Lithium-ion battery packs are constructed by combining individual cells through series and parallel connections, a process that allows customization of voltage and capacity to meet the specific power requirements of devices ranging from smartphones to electric vehicles. Understanding these connections is critical for designing efficient, safe, and reliable battery systems.

　　Series connections involve linking the positive terminal of one cell to the negative terminal of the next, effectively increasing the total voltage of the pack while keeping the capacity (measured in ampere-hours, Ah) unchanged. For example, connecting three 3.7V lithium-ion cells in series results in a total voltage of 11.1V (3.7V × 3), with the capacity remaining equal to that of a single cell. This configuration is essential for devices requiring higher voltages, such as laptops, power tools, and electric vehicles, where the motor or circuitry demands more than the 3.7V (nominal) of a single cell.

　　Parallel connections, by contrast, connect the positive terminals of multiple cells together and the negative terminals together, increasing the total capacity while keeping the voltage the same as a single cell. For instance, three 3.7V cells in parallel maintain 3.7V but triple the capacity. This setup is ideal for devices needing longer runtime, such as portable generators, medical equipment, and solar energy storage systems, where sustained power delivery is more critical than high voltage.

　　In practice, most battery packs use a combination of series and parallel connections (known as a series-parallel configuration) to achieve both the desired voltage and capacity. For example, a 14.8V, 10Ah battery pack might consist of four cells in series (4 × 3.7V = 14.8V) combined with two such series groups in parallel (doubling the capacity to 10Ah).

　　However, these connections require careful balancing to ensure all cells age uniformly. Mismatched cells in a series can lead to overcharging or undercharging of individual cells, reducing lifespan and posing safety risks. Similarly, parallel cells with varying capacities may discharge unevenly. To mitigate this, battery management systems (BMS) are integrated into packs to monitor cell voltages, balance charges, and prevent overcurrent or overvoltage scenarios, ensuring the longevity and safety of the entire system.

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