Electrical Connections &
Interconnect Design
From busbars and cell-level strips to welding methods, fuses, connectors, and the contactor pre-charge sequence — every element of the electrical path that carries current safely from cells to load.
Every battery pack is, at its core, an electrical system — and like every electrical system, it must have a deliberate, engineered path for current to flow from the cells to the load and back again. Every wire, busbar, strip, connector, fuse, and contact point in the pack is part of the electrical path design, and every one of them directly influences the pack's performance, efficiency, safety, and reliability.
The starting point for any electrical path design is the current specification. The designer must know three values: maximum continuous current, peak current for short bursts, and fault current — the current that will flow during a short circuit that the protection system must interrupt.
A single poor connection with just 1 mΩ of contact resistance carrying 200 A generates 40 W of heat continuously — enough to create a hotspot, accelerate local degradation, and in worst cases initiate a fire. Resistance minimisation is the governing principle of all electrical path design in batteries.
Busbars are the primary current-carrying conductors in a battery pack — rigid or semi-rigid metallic strips or bars that connect cells, modules, or the pack to external terminals. They are the backbone of the pack's electrical architecture, responsible for carrying the full pack current with minimal resistive loss.
| Material | Resistivity | Density | Key Advantage | Key Limitation | Best Use |
|---|---|---|---|---|---|
| Copper | 1.72×10⁻⁸ Ω·m | 8.96 g/cm³ | Lowest resistance | Heavy, expensive | Automotive packs |
| Aluminium | 2.82×10⁻⁸ Ω·m | 2.70 g/cm³ | Lightest option | Oxide layer, complex joining | Weight-critical apps |
| Nickel | 6.99×10⁻⁸ Ω·m | 8.91 g/cm³ | Easy to spot-weld | High resistance | Consumer electronics |
An aluminium busbar of the same resistance as a copper busbar is significantly lighter but physically larger — approximately 1.6× the cross-sectional area required. This larger size may be acceptable in an ESS enclosure with ample space but problematic in a compact EV module. Aluminium also requires special joining techniques to prevent galvanic corrosion at copper-aluminium interfaces.
At the cell level — where individual cells are connected to form parallel groups and series strings — interconnects are typically thinner and more flexible than main power busbars. The choice of cell-level interconnect material is one of the most practically consequential decisions in pack design, directly affecting resistance, heat generation, cost, and manufacturability.
| Interconnect Type | Typical Thickness | Resistance vs Cu | Weldability | Current Range | Typical Application |
|---|---|---|---|---|---|
| Pure Nickel Strip | 0.1–0.2 mm | 4× higher | Excellent (spot) | <10 A/cell | Consumer electronics, small packs |
| Pure Copper Strip | 0.1–0.3 mm | Baseline | Hard (laser only) | 10–30 A/cell | Automotive, power tools, EV |
| Aluminium Strip | 0.2–0.4 mm | 1.6× higher | Laser / ultrasonic | 8–25 A/cell | Weight-critical EV modules |
| Cu-Clad Nickel | 0.1–0.25 mm | 1.5–2× higher | Good (spot) | 5–20 A/cell | Mid-range packs, e-bikes, BESS |
The general rule: use nickel when spot welding is required and current is low (<10 A/cell). Use copper-clad nickel when you want spot weldability with better conductance. Use pure copper when currents exceed 10–15 A per cell and laser welding is available. Use aluminium only when weight is critical and the additional joining complexity is acceptable.
The quality of connections between cells and interconnects is one of the most critical determinants of a battery pack's electrical performance, thermal behaviour, and long-term reliability. The joining method must produce low contact resistance, adequate mechanical strength, and minimal thermal damage to the cell — all simultaneously.
Use spot welding for nickel strips on cylindrical cells in cost-sensitive applications. Use laser welding for copper interconnects and all automotive-grade packs. Use ultrasonic welding for aluminium tab connections in pouch and prismatic cell modules. Use bolted connections for all module-to-module and pack-level junctions that may need servicing.
