Battery Packs

How cells are combined in series and parallel into modules and packs to deliver target voltage and capacity for EVs and storage.

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.

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.

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.

Notation: xSyP

Battery configurations are described using xSyP notation:

• S = number of cells in series
• P = number of cells in parallel

Configuration	Meaning	Result (using 3.7V, 5Ah cells)
4S1P	4 cells in series	14.8V, 5Ah
1S4P	4 cells in parallel	3.7V, 20Ah
4S2P	4 series × 2 parallel	14.8V, 10Ah
96S4P	96 series × 4 parallel	355V, 20Ah (typical EV)

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.

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.

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

Why Use Modules?

Benefit	Description
Manufacturability	Easier to assemble and test smaller units
Serviceability	Replace a module instead of the entire pack
Scalability	Combine modules to create different pack sizes
Safety	Contain thermal events within a module

Pack Architecture

The complete battery pack integrates:

Component	Function
Cells	Store and release electrochemical energy
Modules	Group cells mechanically and electrically
BMS	Monitor and control the pack (details)
Thermal system	Maintain cells within safe temperature range
Contactors	High-voltage switches for isolation
Fuses	Overcurrent protection
Enclosure	Mechanical protection and sealing
Connectors	High-voltage and communication interfaces

Example: Electric Vehicle Pack

A typical EV battery pack might be configured as:

96S4P configuration using 5Ah pouch cells:
- 4 cells in parallel = 1 parallel group (5Ah × 4 = 20Ah)
- 12 parallel groups in series = 1 module (44.4V nominal)
- 8 modules in series = 1 pack (355V nominal, 20Ah, ~7 kWh)

Cell-to-Pack (CTP) Design

Modern designs increasingly use Cell-to-Pack (CTP) architecture, which eliminates the module level:

Traditional	Cell-to-Pack
Cell → Module → Pack	Cell → Pack
More structural components	Fewer components, higher energy density
Easier serviceability	Lower cost, better space utilization

CTP designs use larger cells (often blade or prismatic format) that provide structural rigidity, reducing the need for intermediate module housings.

Related Topics

• Battery Management Systems—the electronic brain that monitors and controls the pack
• Thermal Modelling—managing heat at the pack level
• State of Charge—estimating charge across cells in a pack
• Internal Resistance—how cell resistance affects pack performance