M6 busbar straps are a very common and versatile solution for connecting flexible battery cells (like prismatic or pouch cells) into battery packs. They offer a robust, reliable, and often more cost-effective alternative to laser welding for many applications.
Here’s a comprehensive breakdown of everything you need to know about using them for your project.
What is an M6 Busbar Strap?
It's a pre-formed, flat strip of conductive metal (usually copper or aluminum) with an M6-sized hole at each end. The "M6" refers to the metric thread size of the bolt that will be used to secure it.
Busbar: The conductive link that carries high current between cells.
Strap: Indicates it's a flexible, flat connector designed to span a specific distance.
M6: The standardized connection method (6mm bolt diameter).
Key Advantages for Flexible Cell Connections
Flexibility & Tolerance Absorption: The strap can bend and flex slightly, accommodating small variations in cell terminal height or alignment that are common with flexible cells after swelling or during assembly. This prevents stress on the terminals.
Serviceability: The biggest advantage over welding. You can easily disconnect a single cell for testing, replacement, or maintenance without damaging the pack.
No Specialized Equipment: Requires only standard tools (torque wrench, socket) instead of expensive laser welders or ultrasonic welders.
Excellent Conductivity: With proper sizing and torque, they provide a very low-resistance, high-current connection.
Thermal Management: Thicker straps can act as a heat sink, and the connection point allows for the addition of temperature sensors.
Critical Design & Installation Considerations
1. Material Choice:
Copper (C1100/ETP): Most common. Excellent conductivity (~100% IACS). Must be tin-plated or nickel-plated to prevent oxidation and ensure a good, low-resistance contact surface.
Aluminum: Lighter and cheaper, but lower conductivity. Used often when connecting to aluminum cell terminals to avoid galvanic corrosion. Also requires plating or anti-oxidant compound.
2. Sizing the Strap:
Cross-Sectional Area (mm²): This is crucial for current handling. You must calculate based on your pack's maximum continuous and peak current.
Rule of Thumb: For copper, a common starting point is 1-2 mm² per amp of continuous current (e.g., a 100A continuous system needs a 100-200 mm² strap).
Example: A strap 20mm wide x 3mm thick = 60 mm² cross-section.
Length: Center-to-center distance between the cell terminals. Must be specified accurately.
3. Hardware (The "M6" Part):
Bolts: Use steel, grade 8.8 or higher (e.g., 10.9, 12.9). They must be zinc-plated or stainless steel to resist corrosion.
Nuts: Self-locking (nyloc) nuts or nuts with integrated star washers are ESSENTIAL to prevent loosening from vibration and thermal cycling.
Washers:
Flat Washers: Under the bolt head and nut to distribute load.
Belleville (Spring) Washers: Often used in high-vibration environments to maintain clamping force.
Star Washers (Serrated): Help bite into the busbar and terminal to improve electrical contact and prevent rotation.
4. Terminal Preparation:
The cell terminal (usually aluminum or copper) must have a properly tapped M6 hole or a welded stud. Do not overtap or force a bolt into a soft terminal.
The contact surface must be clean, flat, and free of oxidation. Use a wire brush or sandpaper designed for the metal, followed by cleaning with contact cleaner or isopropyl alcohol.
5. Torque Management - THE MOST IMPORTANT STEP:
Under-torquing: Leads to high resistance, hot spots, energy loss, and potential fire.
Over-torquing: Can strip the threads in the soft cell terminal, causing catastrophic failure.
You MUST obtain and follow the cell manufacturer's specified torque value for an M6 connection. If unavailable, a typical range for an M6 bolt into an aluminum terminal is 4-6 Nm (35-53 in-lb), but this is only a guideline. Always use a calibrated torque wrench.
Standard Connection Order (Stack-Up)
From the cell terminal upward:
Cell Terminal (clean)
Star Washer (optional, but recommended)
Busbar Strap
Flat Washer
Belleville Washer (if used)
Nut (torqued to spec)
Common Applications
Energy Storage Systems (ESS): Home and grid-scale battery banks.
Electric Vehicles (EVs): Traction packs, especially in lower-volume or prototype builds.
Robotics & Marine: Where serviceability is key.
Golf Carts & Material Handling: Rugged, repairable packs.
Where to Source
Electrical Suppliers: Panduit, Eaton, ABB, Schneider Electric.
Battery Specialist Suppliers: Companies like Maco Technology or Eltek offer custom and standard straps specifically for battery packs.
Custom Fabricators: Many machine shops can waterjet or laser cut straps to your exact dimensions and plating specifications.
Checklist for Implementation
Calculate required cross-sectional area based on current.
Determine center-to-center length accurately.
Specify material (tin-plated copper recommended).
Source proper hardware (plated bolts, nyloc nuts, washers).
Obtain correct torque specification from cell datasheet.
Prepare terminals (clean, degrease).
Use a calibrated torque wrench for assembly.
Perform a contact resistance test after assembly if possible (milliohm meter).
Implement a regular re-torquing maintenance schedule if the manufacturer recommends it.
By carefully designing and installing your M6 busbar straps, you create a powerful, safe, and serviceable connection system perfectly suited for flexible battery cells.
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