The nail penetration test is one of the most demanding safety evaluations for power batteries, simulating a severe internal short circuit by driving a steel nail through a fully charged lithium-ion cell. This article explains the testing principle, applicable standards, sample requirements, test methods, and acceptance criteria for nail penetration testing of EV traction batteries and energy storage cells. It is intended for battery engineers, quality professionals, and technical managers seeking a clear understanding of how internal short-circuit safety is verified in accredited third-party laboratories and R&D facilities.
Technical Overview
The nail penetration test, also known as the nail penetration abuse test, is a destructive safety test used to evaluate the ability of a power battery to withstand an internal short circuit. In this test, a hardened steel nail is driven through a fully charged cell at a controlled speed, creating a direct conductive bridge between the positive and negative electrodes. The concentrated current flow at the penetration point generates intense localized Joule heating, which can trigger separator collapse, electrolyte decomposition, and, in unfavorable cases, a full thermal runaway chain reaction.
Because the heat source is generated inside the cell rather than externally, nail penetration is widely regarded as more severe than external short-circuit or crush testing, and it is often used as a benchmark for comparing the intrinsic safety of different cell designs and chemistries. In China, the cell-level nail penetration test was specified in GB/T 31485-2015, while the current mandatory national standard GB 38031-2020 no longer lists it as a mandatory certification item, focusing instead on thermal propagation safety at the pack level, requiring an advance warning of at least five minutes before a fire or explosion endangers occupants. Nevertheless, nail penetration remains extensively used in research and development, OEM acceptance specifications, and comparative safety studies. International frameworks such as the IEC 62660 series, ISO 12405 series, and UL 2580 address internal short-circuit safety through complementary abuse tests, while nail penetration continues to serve as a common OEM-specified verification method performed by accredited third-party laboratories.
Testing Samples
Lithium-Ion Cells (Primary Test Objects)
Cell-level samples are the principal objects of nail penetration testing, since the internal short circuit is initiated directly within the electrode stack. Typical samples include prismatic aluminum-case cells, cylindrical cells, and pouch cells used in electric vehicles and energy storage systems. Prior to testing, cells are charged to full state of charge (100% SOC) using the standard charging method specified by the manufacturer, then rested at ambient temperature until electrochemical and thermal stabilization. The number of samples is defined by the applicable standard or client specification, commonly three or more per test condition to account for statistical variability.
Battery Modules
Module-level penetration testing is performed to evaluate whether a triggered internal short circuit in one cell propagates to neighboring cells. In this configuration, the nail is typically driven into a designated target cell within the module, and the response of surrounding cells, busbars, and thermal barriers is monitored. Module testing provides valuable insight into propagation suppression designs such as aerogel pads, mica sheets, and directed venting channels.
Battery Packs and Systems
Pack-level nail penetration is rarely specified as a routine qualification item due to cost and safety considerations, but it is used in research programs and custom OEM specifications, particularly to support thermal propagation demonstrations required by GB 38031-2020. The test may involve penetration through the pack enclosure into a selected cell, with observation of gas management, venting behavior, and warning signal timing.
Testing Items
The nail penetration test evaluates a defined set of parameters and phenomena, which typically include:
- State of charge verification: confirmation that the sample is at 100% SOC before penetration.
- Nail parameters: rust-free steel nail diameter, typically within the 3 mm to 8 mm range specified by GB/T 31485-2015, with controlled tip geometry.
- Penetration speed: controlled within the range of 10 mm/s to 40 mm/s per the standard, with the speed profile recorded by the test machine.
- Penetration position and direction: the nail is driven in the direction perpendicular to the electrode plates, usually through the geometric center of the largest face of the cell.
- Penetration depth: full through-penetration, with the nail remaining inside the cell for the observation period.
- Voltage response: continuous monitoring of terminal voltage to detect sudden collapse, which indicates the establishment of the internal short circuit.
- Temperature response: surface temperature measured by thermocouples at the penetration point and at defined reference locations on the cell surface.
- Visual phenomena: observation of smoke, gas venting, jet flame, fire, or explosion throughout the test and observation period.
- Acceptance criteria: no fire and no explosion during the test and the subsequent observation period, typically one hour, unless otherwise specified.
Testing Methods
Test Equipment
The test is performed using a servo-driven or electromechanical nail penetration machine capable of controlling penetration speed, position, and depth with high repeatability. The equipment set also includes a rigid clamping fixture, certified thermocouples with a multi-channel data acquisition system, voltage measurement channels, high-speed and standard video recording, and a blast-proof test chamber with forced ventilation, gas extraction, and fire suppression capability. All operations are conducted remotely to protect laboratory personnel.
