Busbar laser welding for ESS modules: choosing the laser, setting parameters, proving quality

What does busbar welding do on the line?

Once prismatic cells are stacked into a module, aluminium busbars join the positive and negative terminals of neighbouring cells in series or parallel. With two terminals per cell, a 1P13S module has 26 welds. Those welds carry the module’s full current, and one weak joint can cause local overheating — which makes busbar welding one of the most critical steps in the module segment.

A typical module sequence: stacking and pressing → terminal addressing and polarity check → terminal cleaning → busbar (or CCS) placement → laser welding → post-weld cleaning and inspection → module EOL test.

Choosing the laser and head

Choice	Common practice	When it fits
Laser type	Multimode fibre laser	The mainstream choice for aluminium busbars, typically 3–6 kW
Beam mode	Single spot / ring mode (core + ring, adjustable split)	Ring mode stabilises the keyhole and reduces spatter and porosity
Head	Galvo scanner / fixed head on a robot	A galvo jumps between welds at high speed within its field, ideal for many evenly spaced welds; fixed heads suit long seams
Path	Line, circle, spiral, wobble	Wobble and spiral paths widen the seam and tolerate position error better, at some cost in depth
Shielding gas	Nitrogen or argon	Less oxidation and porosity; protects the optics

Busbars are commonly 1–4 mm thick, with 2 mm aluminium the most frequent. Thicker bars need more power and make heat input harder to control.

Key parameters and the process window

Weld parameters sit inside a window: too shallow and the joint is weak, too deep and the cell is damaged; too narrow and strength suffers, too wide and heat input climbs.

Parameter	Common requirement or range	Notes
Laser power	3–6 kW	6 kW on the 8 PPM line on this site
Welding speed	about 50–100 mm/s	Sets penetration together with power and thickness
Penetration	≥ 1.0 mm and below the datasheet maximum	Some cell datasheets cap it at about 2 mm
Width	≥ 1.5 mm	Sets current-carrying section and strength
Pull / shear force	≥ 1000 N (destructive sample test)	In reference data, a 2 mm busbar on a 3 mm terminal reached about 1,800 N in shear
Contact resistance	micro-ohm range	Consistency across a module matters more than the absolute value

Cell datasheets usually add welding limits — get them before choosing equipment or tuning: the weldable zone on the terminal, maximum penetration, the highest temperature and duration allowed at the terminal and seal, and the maximum force on the top cover during welding.

Before the weld: clean, clamp, locate

Many weld defects start before the laser fires:

1. Laser cleaning. Remove oxide, oil and adhesive residue from terminals and busbars. Uneven surfaces absorb unevenly, and penetration wanders with them.
2. Clamping to close the gap. The busbar-to-terminal gap is usually held within 0.1 mm. Clamps press each terminal hard enough to close the gap without exceeding the terminal and cover limits, and sit a few millimetres away from the weld path to avoid shadowing and sticking.
3. Vision positioning. Stacked cells carry accumulated position error, so each terminal is located by vision before welding — typically to ± 0.05–0.2 mm (≤ ± 0.2 mm addressing accuracy on the 8 PPM line on this site) — with a polarity check to prevent reversed connections.

Common defects and their causes

Defect	Usual causes	Remedies
Lack of fusion, shallow weld	Large gap, contamination, defocus	Clamping and gap checks, pre-cleaning, focus calibration
Burn-through, excessive depth	Too much power, too slow, thin busbar	Process-window validation, incoming thickness checks
Porosity	Contamination, too little gas, unstable keyhole	Cleaning, gas-flow monitoring, ring mode or power modulation
Spatter	Keyhole collapse, power spikes	Beam mode and power ramp tuning, protective windows
Cracks	Heat input and cooling rate, alloy	Adjust heat input, verify busbar alloy grade
Off-position weld	Positioning error, cell drift	Per-weld vision positioning, fixture maintenance

Proving quality: inline monitoring and post-weld checks

Sample pull tests cannot vouch for every weld, so production lines combine:

• Process monitoring: photodiode, plasma or back-reflection signals compared with a reference curve, flagging abnormal welds in real time.
• OCT depth measurement: optical coherence tomography measures keyhole depth directly, within about 4 % of cross-sections; some systems also use it to adjust power in closed loop.
• 3D post-weld inspection: line-laser profilers check seam position, width, undercut and spatter.
• Destructive sampling: cross-sections (depth, width, porosity) and pull tests at a set frequency.
• Electrical confirmation: module EOL checks connection resistance and voltage consistency, catching weld problems indirectly.

Parameters and results for every weld should be tied to cell and module barcodes in MES, so any issue traces back to the exact joint.

Factory acceptance (FAT): a sensible approach

• Sample size and capability: evaluate key dimensions (depth, width) and pull force on at least 30 samples, with Cpk ≥ 1.33.
• First-pass weld yield: measured over continuous production; a common target is 98 % or better, with final yield after rework higher still.
• Takt: run continuously with real cells and modules. At 8 PPM that is about 16 welds a minute, about 3.75 s per weld including scanner and axis motion.
• Edge cases: run the thinnest and thickest busbars and cells at the tolerance limits to confirm the window is wide enough.
• Traceability: spot-check that weld data traces from MES to the barcode.

What we can supply

The busbar welding station on the 8 PPM line on this site uses a 6 kW laser with a galvo head: penetration ≥ 1.0 mm, width ≥ 1.5 mm, pull-out force ≥ 1000 N. As a project needs, we can also supply ring-mode lasers, OCT depth monitoring, pre-weld laser cleaning and 3D post-weld inspection through our supply-chain service, or a stand-alone welding workstation.