Battery Cover Plate Testing

Battery cover plate inspection mainly conducts scientific testing and analysis on the air tightness testing, dimension and geometric tolerance testing, burst pressure testing, insulation resistance and withstand voltage testing, mechanical strength testing and material composition and coating analysis, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Air tightness testing: This is the most core testing item of the battery cover plates, aiming to verify the integrity of their sealing structures. Whether the key parts such as the laser welding between the cover plates and housings, the explosion-proof plate (safety valve) welding, and the insulation sealing between the terminal posts and cover plates have the leakage is mainly tested. Through the testing, ensuring that the internal cells of the battery are isolated from the external environments, preventing the electrolyte leakage, moisture intrusion and abnormal air pressures, which is the basis of guaranteeing the battery safety and service lives.

Dimension and geometric tolerance testing: The precision measurement is carried out on the total thicknesses, terminal post heights, mounting hole position dimensions, flatness and parallelism of the cover plates. The dimension precision directly affects the assembly qualities and welding effects of the cover plates and battery housings, and the geometric tolerances are related to the precision and consistency of the subsequent module assembly. Usually completed with the high-precision equipment such as the coordinate measuring machines, image measuring instruments and laser thickness gauges.

Burst pressure testing: The key safety testing carried out for the explosion-proof plates (safety valves or reversal plates) on the cover plates. The air pressures are continuously increased to the cover plate sealing cavities until the explosion-proof plates act to rupture or reverse, recording their actuation pressure values. This testing aims to verify whether the explosion-proof plates can open timely and accurately within the designed pressure ranges when the abnormal high pressures (such as the thermal runaway) occur inside the battery, thereby releasing the pressures and preventing the battery explosion.

Insulation resistance and withstand voltage testing: The electrical insulation properties between the positive and negative terminal posts, and between the terminal posts and the cover plate metal bodies are tested. The insulation resistance testing verifies whether the resistance values of the insulation materials reach the standards under the specific DC voltages; the withstand voltage testing (also known as the Hi-Pot testing) applies the AC or DC high voltages far higher than the working voltages for a certain time, verifying whether the insulation structures can withstand them without the breakdown, preventing the short circuit risks.

Mechanical strength testing: Evaluating the mechanical properties of the cover plates as a whole and their components (such as the terminal posts and explosion-proof plates). Mainly including the terminal post pull-out force/torque testing, verifying the firmness of the riveting or welding between the terminal posts and cover plates; the overall bending strength testing of the cover plates; as well as the explosion-proof plate fatigue life testing (simulating the micro pressure fluctuations during the multiple charge and discharge cycles), ensuring their structural reliability within the battery life cycles.

Material composition and coating analysis: The material designation conformity verification is carried out on the cover plate base materials (usually the 3003 or 5052 aluminium alloys), terminal post materials (copper-aluminium or nickel-plated copper-aluminium) and sealing rings (such as PFA and PP). At the same time, the surface treatment layers are tested, such as the terminal post plating thicknesses, adhesion and salt spray corrosion resistance, as well as the anodized film thicknesses and hardnesses on the cover plate surfaces, ensuring that the materials meet the design requirements of the corrosion resistance, electrical conductivity and insulation.

Testing Scope

Prismatic battery cover plates: Mainly applied to the prismatic aluminium-shell or steel-shell lithium batteries, with the relatively complex structures, usually integrating the positive and negative terminal posts, explosion-proof plates (reversal valves or scored explosion-proof films), liquid injection holes and short circuit protection devices, etc. The testing scope covers the assembly relationships, sealing properties and overall dimension matching among all the integrated components, and is the most comprehensive cover plate type in the testing items.

Cylindrical battery cover plates (cap assemblies): Used for the cylindrical batteries, usually including the explosion-proof plates (CID, Current Interrupt Device), positive cap, sealing rings and insulation sheets. The testing emphases are the actuation pressures of the CID and the reliability of the current interruption functions, the sealing of the cap and housing crimping, as well as the effectiveness of the internal insulation structures.

Pouch battery aluminium-plastic film sealing edges: The "cover plates" of the pouch batteries are essentially the sealing edges of the aluminium-plastic packaging films. The testing scope concentrates on the sealing qualities of the top sealing and folding edges, including the sealing strengths (peel forces), sealing width consistency, as well as whether the defects such as the wrinkles, foreign matters and punctures exist, ensuring that the various layers of the aluminium-plastic films (nylon, aluminium foil and PP) are completely heat-sealed.

Cover plate component single parts: Before the cover plate assembly testing, the incoming material testing is carried out on its key sub-components. Mainly including the burst pressures and deformation properties of the explosion-proof plates; the dimensions, electrical conductivities and plating qualities of the terminal posts; the material hardnesses, compression set rates and electrolyte corrosion resistance of the sealing rings (gaskets), etc., controlling the quality from the sources.

Welding and connection parts: This is the core area of the cover plate testing. Specifically including: the sealing and strengths of the laser welding/ultrasonic welding seams between the cover plates and battery housings; the laser welding or edge wrapping sealing qualities between the explosion-proof plates and cover plates; the insulation sealing properties of the riveting, welding or ceramic/plastic structures between the terminal posts and cover plates.

Factory full inspection and sampling testing: At the ends of the production lines, the air tightness testing is usually the 100% full inspection item. The items such as the dimensions and appearances may adopt the online vision system full inspection or high-frequency sampling testing. The destructive testing such as the burst pressure and life fatigue strictly carries out the batch sampling inspection according to the sampling plans (such as GB/T 2828.1), to monitor the process stability.

