Cylindrical Power Lithium-Ion Batteries (1): Features, Selection & Safety Guide for 18650 and 21700

In the field of new energy equipment, cylindrical power lithium-ion batteries have become a common power source for power tools, drones, small energy storage systems, and other scenarios, thanks to their standardized design and stable performance. However, when faced with different specifications like 18650 and 21700, many people wonder, "Which one should I choose?" and "How to use it safely?" This article focuses on these two mainstream models, breaking down their core features, applicable scenarios, and safety key points to help you quickly grasp the basic selection logic.

Part1、First, Understand: Basic Knowledge of Cylindrical Power Lithium-Ion Batteries

Cylindrical power lithium-ion batteries (referred to as "cylindrical batteries" for short) are cylindrical lithium batteries that can continuously supply power to high-energy-consuming equipment, distinguishing them from ordinary consumer batteries used in remote controls. Their naming rule is simple: diameter (mm) + height (mm). For example, 18650 means "18mm in diameter and 65mm in height," while 21700 stands for "21mm in diameter and 70mm in height."

The internal structure of such batteries resembles a "rolled-up sandwich": the positive electrode, negative electrode, and separator are alternately stacked and then rolled into a cylinder. Combined with a safety valve, overcurrent protection device, and aluminum casing (steel casings were used in the early days), they form a complete power unit. Among these components, the overcurrent protection device is the core of safety, mainly divided into two types:

• PTC (Positive Temperature Coefficient Thermistor): A resettable overcurrent protection device. When the current is too large and causes the temperature to rise, its resistance increases sharply to limit the current; it resumes conduction when the temperature drops. Suitable for short-term, resettable overcurrent situations, commonly used in power tools, household energy storage, and other scenarios.

• CID (Current Interrupt Device): A one-time, irreversible current protection device. When the internal pressure of the battery rises slightly due to faults such as overcharging, the metal conductive sheet of the CID breaks physically, permanently cutting off the current circuit to prevent the overcurrent from worsening the fault. The "explosion-proof valve" of the battery is an independent pressure relief component, responsible for releasing gas when the pressure is too high to avoid explosion.

Part2、 18650 Battery: A Mature and Reliable "Entry-Level Choice"

The 18650 is the first power cylindrical battery to achieve large-scale application, with decades of technological accumulation. It is still widely used in low-threshold scenarios today.

2.1.  3 Core Advantages, Suitable for Cost-Sensitive Scenarios

• Lowest Cost: The industrial chain is the most mature, with complete supply from raw materials to production equipment. The unit cost is 15%-20% lower than that of 21700, making it suitable for budget-friendly power tools (around $100) and small energy storage systems.

• Good Consistency: The dimensions are fully standardized (with minimal errors among different manufacturers). Fully automated production ensures small differences in capacity and voltage between batches, making the assembly of battery packs more stable.

• Strong Impact Resistance: The cylindrical closed structure can withstand greater bending and extrusion forces. When installed in power tools and exposed to bumpy working conditions, it is more durable than prismatic batteries.

2.2   4 Obvious Shortcomings, Use with Caution in High-Demand Scenarios

• High Battery Pack Cost: If used in large equipment (such as small electric vehicles), hundreds of cells need to be connected in series/parallel, requiring more complex brackets, heat dissipation systems, and BMS (Battery Management Systems). As a result, the overall cost is higher than that of 21700 battery packs.

• Difficult Heat Dissipation: When small-sized cells are densely arranged, heat is prone to accumulate in the middle area. Long-term use will lead to uneven battery degradation (longer lifespan for edge cells and shorter lifespan for middle cells).

• Low Energy Density: The maximum monomer energy density is approximately 260Wh/kg, which is difficult to break through due to size limitations. To improve the equipment's battery life, the only way is to "increase the number of cells," which instead adds weight.

• Limited Battery Life: Taking power tools as an example, the battery life of 18650 battery packs is usually 20%-30% shorter than that of 21700 versions. Frequent battery replacement is required during high-intensity operations.

