Battery Applications in Smart Shoes

Smart shoes integrate sensors, connectivity, and advanced features like heating or gait analysis, requiring batteries that are lightweight, flexible, and durable. Below is a detailed breakdown of battery technologies, use cases, and innovations in smart footwear:

1. Common Battery Types

**(1) Lithium Polymer (Li-Po) Batteries**

• Capacity: 100–500mAh (3–24 hours runtime).
• Voltage: 3.7V (single-cell).
• Advantages:
• Flexible & Thin: Conforms to shoe contours (e.g., heated insoles like ThermaCELL).
• Fast Charging: 1–2 hours via USB-C or wireless charging.
• Use Cases:
• Heated Shoes: Keeps feet warm in extreme cold (e.g., Millet Heated Boots).

• Fitness Tracking: Powers step counters and heart rate monitors (e.g., Under Armour HOVR).

**(2) Lithium-Ion (Li-ion) Batteries**

• Capacity: 500–2000mAh (12–48 hours runtime).
• Voltage: 3.6V (e.g., pouch cells).
• Advantages:
• High Energy Density: Longer runtime for GPS or AR features.
• Durability: Withstands vibrations and impacts (e.g., running shoes).
• Use Cases:
• GPS Tracking: Lost-and-found features (e.g., GPS SmartSole).

• AR Navigation: Heads-up displays in smart boots (e.g., prototype military footwear).

**(3) Flexible Solid-State Batteries** (Experimental)

• Features:
• Bendable Design: Integrates into fabric or soles (e.g., Imprint Energy’s zinc-based cells).
• Safety: No flammable electrolytes.

• Challenges: Limited commercial availability, higher cost.

2. Core Applications & Power Requirements

3. Key Design Challenges

1. Size & Weight Constraints:
• Target: Battery weight <50g (e.g., Adidas GMR soccer tracker: 15g module).
• Innovation: Printed batteries (e.g., Enfucell’s SoftBattery®).
2. Waterproofing:
• IP67/IP68: Essential for sports/outdoor shoes (e.g., Salomon hiking boots).
3. Heat Management:
• Risk: Overheating in heated insoles.

• Solution: Phase-change materials (PCMs) + thermal cutoffs.

4. Charging & Energy Efficiency

1. Wireless Charging:
• Qi-compatible insoles (e.g., Wiivv Heat 360).
2. Kinetic Energy Harvesting:
• Piezoelectric soles convert foot strikes to power (e.g., SolePower work boots).
3. Solar Integration:

• Transparent solar films on shoe surfaces (experimental: Solar Sneakers by Pauline van Dongen).

5. Safety & Sustainability

• Certifications: UL 2054, IEC 62133 for fire/impact resistance.
• Recycling: Li-Po/Li-ion recycling programs (e.g., Call2Recycle).

• Biodegradable Batteries: Research on cellulose-based electrolytes (e.g., Uppsala University prototypes).

6. Case Studies

1. Digitsole Smart Shoes:
• Battery: 3.7V 400mAh Li-Po (8-hour heating).
• Features: Self-lacing, app-controlled temperature.
2. Vixole LED Sneakers:
• Battery: 1000mAh Li-ion (6-hour customizable lighting).
• Charging: 2-hour USB-C.
3. Lechal GPS Smart Shoes:

• Battery: 3.7V 600mAh Li-Po (vibration navigation for visually impaired users).

7. Future Trends

1. Self-Charging Shoes:
• Hybrid systems combining kinetic, solar, and thermal energy (DARPA-funded projects).
2. AI-Optimized Power Use:
• Predictive algorithms to extend battery life (e.g., reducing GPS polling when stationary).
3. 3D-Printed Batteries:

• Custom-shaped cells integrated during shoe manufacturing (e.g., New Balance collaborations).