Mechanical testing for wearable and flexible batteries requires specialized approaches distinct from traditional rigid cell evaluations. These batteries must endure dynamic stresses such as bending, folding, and stretching while maintaining electrochemical performance and safety. Key assessments include cyclic mechanical deformation tests, skin-contact compatibility evaluations, and structural integrity analyses under strain.

Cyclic Bend and Fold Testing
Wearable batteries undergo repeated bending or folding during use, necessitating rigorous cycling tests to simulate real-world conditions. Bend cycling involves clamping the battery at one end and subjecting it to controlled angular deflection, typically between 90 and 180 degrees, at a fixed radius. The radius is critical; for skin-worn devices, a 5 mm to 10 mm radius is common to mimic human joint movement. Fold cycling, more extreme, involves compressing the battery to a near-zero radius, often with a creasing apparatus. Performance metrics include capacity retention, internal resistance shifts, and mechanical failure points after thousands of cycles.

For instance, a lithium-polymer flexible cell may retain over 90% capacity after 10,000 bend cycles at a 5 mm radius but degrade to 80% after 1,000 fold cycles at a 1 mm radius. Delamination of electrode layers or fracture of current collectors often limits cycle life.

Tensile and Compression Tests
Unlike rigid cells, flexible batteries must withstand tensile forces without cracking. Uniaxial tensile tests measure elongation at break, with high-performance flexible electrodes often enduring strains exceeding 10% before failure. Compression tests evaluate resilience against pressure, such as from wearable straps or embedded textiles. A typical protocol applies 50 kPa to 200 kPa of pressure while monitoring for short circuits or performance decay.

Skin-Contact Safety Assessments
Wearable batteries must avoid hazardous reactions upon contact with sweat or abrasion. Key evaluations include:
- **Leakage Tests**: Submersion in artificial sweat (pH 4.5–6.5) for 24–48 hours to check electrolyte seepage.
- **Biocompatibility**: Cytotoxicity assays per ISO 10993-5, ensuring no harmful leachables.
- **Thermal Stability**: Surface temperature monitoring during operation to prevent burns, with limits set at 41°C for prolonged skin contact.

Structural and Interconnect Reliability
Flexible batteries use thin-film or segmented designs, requiring robust interconnects. Peel tests measure adhesion strength between layers, with values below 0.5 N/cm indicating delamination risk. Dynamic shear tests assess solder or conductive adhesive joints under repetitive motion.

Environmental Stress Screening
Combined mechanical and environmental stresses are critical. Tests include:
- **Temperature Cycling**: -20°C to 60°C with concurrent bending to simulate outdoor use.
- **Humidity Exposure**: 85% relative humidity with cyclic deformation to assess corrosion susceptibility.

Data from these tests inform design improvements, such as strain-tolerant electrodes or encapsulation barriers. Unlike rigid cells, where mechanical tests focus on crush resistance or vibration, wearable battery evaluations prioritize flexibility endurance and user safety.

Standardization gaps persist, but emerging protocols from IEC 63215 and ASTM F2921 provide frameworks for consistent testing. Future developments may integrate real-time strain sensing to correlate mechanical stress with electrochemical degradation dynamically.

This focus on dynamic mechanical reliability ensures wearable batteries meet the demands of next-generation flexible electronics without compromising safety or performance.