Sample Preparation
Samples are conditioned at ambient temperature, charged to full SOC using the standard charging procedure defined by the cell specification, and rested, typically for one hour, to allow relaxation of the electrochemical system. Thermocouples are attached at the nail entry point and at reference points on the cell surface, and the voltage monitoring leads are connected before the sample is secured in the fixture.
Penetration Procedure
The steel nail is aligned perpendicular to the electrode plates and driven through the geometric center of the cell at the specified constant speed until full penetration is achieved. The nail remains inside the cell for the duration of the observation period. During the test, voltage, temperature, and visual phenomena are recorded continuously. After penetration, the sample is observed for the period defined by the applicable standard, typically one hour or until the surface temperature has clearly decreased and stabilized. Post-test inspection documents deformation, venting, rupture, and any residual combustion.
Result Evaluation
A sample is judged to pass if no fire and no explosion occur during penetration and the observation period; smoke generation and temperature rise are recorded but do not constitute failure under GB/T 31485-2015. Some OEM specifications impose additional limits on maximum surface temperature or require the absence of full thermal runaway. Because results are highly sensitive to nail diameter, speed, SOC, and penetration location, strict parameter control and adequate sample quantities are essential for reproducible conclusions, which is one of the reasons the test was transitioned out of mandatory certification in favor of system-level thermal propagation requirements.
Applications
Nail penetration testing supports battery safety across the entire development and qualification lifecycle. Typical applications include:
- Cell design validation: assessing separator robustness, electrolyte formulation, vent design, and electrode stack architecture against internal short-circuit abuse.
- Chemistry and supplier comparison: benchmarking lithium iron phosphate (LFP) cells against ternary NCM or NCA cells, with LFP generally exhibiting higher pass rates due to the thermal stability of the olivine cathode structure and lower total heat release.
- Thermal runaway and propagation research: using nail penetration as a controlled trigger method for module- and pack-level propagation studies supporting GB 38031-2020 thermal diffusion compliance.
- Quality control: periodic safety sampling of production batches and verification of process changes affecting electrode burr control, weld quality, and foreign-particle management.
- Failure analysis: reproducing field failure modes associated with metallic contaminants, lithium dendrites, or mechanical crush damage.
- New technology development: evaluating solid-state electrolytes, ceramic-coated separators, flame-retardant electrolytes, and other next-generation safety technologies.
- Broader product coverage: energy storage system cells, commercial vehicle batteries, and light-duty or two-wheeler battery platforms.
FAQ
What does the nail penetration test simulate?
It simulates a severe internal short circuit inside a lithium-ion cell, similar to those caused by metallic particle contamination, lithium dendrite growth, or mechanical deformation after a crash. Because the short circuit is created inside the cell, it represents one of the most challenging abuse scenarios for battery safety.
What are the pass criteria for the test?
Under GB/T 31485-2015, the sample passes if no fire and no explosion occur during the penetration and the subsequent observation period. Voltage and temperature behavior, smoke, and venting are recorded as supporting data, and some client specifications add maximum temperature limits.
Is nail penetration still a mandatory test in China?
No. The mandatory standard GB 38031-2020 removed the cell-level nail penetration test from the certification sequence and instead requires thermal propagation safety at the system level, ensuring occupants receive a warning signal at least five minutes before a hazardous event. Nail penetration remains widely used in R&D, OEM specifications, and comparative safety studies.
What nail diameter and speed are used?
GB/T 31485-2015 specifies a rust-free steel nail with a diameter between 3 mm and 8 mm and a penetration speed between 10 mm/s and 40 mm/s, applied perpendicular to the electrode plates through the cell, with the nail remaining in place during observation.
Why do LFP cells generally perform better than ternary cells?
Lithium iron phosphate cathodes release less oxygen and less total heat during decomposition, have higher thermal runaway onset temperatures, and generate lower peak temperatures during internal shorting. These factors give LFP cells a significantly higher probability of passing nail penetration without fire or explosion.
Where can nail penetration testing be performed?
Testing is conducted by accredited third-party laboratories and specialized battery abuse-test facilities equipped with servo-controlled penetration machines, blast-proof enclosures, gas handling systems, and remote operation infrastructure, ensuring both result reliability and personnel safety.