The European customer called us in January. They were launching a new line of 150kW DC fast chargers for highway rest stops, and they needed a custom copper bus bar that could carry 200 amps continuously, fit inside a 120mm-wide enclosure, and survive 10 years of outdoor temperature cycling. Their previous supplier was making the bus bars from flat copper stock, bent by hand, and the tolerance was ±0.5mm. That was too loose for their automated assembly line - the bus bars were not seating properly in the charger's power module, and 8% of them were being rejected.
We redesigned the bus bar as a punched and formed part, from 2mm-thick C11000 copper, with a tin-plated finish. The punch die held the tolerance to ±0.1mm. We also added a chamfer on the terminal holes, so the bolts would align without the operator having to fight them. The first sample was approved in 2 weeks, and production started in April. We just shipped the 100,000th unit.
This is our first major program in the European EV charging market, and it has taught us a lot about what a fast-charger manufacturer actually needs from a bus bar supplier.
What a fast-charger bus bar has to do
A bus bar in a 150kW DC charger carries the current from the input terminals to the power modules, and from the power modules to the output connector. The current is high - 200 amps at 400V DC - and the voltage drop across the bus bar has to be below 50 millivolts, otherwise the charger loses efficiency and overheats.
The material is oxygen-free copper (C11000), which has 101% IACS conductivity. The thickness is 2mm, which gives enough cross-section to carry 200 amps with a temperature rise of less than 30C above ambient. We considered 1.5mm, but the temperature rise was 45C, which would have pushed the bus bar above its rated temperature at 40C ambient.
Tin-plated copper bus bars ready for shipment to a European EV charger manufacturer.
The finish is matte tin plating, 5-8um thick. Tin is the standard finish for bus bars in EV applications because it does not tarnish like silver, and it has good contact resistance at the bolted joint. We also considered nickel plating, but nickel is harder to bolt down - you need more torque to get a low-resistance joint, which can crack the insulator around the terminal.
What we changed from the previous supplier
The previous supplier was bending flat copper stock by hand, using a manual brake. The bends were inconsistent, and the hole positions varied by ±0.5mm. On an automated assembly line, that means the robot cannot pick up the bus bar and place it accurately - the holes do not line up with the bolts, and the operator has to adjust each one by hand.
We made two changes. First, we punched the holes and the outer profile in a single die, so the hole positions are held to ±0.1mm. Second, we formed the bends in a progressive die, so the bend angles are consistent to ±0.5 degrees. The result is a bus bar that the robot can pick up, place, and bolt down without any manual adjustment.
We also added a silver-plated contact area at the terminal holes. The rest of the bus bar is tin-plated, but the 10mm around each hole is silver-plated to 2um. This gives the bolted joint a contact resistance of below 100 micro-ohms, which is what the charger manufacturer's thermal model requires.
Four-legged stainless steel domes used in the EV charger's user interface keypad.
How we test a bus bar
Every batch goes through three tests before it ships.
Dimensional inspection - we measure the hole positions, the bend angles, and the overall length on a coordinate measuring machine (CMM). We check 5 samples from each batch, and all dimensions must be within ±0.1mm.
Contact resistance - we bolt a test lug to each terminal hole, and measure the resistance across the joint. It must be below 100 micro-ohms. If it is higher, the plating is too thin or there is contamination on the contact surface.
Salt spray test - we run a sample of bus bars through 240 hours of neutral salt spray, then re-measure the contact resistance. It must not drift by more than 20 micro-ohms. This tests whether the tin plating will protect the copper from corrosion in an outdoor charger, where the bus bar is exposed to rain, road salt, and humidity.
What this means for our customers
If you are building EV chargers, energy storage systems, or industrial power distribution equipment, we can produce custom bus bars in volumes from 1,000 to 500,000 units per year. We work with C11000 copper, aluminum, and brass, with tin, nickel, or silver plating. We also produce the metal dome keypads and membrane switch panels that go on the outside of the charger.
If you have a new power distribution project and need a bus bar supplier that can hold ±0.1mm tolerance and support automated assembly, send us your drawing. We will turn around a sample in 2 weeks, and we can support production volumes up to 500,000 units per year.
- John Smith, Senior Application Engineer, Shanyo