Testing Methods

Differential pressure method air tightness testing: Currently the mainstream non-destructive and quantitative testing method. The cover plates or battery finished products are placed in the closed test cavities, filled with the clean and dry gas (such as the air or nitrogen) of a certain pressure. The pressure difference changes between the test cavities and reference cavities are measured through the high-precision transducers, thereby calculating the leakage rates. This method has the high sensitivity (reaching the order of 10^-3 Pa·m³/s), can distinguish the large leaks and micro leaks, and has the fast testing speed, easy to integrate the automation.

Helium mass spectrometer leak detection method: The absolute leak detection method with the highest precision, often used to verify the accuracy of the differential pressure method or detect the micro leaks with the extremely high requirements. After the cover plates are sealed and filled with the helium of a certain pressure, the helium mass spectrometer leak detectors are used externally with the sniffer probes or the vacuum hood method is adopted to collect the leaked helium molecules. This method has the detection precision up to 10^-12 Pa·m³/s, but the equipment is expensive and the testing cycle is long, mostly used for the laboratory, process verification and sampling testing.

Burst pressure testing method: The dedicated burst pressure testers are adopted. The explosion-proof plate side of the cover plates is sealed and connected, and the pressure is increased to the sealing cavities at the constant rates (such as ≤10 kPa/s, usually using the water or oil as the media). The pressure curves are monitored in real time through the pressure transducers until the explosion-proof plates rupture or reverse, recording the peak pressures as the burst pressures, and comparing with the design windows.

Optical dimension measurement method: The non-contact measurement using the optical principles. The automatic image measuring instruments are mostly used for the two-dimensional dimensions and appearance defects, capturing the contours through the high-resolution CCD cameras for the comparison measurement. For the three-dimensional morphologies such as the flatness and warpage, the laser scanners or white light interferometers are used to quickly obtain the surface point cloud data for the analysis, with the high efficiency and no contact damage.

Electrical property testing method: The insulation resistance testing usually uses the insulation resistance testers (megohmmeters), applying the 500V or 1000V DC voltages and reading the stable resistance values. The withstand voltage testing uses the withstand voltage testers, applying the AC or DC high voltages of several kilovolts (such as 3000V AC for 1 minute between the terminal posts and housings), monitoring whether the breakdown or excessive leakage currents occur.

Mechanical property testing method: The servo tension and compression testing machines or torque testers are used for the terminal post pull-out/torque testing, applying the forces/torques at the constant rates until the failure, recording the maximum values. For the sealing ring compression set rate testing, the rings are placed in the standard fixtures and compressed for a certain time (such as 22 hours at 70°C), and after the unloading, the thickness recovery rates are measured, evaluating their elastic retention abilities.

Testing Standards

Domestic Standards:

1. GB 38031, Electric vehicles traction battery safety requirements

2. GB/T 18287, General specification of lithium-ion cells and batteries for mobile phone

3. GB/T 2828.1, Sampling procedures for inspection by attributes — Part 1: Sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection

4. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

International Standards:

1. IEC 62660-1, Secondary lithium-ion cells for the propulsion of electric road vehicles — Part 1: Performance testing

2. UL 1642, Standard for safety: Lithium batteries

3. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Air tightness tester: The core equipment is the differential pressure type air tightness leak detectors, with the high-precision pressure transducers, pressure stabilizing control systems and dedicated tooling fixtures. The modern equipment integrates the PLCs and touch screens, can store the multiple product testing parameters, automatically complete the whole processes of the inflation, pressure stabilization, testing, judgment and exhaust, and has the data traceability function. It is the key to realizing the automatic full inspection of the production lines.

Burst pressure tester: Composed of the pressure generation systems (electric or pneumatic booster pumps), high-precision pressure transducers, sealing test cavities and data acquisition and control systems. The equipment can precisely control the pressurization rates, draw the pressure-time curves in real time, and accurately capture the pressure peaks at the moment of the burst. Some equipment can also test the reset properties of the explosion-proof plates.

Coordinate measuring machine: Used for the precision testing of the three-dimensional dimensions and geometric tolerances of the cover plates. The probes contact the characteristic points of the cover plate surfaces to obtain the spatial coordinates, and the parameters such as the lengths, diameters, flatness, position degrees and parallelism are calculated through the software. Its measurement precision can reach the micrometre level, and it is the equipment for the die acceptance and first-article testing.

Automatic image measuring instrument: The two-dimensional dimension fast testing equipment based on the machine vision. Through the high-magnification lenses and telecentric light sources, the characteristics such as the cover plate contours, hole positions and engraved codes are detected, and the software automatically extracts the edges and measures. Suitable for the batch testing of the terminal post diameters, hole center distances, character recognition, as well as the appearance defects such as the surface scratches and stains.

Insulation and withstand voltage tester: The electrical safety tester integrating the insulation resistance testing and AC/DC withstand voltage testing functions. The equipment can set the parameters such as the test voltages, ramp-up times, test times and leakage current upper limits, automatically switch the test modes, and has the safety interlock and alarm functions, ensuring the operation safety and reliable test results.

Material analysis instruments: Including the direct-reading spectrometers for the composition analysis; X-ray fluorescence thickness gauges or coulometric thickness gauges for the plating thickness measurement; salt spray test chambers for the salt spray corrosion testing; as well as the dedicated equipment for the thermogravimetric analysis and hardness testing of the sealing ring materials. These instruments provide the scientific data support for the conformity of the cover plate materials and processes.