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A single battery cell has limited voltage and capacity. To power applications like electric vehicles or grid storage systems, cells must be combined into larger assemblies. This page explains the hierarchy from cells to packs and the key considerations at each level.

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The Cell-to-Pack Hierarchy

1

Cell

The fundamental electrochemical unit. A single cell typically provides 3-4V nominal voltage (depending on chemistry) and a fixed capacity (e.g., 50 Ah).
2

Parallel Group

Multiple cells connected in parallel to increase capacity while maintaining the same voltage.
3

String

Cells or parallel groups connected in series to increase voltage while maintaining the same capacity.
4

Module

A mechanical assembly containing multiple cells, often with its own monitoring electronics and thermal management.
5

Pack

The complete battery system including modules, the BMS, cooling system, contactors, and enclosure.

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Series vs Parallel Connections

Series Connection

Cells connected positive-to-negative. Voltages add, capacity stays the same. Used to reach the system voltage requirement.

Parallel Connection

Cells connected positive-to-positive and negative-to-negative. Capacities add, voltage stays the same. Used to increase energy and current capability.

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Notation: xSyP

Battery configurations are described using xSyP notation:
  • S = number of cells in series
  • P = number of cells in parallel

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Parallel Group Considerations

When cells are connected in parallel, they share the same voltage, which causes them to naturally self-balance. This provides increased capacity and higher current capability since the load is shared across multiple cells. However, parallel groups require careful cell matching. Cells with different capacities or resistances will experience uneven current sharing—a cell with higher resistance carries less current and ages differently than its neighbors. Additionally, individual cell voltages cannot be monitored separately, making it harder to detect a failing cell.
Most lithium-ion packs use a “parallel-first” configuration, where cells are first grouped in parallel, then these parallel groups are connected in series. This approach benefits from the self-balancing of parallel cells while achieving the required system voltage.

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Series String Considerations

When cells are connected in series, their voltages add together while sharing the same current. This enables higher system voltages (reducing current for a given power level) and simplifies current measurement since only one sensor is needed. The main challenge with series connections is that cells drift apart in their state of charge over time due to manufacturing variations and temperature differences. Without intervention, a single cell reaching its voltage limit forces the entire string to stop—even if other cells have capacity remaining.
The weakest cell in a series string determines pack performance. This is why cell balancing is essential for series-connected packs.

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Module Design

A module is a sub-assembly that groups cells together with:
  • Mechanical structure: Holds cells in place, often with compression
  • Electrical connections: Busbars connecting cells in the desired configuration
  • Thermal interface: Cooling plates or air channels
  • Sensing: Voltage taps and temperature sensors for the BMS

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Why Use Modules?

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Pack Architecture

The complete battery pack integrates:

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Example: Electric Vehicle Pack

A typical EV battery pack might be configured as:

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Cell-to-Pack (CTP) Design

Modern designs increasingly use Cell-to-Pack (CTP) architecture, which eliminates the module level: CTP designs use larger cells (often blade or prismatic format) that provide structural rigidity, reducing the need for intermediate module housings.