Series vs. Parallel:

At its heart, a battery pack is a collection of individual battery cells (or even smaller battery modules) connected together to achieve specific electrical characteristics. The way these cells are connected dictates the overall voltage and capacity of the pack.

Building Battery Packs in Series: Boosting Voltage

When you connect battery cells in series, you’re essentially lining them up end-to-end, positive terminal of one cell to the negative terminal of the next. Think of it like a train: each car adds to the total length.

Here’s what happens when you connect cells in series:

  • Voltage Adds Up: The total voltage of the pack is the sum of the individual cell voltages. If you have four 3.7V lithium-ion cells connected in series, the total pack voltage will be 4×3.7V=14.8V. This is why series connections are used when you need a higher operating voltage for your device.
  • Capacity Remains the Same: The overall capacity (measured in Ampere-hours, Ah, or milliampere-hours, mAh) of the pack remains the same as the capacity of a single cell. If each of those four 3.7V cells has a capacity of 2500mAh, the 14.8V series pack will still have a capacity of 2500mAh.
  • Applications: Series configurations are common in devices requiring higher voltages, such as power tools, laptops, electric vehicles (where many cells are in series to achieve hundreds of volts), and flashlights that take multiple batteries.

Building Battery Packs in Parallel: Increasing Capacity

Connecting battery cells in parallel is like creating multiple lanes on a highway. You connect all the positive terminals together and all the negative terminals together.

Here’s what happens when you connect cells in parallel:

  • Capacity Adds Up: The total capacity of the pack is the sum of the individual cell capacities. If you have four 2500mAh lithium-ion cells connected in parallel, the total pack capacity will be 4×2500mAh=10000mAh (or 10Ah). This means the pack can deliver current for a longer duration.
  • Voltage Remains the Same: The overall voltage of the pack remains the same as the voltage of a single cell. If each of those four cells is 3.7V, the parallel pack will also be 3.7V.
  • Applications: Parallel configurations are used when you need longer runtimes or higher current output at a lower voltage, such as in portable power banks, uninterruptible power supplies (UPS), and some low-voltage, high-drain devices.

Hybrid Configurations: Series-Parallel (S-P) and Parallel-Series (P-S)

For many applications, particularly those requiring both higher voltage and extended runtime, battery packs are built using a combination of series and parallel connections.

These are often denoted as S-P or P-S configurations.

  • S-P (Series-Parallel): This involves connecting multiple parallel strings of cells in series. For example, a “2S2P” pack would have two parallel groups, and these two groups are then connected in series. This provides both increased voltage (from the series connection) and increased capacity (from the parallel connection within each group).
  • P-S (Parallel-Series): This involves connecting multiple series strings of cells in parallel. For example, a “2P2S” pack would have two series groups, and these two groups are then connected in parallel. While the end result is often functionally similar to an S-P configuration, the internal wiring and fault tolerance can differ slightly.