Every conductor in the battery pack has a maximum current it can carry without excessive heating — called the current carrying capacity or ampacity. Correctly sizing every conductor is a fundamental responsibility of the pack designer.
Connection resistance — the resistance at every interface between conductors — is one of the most significant and most frequently underestimated sources of energy loss and heat generation in battery pack design. Every weld, crimp, bolt, socket, or press-fit has a small but non-zero resistance that contributes to the pack's total internal resistance and heat generation.
| Practice | Applies To | Effect |
|---|---|---|
| Clean surfaces before joining | All joints | Removes contamination & oxide |
| Nickel plating on copper terminals | Bolted & press-fit | Stable, low-resistance surface |
| Silver plating on HV contacts | High-current connectors | Lowest possible contact resistance |
| Correct torque on bolted joints | Bolted busbars | Under-torque = high R; over-torque = damage |
| Conductive anti-corrosion grease | Dissimilar metal junctions | Prevents galvanic corrosion over time |
| 4-wire (Kelvin) resistance measurement | Production QC | Accurate mΩ-level measurement |
Fuses and protection elements serve one essential function: interrupting dangerous overcurrent conditions before they cause permanent damage or create a safety hazard. Every battery pack, without exception, requires overcurrent protection.
Standard automotive fuses designed for 12V systems absolutely cannot be used in high-voltage battery packs. When a high-voltage DC circuit is interrupted under load, the resulting electrical arc is extremely energetic and difficult to extinguish — a 12V fuse will arc through and fail to interrupt the circuit, making the fault condition significantly worse. Always use fuses rated for the full pack voltage.
Connectors in a battery pack provide separable electrical connections for assembly, installation, servicing, and replacement. In high-voltage applications they also provide critical safety interlock functions that prevent accidental contact with live conductors.
Battery packs operate in two distinct electrical domains simultaneously — the high-voltage domain of the cell stack and power electronics, and the low-voltage domain of the BMS, sensors, and control systems. Managing the boundary between these domains safely and reliably is one of the most important aspects of battery pack electrical design.
An IMD continuously measures insulation resistance between the high-voltage domain and the vehicle chassis. A healthy pack has insulation resistance of megaohms or higher. If insulation resistance falls below a threshold — indicating HV is contacting the chassis through a fault — the IMD generates a fault signal, typically triggering a driver warning and potentially de-energising the pack. Required by ISO 6469-3 and FMVSS 305 for all road-going EVs.
Every high-voltage conductor that is not protected by a rated connector or enclosed busbar system must be insulated. Insulation and isolation design is an area where many beginner designers underestimate requirements — with potentially catastrophic consequences.
Contactors are the switching elements that connect and disconnect the battery pack from the high-voltage electrical system. They are safety-critical components — failure to open when commanded, or inadvertent opening under load, can both result in dangerous conditions.
| Contactor Rating | What It Governs | Design Consequence |
|---|---|---|
| Continuous current | Max sustained pack current at temperature | Size to max continuous + derating factor |
| Make current | Inrush when closing into uncharged capacitors | Pre-charge circuit required to limit this |
| Break current | Current contactor must interrupt when opening under load | Must match maximum fault scenario |
| DC voltage rating | Full pack voltage including surge | AC-rated contactors CANNOT be used in DC circuits |
When main contactors close, inverter bus capacitors are initially uncharged. Connecting a fully charged 400V pack directly to uncharged capacitors creates an enormous inrush current — potentially thousands of amperes for tens of milliseconds. This can weld the contactor contacts shut, damage capacitors, and cause severe voltage transients. The pre-charge circuit limits this inrush to a safe level using a series resistor.
If a contactor is exposed to current beyond its break rating during a severe fault, the resulting arc can weld the contacts shut permanently. A welded contactor cannot be opened — the HV bus cannot be de-energised, creating a serious safety hazard for service personnel. Prevention: size contactors correctly, ensure pre-charge limits make current, and size the main fuse to interrupt fault current before it can weld contactor contacts.
