A battery cell interconnect system joins the individual cells in a battery pack into series and parallel strings and carries sensing signals to the battery management system (BMS). Many electric vehicle packs use a cell contacting system (CCS), a module that combines busbars for current with a flexible printed circuit (FPC) for voltage and temperature sensing, in place of a wiring harness.
In short:
- The interconnect has two jobs: carry high current between cells, and carry small sensing signals to the BMS.
- Busbars are copper, aluminum or nickel-plated copper. Aluminum is lighter but needs a larger cross-section for the same performance.
- Cells are usually joined by laser welding or ultrasonic wire bonding, because both keep contact resistance low.
- Cell format (cylindrical, prismatic or pouch) changes the design of the interconnect.
What a Cell Interconnect System Does
The interconnect has two separate electrical paths.
- The power path. Busbars link cells in series and parallel, so the pack reaches its voltage and capacity. High current flows through them during fast charging and driving.
- The sensing path. Small currents flow through traces on a printed circuit that report cell voltage and temperature to the BMS. A typical design places an NTC thermistor next to a nickel sheet that is soldered to a busbar. As the temperature changes, the thermistor's resistance changes, and the BMS reads it.
A pack-level connector then links this assembly to the rest of the vehicle. This article covers the cell level. For the connectors at pack level, see our guides to battery connector systems for electric vehicles and high-voltage EV connectors.
Parts of a Cell Contacting System
A cell contacting system (CCS) is built from a few standard parts. This description follows a manufacturer's overview of the technology, so check details against your supplier's datasheet.
- Busbars. Copper is the most common choice. Aluminum and nickel-plated copper are also used. The busbar material often follows the cell terminal: aluminum for pure nickel poles, copper for copper poles.
- FPC or rigid PCB. The signal board carries the sensing traces, thermistors and a connector to the BMS. The flexible version (FPC) is polyimide-based and is the most common. A rigid FR4 board is the alternative.
- Nickel sheets. These small parts connect the signal board to the busbars, by laser soldering or screw riveting.
- Insulation films. Thin plastic films, such as PET, insulate the assembly from the pack housing.
Compared with a wiring harness, a printed-circuit design cuts cable count and weight, and it suits automated assembly. Harness-based designs are heavier, take more space and rely on manual terminal fixing.
Copper vs Aluminum Busbars
Material choice is the main trade-off in busbar design. The figures below come from a busbar supplier's published guidance.
- Thermal conductivity — Copper: 401 W/mK; Aluminum: 237 W/mK
- Thermal expansion — Copper: 16.5 ppm/K; Aluminum: 23.1 ppm/K
- Weight — Copper: Baseline; Aluminum: Typically about 50% lighter
- Typical cell-connection thickness — Copper: 0.5 to 2.5 mm; Aluminum: 1.0 to 2.0 mm
For equal performance, aluminum needs a larger cross-section. The supplier's example is a 1 mm copper conductor matched by a 2 mm aluminum one. Busbars that connect a whole module need to be thicker than those that link single cells, because they carry more current.
When copper and aluminum meet in one joint, watch the materials: the two metals expand at different rates, and dissimilar metals invite corrosion. Our article on galvanic corrosion and plating incompatibility explains how to manage that.
How Cells Are Joined
Two methods dominate: laser welding and ultrasonic wire bonding. Both keep contact resistance low, which matters because resistance turns into heat. Suppliers recommend laser welding over soldering for this reason, and it suits automated high-volume production. A single busbar may connect hundreds of cells, and some packs contain thousands.
What a laser-weld study found
A 2021 University of Warwick study tested laser-welded tab-to-busbar joints in different material pairs. These were lab test coupons, so in-pack temperatures will differ, but the pattern is useful:
- Strength: nickel tabs gave the strongest joints, at about 930 N on aluminum and about 1,320 N on copper. Copper tab on aluminum busbar reached about 560 N, and aluminum tab on copper busbar about 520 N.
