21700 Batteries

What is a 21700 Battery?

A 21700 battery is a rechargeable lithium-ion cell with a cylindrical shape and standardized dimensions of 21mm in diameter and 70mm in length. It was developed as an improvement over the 18650 battery, offering higher capacity, better energy density, and improved efficiency, making it ideal for high-power applications such as electric vehicles and energy storage systems.

Technical Specifications

• Chemistry: Like 18650 batteries, 21700 batteries come in various lithium-ion chemistries, including:
  • Lithium cobalt oxide (LiCoO₂ - LCO): High energy density but lower discharge rates.
  • Lithium manganese oxide (LiMn₂O₄ - LMO): Improved safety and thermal stability.
  • Lithium nickel manganese cobalt oxide (LiNiMnCoO₂ - NMC): Balanced energy density, power output, and safety.
  • Lithium iron phosphate (LiFePO₄ - LFP): Excellent safety and long cycle life but lower energy density.
• Voltage: Typically 3.6V or 3.7V nominal, with a full charge voltage of 4.2V and a discharged state at 2.5V to 3.0V.
• Capacity: Ranges from 3000mAh to over 5000mAh, depending on the chemistry and manufacturer.
• Discharge rate (C-rating): Some 21700 cells are designed for high-drain applications, offering continuous discharge rates (CDR) up to 45A or more.
• Cycle life: Can last between 500 to 1500+ charge cycles, depending on usage and chemistry.

Protected vs. Unprotected Cells

• Protected 21700 batteries: Feature an internal protection circuit to prevent overcharging, over-discharging, and short circuits, making them slightly longer than 70mm.
• Unprotected 21700 batteries: Lack built-in safety features and are generally used in battery packs with external battery management systems (BMS).

Common Applications

21700 batteries are widely used in:

• Electric vehicles, with companies like Tesla and other manufacturers using them in battery packs for improved energy storage and performance.
• High-power consumer electronics such as flashlights and power banks.
• Power tools, including drills, saws, and industrial-grade equipment.
• Renewable energy storage, such as solar power storage systems and backup power supplies (UPS).

Physical Variations

Some 21700 batteries come with slight design variations affecting their compatibility:

• Button top vs. flat top: Button top batteries have an extended positive terminal, while flat tops are flush.
• Protected vs. unprotected: Protected cells are slightly longer due to the built-in safety circuit.

As with any lithium-ion battery, it is essential to check the device manufacturer's specifications before using a 21700 battery to ensure compatibility and optimal performance.