One milliohm too many can trigger a battery fire. That sounds dramatic - but it's physically precise. In the high-current connections of modern traction batteries, the quality of every single bolted joint determines whether power loss and heat stay within acceptable limits or whether a slow degradation process begins - one that may not surface until months later as capacity loss or a safety hazard.
This article explains the physical relationship between torque, contact pressure, contact resistance, and resistive heating - and shows how to reliably secure your assembly process using the prevailing torque test method per VDI/VDE 2645-3.
The Physics Behind the Connection: Contact Pressure and Contact Resistance
Every bolted joint on a busbar or cell terminal serves two functions simultaneously: it holds components together mechanically and conducts electrical current. Both functions depend on the same variable - the clamping force generated by the applied torque.
At the microscopic level, two metal surfaces never make full-area contact. The so-called constriction resistance arises from the microscopic irregularities of the contact surface; the effective contact area is therefore smaller, and current flow is constricted. The actual size of the contact spots is determined by the contact normal force and the hardness of the surface material.
This means: The higher the clamping force, the larger the true contact area and the lower the contact resistance. The coefficient of friction has a significant influence on the assembly force in the joint, which in turn has a significant influence on contact resistance - insufficient clamping forces lead to an increase in contact resistance.
In operation, this directly affects power loss: Contact resistance is proportional to power loss in the system according to the formula P = I² · R. [1] At the current levels found in a traction battery - several hundred amperes under load - even a marginally elevated contact resistance produces measurable heat generation at the joint.
The Aging Cycle: When Heat Generates More Heat
As electrical connections age, the resistance at the contact point increases over time. Contributing factors include a reduction in contact force, the growth of foreign layers, fretting corrosion, and the resulting further rise in temperature.
Contact force decreases as the preload relaxes, which reduces the area available for current flow. Rising temperature and external influences accelerate the growth of foreign layers, causing oxide films to build up.
This cycle is particularly critical with aluminum busbars: aluminum oxide is a relatively poor electrical conductor - it restricts current flow, increases contact resistance, and heats the joint, which in turn accelerates oxidation, creating a negative feedback loop. This effect was overlooked in the early days of aluminum as an electrical conductor and has led to fires.
Under-Torque and Over-Torque: Two Different Failures, One Outcome
In practice, both extremes are dangerous - though for different reasons.
Under-Torque: Insufficient Clamping Force
Insufficient clamping forces lead to increased contact resistance. There is also the risk of micro-motion: the clamping force achieved and the friction between the mating surfaces influence the lateral force, sliding behavior, and therefore fretting in the contact zone. A general rule holds that sliding or micro-motion in an electrical contact must be avoided.
A loose electrical bolted connection first produces elevated contact resistance and then a voltage breakdown. In practice, the resulting damage patterns range from overheating at the contact point to burning of the contact itself or adjacent components.
Over-Torque: Too Much Is Also Too Much
On the other side, the materials themselves set an upper limit on permissible torque. Highly conductive materials have a limited allowable surface pressure, which can constrain the torque. In some cases, the tightening torque is also limited by the hardware or battery cell manufacturer.
Aluminum is particularly sensitive here: bolted joints must account for the low strength and tendency to creep of pure aluminum, and the correspondingly low allowable surface pressure. Under sustained load, aluminum is prone to long-term creep - bolted joints can loosen over time as a result.
Over-torquing cell contact connections can mechanically damage cell housings or permanently deform the contact surface. Manufacturer specifications for maximum tightening torque are binding — not as a guideline, but as a hard limit.
Clean Contact Surfaces: The Underestimated Prerequisite
The most precise torque value is of little use if the contact surfaces are contaminated. Contact resistance depends on the force pressing the two mating surfaces together, the deformability and hardness of the contact partners, surface roughness, and any surface layers present.
With aluminum busbars, a non-conductive oxide layer forms within minutes: unprotected aluminum surfaces are quickly covered by a hard oxide film. This layer is not electrically conductive, making clean contacts considerably more difficult to achieve. If the oxide layer is not removed, contact resistance rises - the joint heats up, and the risk of fire increases significantly.
Fretting corrosion caused by micro-motion during operation is also a risk factor: fretting corrosion, driven by mechanical displacement or thermal expansion, causes the contact surfaces to shift against each other, destroying existing micro-contacts.
Practical implications for assembly:
- Clean contact surfaces immediately before fastening
- Mechanically remove oxide layers (especially on aluminum)
- Do not apply grease or oil to the actual contact surface, as this uncontrollably alters the friction coefficient
- Document cleanliness and establish it as a defined process step
Friction Coefficient Variation: The Invisible Source of Error
A frequently underestimated problem: even with identical applied torque, the actual clamping force achieved can vary significantly. Friction coefficients measured and calculated during the assembly process can range from 0.09 to 0.2. The resulting clamping force varies accordingly - from 13 kN down to 6.6 kN.
With a friction coefficient difference of 0.09 versus 0.2, the achieved clamping force is cut in half despite identical applied torque. [1]
Friction coefficients have a considerable influence on the assembly force in the joint, which in turn has a significant influence on contact resistance. By calculating the friction coefficient, the torque specification can be adjusted to compensate for the variation introduced by friction scatter.
