advancing world of aerospace, electric vehicles (EVs), and consumer electronics, battery safety is paramount. One often underestimated component is the battery busbar—the metallic strip or bar that conducts electricity between battery cells, terminals, and power systems. While it may seem like a straightforward element, the truth is far more complex.

Many engineers, especially in early-stage design, focus on compliance with electrical parameters such as current ratings or material conductivity. But real-world failures tell a more cautionary tale. Battery busbar design failures—from overlooked thermal expansion to improper fastening—can rapidly evolve into catastrophic events, triggering cell degradation, circuit failure, or even thermal runaway.

Common Lithium Battery Busbar Design Mistakes

1. Undersized Cross-Sections

Utilizing thin or narrow conductors increases electrical resistance, leading to localized heating, voltage drops, and accelerated degradation. In severe cases, the nickel plating may deteriorate, compromising both performance and safety.

2. Ignoring Thermal Expansion Mismatches

Combining metals like copper and aluminum without accounting for their different thermal expansion rates can cause stress during temperature fluctuations. This stress may result in cracks at weld joints or riveted areas, elevating contact resistance unexpectedly.

3. Welding and Bonding Defects

Improper laser or ultrasonic welding can create voids within connections, weakening the structural integrity. These defects are often invisible to the naked eye, making advanced inspection methods like CT scanning essential.

4. Loose Mechanical Fastening

Insufficient torque during bolt tightening or improper crimping can lead to micro-vibrations during operation. These vibrations generate heat, cause contact erosion, and may eventually melt connectors.

5. Passive Thermal Management Only

Relying solely on the housing for heat dissipation is inadequate. In EV applications, busbar temperatures can exceed 125°C, leading to automatic power reduction or irreversible degradation.

6. Misjudged Current Rating Limits

Even if a busbar meets standard current ratings, internal hotspots can form during high-rate charging or discharging. This underscores the importance of validating thermal profiles under load conditions.

7. Layout-Induced Skin Effect or Hotspots

Poor busbar geometry can cause uneven current distribution, especially at high frequencies. The skin effect, where current travels along the conductor’s surface, can raise localized temperatures and damage insulation.

8. Lack of Single-Point Failure Isolation

A single shorted tab can trigger a chain reaction, especially without isolation design. This was evident in the Boeing 787 incident, where one cell’s failure cascaded through the entire battery pack.

9. Insufficient Clearance and Insulation

In compact designs, even minor protrusions can pierce separators under vibration, leading to internal short circuits—as seen in the Galaxy Note 7 explosions.

10. Absence of Abuse Testing Protocols

Skipping standardized abuse testing like SAE J2464 until late-stage prototyping can embed flaws in heat management or structural robustness, making fixes costly and inefficient.

Case Studies of Battery Busbar Failures

Cylindrical Cell Tab Failure Due to Voids

A CT scan analysis revealed "keyhole" voids in the welded region of cylindrical cells. After 300 cycles and a 4C discharge rate, these voids formed cracks, causing a 40% rise in internal resistance and eventually overheating and failure.

Boeing 787 Thermal Runaway Incident

The 787 used eight large lithium-ion prismatic cells housed closely without proper thermal partitioning. One short led to a thermal chain reaction, igniting the aircraft’s underbelly. Investigations pointed to the absence of thermal and fault isolation design as the root cause.

Samsung Galaxy Note 7 Explosions

Two separate battery suppliers were found to have structural defects: one caused by tight internal clearances leading to bent electrodes, and the other by insulation tape omission and burrs that pierced separators—both resulting in internal short circuits.

Best Practices for Battery Busbar Design

Geometry and Material Selection

Use a current density of 2–3 A/mm² for safety. For high-current systems, prefer laminated or composite busbars to reduce both resistance and inductance. Select copper with nickel or silver plating to resist corrosion and ensure consistent contact.

Busbar geometry should favor wide and thin designs to maximize heat dissipation. A design iteration using the formula:

I ⋅ ρ ⋅ L / (ΔT ⋅ k)

helps optimize width and thickness relative to expected thermal conditions.

Thermal-Electrical-Mechanical Simulation Integration

Use multi-physics software like Ansys Maxwell or Icepak to simulate hotspots, inductance, and mechanical stress. For mixed-metal designs (e.g., Cu-Al), ensure stress relief grooves or floating zones are included to accommodate differential expansion.

Connection and Manufacturing Quality

Control energy density precisely in laser or ultrasonic welding. Post-weld, use X-ray or CT inspection to detect hidden voids. Mechanical connections must follow torque-angle guidelines and use conductive anti-loosening coatings.

If contact resistance exceeds 100 µΩ, rework the part—do not proceed to assembly.

Online Monitoring and Redundancy Design

Place NTC thermistors or Hall sensors near high-current paths. Integrate with BMS ICs like TI’s BQ79616 to enable real-time temperature monitoring and thermal shutdown logic.

Include redundant fuses or pressure-activated solder joints to localize faults and prevent system-wide failure.

Quality Control and Industry Standards

Follow TE Connectivity’s Busbar QRG to verify current, voltage, and thermal conditions. Conduct IPC-T50 level spot-checks on coating thickness, insulation, and alignment based on guidelines from NXP AN12969.

Conclusion

Understanding the intricacies of battery busbar design and connector selection is essential for the safety and performance of lithium battery systems. By adhering to best practices and leveraging high-quality components, you can ensure the longevity and reliability of your battery applications.