Battery Pack Calculation Formula

$$\text{If in Series: } V_{total} = N \times V_{cell}, \, C_{total} = C_{cell}$$ $$\text{If in Parallel: } V_{total} = V_{cell}, \, C_{total} = N \times C_{cell}$$

• V_total – Total Voltage (V)
• C_total – Total Capacity (Ah)
• N – Number of 18650 Cells
• V_cell – Voltage per Cell (V)
• C_cell – Capacity per Cell (Ah)

Introduction to 18650 Battery

The 18650 battery is a common type of lithium-ion battery, named after its dimensions: 18mm in diameter and 65mm in length (the "0" indicates a cylindrical shape). It is widely used in consumer electronics, electric vehicles, energy storage systems, and more, known for its high energy density, long lifespan, and reliability.

1. Basic Characteristics

• Dimensions: 18mm in diameter, 65mm in length.
• Voltage: Nominal voltage is typically 3.7V, with a full charge voltage of 4.2V and a discharge cutoff voltage of 2.5V-3.0V.
• Capacity: Common capacity ranges from 1500mAh to 3500mAh, with some high-performance batteries exceeding 4000mAh.
• Chemistry: Primarily lithium-ion (Li-ion) or lithium iron phosphate (LiFePO4).
• Weight: Approximately 40-50 grams, depending on capacity and materials.

2. Advantages

• High Energy Density: Stores a large amount of energy, suitable for high-power devices.
• Long Cycle Life: Typically lasts 500-1000 charge cycles, with high-quality batteries exceeding 2000 cycles.
• Good Stability: Uses mature lithium-ion technology, offering high safety.
• Wide Applications: Used in laptops, power tools, electric vehicles, energy storage systems, and more.

3. Disadvantages

• Fixed Size: Limited design flexibility due to standardized dimensions.
• Requires Protection Circuit: Overcharging, over-discharging, or short circuits can cause safety issues, necessitating a protection circuit board (PCB).
• Higher Cost: More expensive compared to nickel-metal hydride (NiMH) or lead-acid batteries.

4. Main Applications

• Consumer Electronics: Laptops, flashlights, drones, etc.
• Power Tools: Drills, saws, etc.
• Electric Vehicles: Battery packs in Tesla and other EVs consist of thousands of 18650 cells.
• Energy Storage Systems: Home energy storage, solar energy storage, etc.

5. Charging and Discharging

• Charging Voltage: 4.2V (Li-ion) or 3.65V (LiFePO4).
• Discharge Cutoff Voltage: 2.5V-3.0V to avoid over-discharging.
• Charging Current: Typically 0.5C (e.g., 1.25A for a 2500mAh battery), with fast charging up to 1C.
• Discharge Current: Standard batteries support 1C, while high-rate batteries can handle 10C or more.

6. Safety Precautions

• Avoid Overcharging/Over-Discharging: Use chargers with protection circuits to prevent damage.
• Prevent Short Circuits: Short circuits can cause overheating, fire, or explosion.
• Control Temperature: Avoid charging or using in extreme temperatures.
• Regular Inspection: Stop using the battery if swelling, leakage, or performance degradation is observed.

7. Popular Brands

• International Brands: Panasonic, Samsung SDI, LG Chem, Sony.
• Chinese Brands: Lishen, BAK, CATL.

8. Purchasing Tips

• Buy from Reliable Sources: Avoid counterfeit or low-quality batteries.
• Choose Based on Needs:
  • High-capacity batteries: Ideal for devices requiring long runtime.
  • High-rate batteries: Suitable for high-power devices (e.g., power tools).
• Check for Protection Circuit: Ensure the battery includes a protection circuit board (PCB) for safety.

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