EV and battery applications: busbars, interconnects and thermal parts

What a battery pack asks of its parts

A traction battery is built from cells, modules or a cell to pack structure, a current path, a sensing path, a thermal path and an enclosure. Each path uses metal and plastic parts that look simple on a drawing and become difficult in a pack. The current path must carry hundreds of amps with a low voltage drop. The thermal path must pull heat out of cells that are packed closely. The enclosure must keep water and dust out for years.

The voltage class sets the rules. A 400 V pack and an 800 V pack use different creepage and clearance distances, different insulation coordination and different test levels. A supplier who has only built low voltage busbars will need time to understand those calls. Ask for the insulation standard and the test plan before discussing price.

The parts in the current path

The current path is a chain of conductors and joints. Each element has a different job, and a weak joint in the middle of the chain is harder to find than a weak part at the end.

Joining is the real process

Most pack failures that reach a test bench start at a joint. Laser welding, ultrasonic welding, resistance welding and bolted joints each leave a different signature, and each needs a different check. A laser weld needs a clean surface, a controlled gap and a weld footprint that suits the conductor thickness. An ultrasonic weld needs the right anvil and a consistent clamp. A bolted joint needs a defined torque and a contact surface that will not oxidise.

The drawing should say which process is allowed, because the supplier may otherwise choose the cheapest option. It should also say how the joint is verified: a cross-section, a pull test, a millivolt drop reading or a visual standard with photographs. A weld that looks acceptable and carries current poorly is the most expensive kind of joint, because it passes the visual check and fails in service.

Thermal parts and the interface

Heat leaves a cell through a thin interface and a cold plate. The interface material, the contact pressure and the flatness of the plate decide how well that path works. A cold plate with a good alloy and a poor mounting face is a bottleneck. A gap pad that is compressed unevenly creates a hot cell and a cool neighbour, and the pack management system sees an average that hides both.

Extruded and machined cold plates are both used. An extrusion gives a constant cross-section and a lower tooling cost, while a machined plate allows a shaped channel and tighter flatness. Friction stir welding is common for closing a machined plate, and the weld quality matters because a leak path inside a pack is a safety issue, not only a quality defect.

Insulation, tracking and clearance

High voltage parts need insulation that survives heat, contamination and time. The comparative tracking index of a plastic tells you how well it resists surface tracking, and the flame rating tells you how it behaves if a fault starts. Neither number is useful on its own. A housing with a good tracking index but a creepage path that collects conductive dust will still fail.

The practical work is to draw the creepage and clearance path on the actual geometry, including the joints, the fixings and the cable exits. Then check the coating or the barrier that protects it. A conformal coating over a sharp edge is thinner than the drawing says, and a busbar that is coated after bending may have a thin patch on the outside of the bend.

Test and inspection

A pack part is not proven by dimensions alone. The inspection plan should cover the current path, the insulation path and the thermal path. That means a resistance or millivolt drop check on the joints, an insulation resistance and hi-pot test on the assembly, and a flatness or contact check on the thermal faces.

For the first article, add a cross-section of a welded joint and a material certificate for the conductor and the plating. If the part is coated, check the thickness on a bend and not only on a flat face. If the pack is sealed, a leak test with a recorded pressure and decay limit is more useful than a visual check of the gasket.

• Conductor grade, temper and plating thickness
• Joint cross-section, pull or shear result
• Millivolt drop at the stated test current
• Insulation resistance and hi-pot result
• Creepage and clearance measured on the part
• Cold plate flatness and leak test record
• Coating thickness on bends and edges

What to verify in an Indian EV supplier

India has a growing base of busbar, stamping, machining and moulding suppliers moving into EV work. The process equipment is often familiar. The gap is usually in high voltage discipline: test coverage, traceability and change control on a part where a small deviation has a safety consequence.

Ask for the test equipment list and the actual test records, not a capability statement. Check how the supplier controls plating and whether it can show thickness and adhesion results. Confirm who welds the joint, on which machine, and how the weld parameters are monitored. Then ask what happens when a cell terminal supplier changes a dimension, because that is the change that quietly breaks a pack.

• High voltage test equipment and records
• Welding process, parameter control and monitoring
• Plating thickness and adhesion evidence
• Material traceability from coil or strip to pack
• Cleanliness control for parts near cells
• Change notification for material and sub-suppliers