This is why torque-only control is not sufficient in battery assembly: the tool can hit the target torque exactly - and the joint can still be defective because the friction coefficient fell outside the expected range.
Process Validation: The Prevailing Torque Test per VDI/VDE 2645-3
The answer to these challenges is the process capability study (PCS) with prevailing torque measurement. VDI/VDE 2645-3 describes methods for process capability studies (PCS) for prevailing torques of bolted connections with preload. [2]
The goal of a process capability study for bolted joints is to evaluate and document the quality capability of a fastening process under production conditions. A PCS provides guidance for assessing and continuously improving the fastening process under series production conditions.
Unlike a machine capability study (MCS), a process capability study accounts not only for machine influence but also for the additional influence categories of personnel, material, method, and environment.
This distinction is critical: in battery assembly, friction coefficients vary depending on batch, surface condition, and ambient temperature. A machine capability study alone does not fully capture these influences.
What the Prevailing Torque Measurement Actually Measures
The breakaway torque during further rotation is the torque required - after the fastening operation is complete - to overcome the static friction of the fastener.
During measurement, it is essential to capture the exact point at which the screw or nut just begins to turn. The further the screw is turned, the more the torque increases, and the measured result then depends on how far the inspector has rotated the fastener.
This is precisely where the requirement for the test tool lies: it must accurately capture the breakaway point - not the torque after several degrees of additional rotation.
The Q-CHECK® as a QA and Audit Tool in Battery Assembly
The Q-CHECK® from GWK is purpose-built for this task: prevailing torque measurements for process capability studies per VDI/VDE 2645-3 - directly on the assembly line, with no laboratory overhead.
Q-CHECK® key technical specifications:
- Measurement range: 3 to 1,000 Nm
- Accuracy: ±1% between 10 and 100% of the nominal range
- Storage: 2 GB internal data storage
- Application: QA inspection and audit tasks on the line
Important distinction: The Q-CHECK® is a QA and audit tool for prevailing torque measurements - not a calibration device. DAkkS-accredited calibration of torque and angle wrenches is performed using the DWPM calibration machine (accuracy class 0.2) in the GWK calibration laboratory. Both instruments have clearly defined, distinct roles within the quality system.
Q-CHECK® vs. DWPM 1000c – choosing the right tool:
| Task | Tool |
|---|---|
| Prevailing torque testing / PFU per VDI/VDE 2645-3 | Q-CHECK® |
| DAkkS-accredited calibration of torque tools | DWPM 1000c |
Both instruments are part of a complete quality system — but they are not interchangeable.
Applications in Battery Assembly: Concrete Use Cases
Busbar fastening (cell-to-cell, module-to-module): Torque values here are often in the low range (a few Nm up to approximately 20 Nm). Friction coefficient variations caused by different surface coatings (nickel, tin, silver) have a particularly strong effect on clamping force. The prevailing torque test reveals whether the assembly tool is actually generating the required preload.
High-voltage connections (battery-to-inverter, battery-to-charge port): Larger fastener sizes (M8-M12), higher torque values, but also higher currents - and therefore higher demands on contact resistance. Regular PCS sampling secures process stability across shifts and batches.
Audit inspections: The Q-CHECK® is well suited for spot-check verification of already-assembled joints - for example, at production launch, after a tool change, or following a material change (new fastener batch, new lubricant).
Process Assurance in Practice: What a Complete System Must Deliver
Under series production conditions, a wide range of technical influences in the fastener and the assembly tool can result in joints that are not assembled in a process-capable manner. To ensure process reliability, fasteners, tools, and assembly processes must all be monitored.
A complete quality system for electrical contact fastening in battery assembly includes:
Calculate tightening torque based on the required clamp force (VDI 2230). Determine friction coefficients for the material pairings used (Cu/Cu, Cu/Al, Al/Al) and any coatings. Account for manufacturer specifications on maximum surface pressure.
Define a cleaning protocol for contact surfaces. Remove oxide layers on aluminum busbars immediately before assembly. Establish cleanliness as a documented process step.
Assess assembly tools for machine capability (VDI/VDE 2645-2). Ensure the tool produces the target torque with sufficient repeatability.
Perform prevailing torque measurements per VDI/VDE 2645-3. Define sample sizes and inspection intervals. Calculate and document Cp/Cpk values. Re-evaluate after any process changes (material changes, tool changes).
Calibrate Q-CHECK® and assembly tools on a regular basis — using the DWPM 1000c at the DAkkS-accredited GWK calibration laboratory or via the mobile on-site calibration service.
Archive measurement data without gaps. The Q-CHECK®'s 2 GB storage enables extensive data sets to be stored directly on the tool. Use data for audit evidence and continuous process improvement.
Conclusion: Torque Is Not Clamping Force - and Clamping Force Is Not Contact Resistance
The chain from tool rotation to electrical resistance at the contact point is longer than it first appears. Friction coefficients, surface condition, material properties, and process variation influence every step. Lower contact resistance reduces thermal stress and thereby increases reliability, service life, and energy efficiency.
The prevailing torque test with the Q-CHECK® closes the gap between what the assembly tool displays and what has actually been achieved in the joint. It is not a bureaucratic exercise - it is the only method that allows you to demonstrate, under series production conditions, that your electrical contact fastenings meet the required quality standard.
Accuracy by GWK.