Part3、 21700 Battery: A Performance-Upgraded "Mainstream Choice"

Promoted by Tesla to gain popularity, the 21700 has comprehensively optimized the shortcomings of the 18650 and is currently the first choice for mid-to-high-end equipment.

3.1.   4 Key Upgrades, Adapted to High-Power Demands

• 20% Longer Battery Life: With the same materials, the monomer capacity is 35% higher than that of 18650 (e.g., 18650 has a capacity of 3.5Ah, while 21700 can reach 5Ah), extending the flight time of drones by 15%-20%.

• 9% Lower Battery Pack Cost: Due to the larger monomer capacity, fewer batteries are needed for the same energy output (600 21700 cells are required where 1000 18650 cells are needed). This reduces the use of brackets, connecting wires, and other components, resulting in lower overall costs.

• Faster Charging: With a larger diameter (21mm vs. 18mm), it can adopt a "multi-tab structure," reducing internal resistance by 15%-20% compared to 18650. It can be charged to 80% in 30 minutes, nearly twice as fast as 18650.

• Flexible Heat Dissipation: The larger size can accommodate larger-area heat sinks, which can be directly attached to the side of the battery. This avoids the "difficult heat dissipation of dense 18650 cells" and ensures more stable service life in long-term use.

3.2.  18650 vs. 21700: A Table to See the Differences      

 Part4、Safe Use: 4 Universal Key Points for Both Batteries

Whether you choose 18650 or 21700, improper use will shorten the service life or even cause risks. Remember these key operations:

4.1.  Charging: 3 "Absolute Must-Nots"

• Do not use ordinary chargers: Must use lithium-ion-specific chargers. Ordinary chargers lack balancing functions, which will cause voltage imbalance of the battery.

• Do not overcharge: The nominal voltage of a single cell is 3.7V, and the full-charge voltage is 4.2V. Unplug the charger immediately when it shows "fully charged"; do not leave it in a long-term trickle charge state.

• Do not leave charging unattended: Place the battery in a fireproof bag during charging and monitor the process throughout. If smoke, swelling, or other abnormalities occur, cut off the power immediately.

4.2.  Storage: 2 "Key Actions"

• Store with half charge:When not in use for a long time, charge the battery to 3.8V per cell (total voltage 11.4V). Do not store it fully charged (accelerates aging) or fully discharged (causes permanent damage).

• Store in a cool and dry place: Keep it in an environment of 15-25℃, away from balconies (high-temperature exposure), refrigerators (humidity), heaters, and stoves (high-temperature heat sources).

4.3.  Discharging: 2 "Must-Avoids"

• Do not over-discharge: Stop discharging when the voltage drops to 3.0V per cell (total voltage 9.0V). For example, replace the battery when the power tool indicates low power; do not use it until it shuts down automatically.

• Do not exceed the discharge rate: Use the battery according to the marked maximum discharge rate. For example, the maximum output current of a 5C battery should not exceed 25A; do not force it to drive an over-powered motor.

4.4.  Maintenance: 1 "Regular Check"

Check the battery status before each use: If swelling, leakage occurs, or the voltage difference between individual cells exceeds 0.3V (measured with a multimeter), stop using and replace it immediately.

Note: In practical applications, the discharge protection threshold of lithium iron phosphate (LiFePO4) batteries is usually set to 2.7-2.8V per cell to avoid irreversible damage caused by reaching the absolute cutoff voltage of 2.5V.

Part5、 Selection Summary: How to Choose Between 18650 and 21700?

• Choose 18650: If you have a limited budget (e.g., for budget-friendly power tools), limited equipment space (e.g., for mini drones), or low requirements for battery life (e.g., for household small energy storage).

• Choose 21700: If you pursue longer battery life (e.g., for mid-to-high-end drones), high power (e.g., for racing power tools), or cost reduction in bulk installation (e.g., for equipment manufacturers' bulk procurement).

If you need to further calculate the battery life of specific equipment or customize battery pack solutions, please contact us for technical support. The next article will focus on 32140 and 4680, two models suitable for large-scale scenarios, to help you understand the future trends of cylindrical batteries.