- Defects: copper-to-aluminum welds showed micro-cracks, which the authors link to the two metals' different thermal expansion. Nickel-to-aluminum welds formed intermetallic compounds that the authors link to pores. Nickel-to-copper welds showed none.
- Electrical resistance: copper-to-copper had the lowest contact resistance, about 0.042 milliohms at the start, rising to about 0.050 milliohms at 200 A. Copper-to-aluminum measured 16% to 20% higher.
- Heat: the nickel tab overheated at 200 A, reaching about 400 °C after 30 seconds. Copper-to-copper stayed coolest, at 68.4 °C at 200 A.
The authors concluded that copper and nickel-plated copper tabs gave the best combined electrical and thermal performance, and that nickel is strong but expensive with poor electrical and thermal performance. For more on why contact resistance matters, see our guide to connector contact resistance.
How Cell Format Changes the Design
The interconnect looks different for each cell format.
- Cylindrical cells. The busbar is usually flat, separate conductors or a laminated structure, with connections on one side only. Aluminum conductors from 0.25 to 1.0 mm thick are common. Individual cells are joined by laser weld or wire bond.
- Prismatic cells. Screw-attached lugs in an integrated housing are typical. Some hybrid busbars also carry sensor signals to the BMS.
- Pouch cells. Often there is no busbar at all. Up to four cells are commonly welded into a small module, which is then placed in a metal case.
Design Considerations
- Connect one side or both. Connecting cells at the top and bottom can block efficient air or liquid cooling, so some designs connect on one side only, at the cost of a more complex busbar. A laminated busbar for a one-sided design uses two conductors separated by an insulating film, which also seals the assembly together.
- Voltage and spacing. Future cells may reach up to 5.0 V each, which makes clearance and creepage distances more important. See our guide to voltage rating, creepage and clearance.
- Vibration and expansion. The assembly has to stay rigid enough to hold the module together and flexible enough to absorb thermal expansion and shock loads.
- The connector to the BMS. The signal board ends in a connector that has to survive vibration. See our guides to FPC connector selection and locking mechanisms for vibration.
- Heat. Joint resistance and current set the temperature. Our article on thermal derating curves shows how to read a current rating at temperature.
Questions to Ask a Supplier
- Which cell format and joining method (laser weld, wire bond or screw) is the design validated for?
- What are the busbar material and thickness, and which cell terminal materials does it join?
- What contact resistance was measured on the joint, and at what current?
- How are the sensing traces insulated from the busbars, and what clearance does the design keep?
- Which connector joins the signal board to the BMS, and how is it locked?
Frequently Asked Questions
What is a cell contacting system (CCS)? A CCS is the module that electrically joins the cells in a pack and links them to the battery management system. It combines busbars for current with a printed circuit for sensing.
Why use an FPC instead of a wiring harness? A printed-circuit design cuts cable count and weight, and it suits automated assembly. Harness designs are heavier and need manual terminal fixing.
Copper or aluminum busbars? Aluminum is typically about 50% lighter, but it needs a larger cross-section for the same performance. Copper conducts heat better and expands less. The right choice depends on cell terminal material, weight targets and the joint.
Why is laser welding preferred? It keeps contact resistance low, which reduces electrical loss and heat, and it suits automated, high-volume production.
How is a CCS different from a battery connector? A CCS works inside the pack at cell level. A battery connector links the pack to the vehicle. See our guide to battery connector systems for electric vehicles and HVIL design.
Sources
- Battery cell contact system (PCBONLINE): CCS parts and functions. Manufacturer overview.
- Considerations for choosing the optimal busbar for your EV battery system (Rogers): material properties, thickness, cell formats.
- Optimal interconnection for cylindrical battery systems (Rogers): cylindrical cell joining.
- In-depth evaluation of laser welded tab-to-busbar interconnects (University of Warwick, 2021): weld strength and resistance results.