> ## Documentation Index
> Fetch the complete documentation index at: https://docs.ionworks.com/llms.txt
> Use this file to discover all available pages before exploring further.
# 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
The fundamental electrochemical unit. A single cell typically provides 3-4V
nominal voltage (depending on chemistry) and a fixed capacity (e.g., 50 Ah).
Multiple cells connected in parallel to increase capacity while maintaining
the same voltage.
Cells or parallel groups connected in series to increase voltage while
maintaining the same capacity.
A mechanical assembly containing multiple cells, often with its own
monitoring electronics and thermal management.
The complete battery system including modules, the BMS, cooling system,
contactors, and enclosure.
## Series vs Parallel Connections
Cells connected positive-to-negative. **Voltages add**, capacity stays the
same. Used to reach the system voltage requirement.
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](/guide/batteries-101/battery-management-systems#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](/guide/batteries-101/battery-management-systems)) |
| **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](/guide/batteries-101/battery-management-systems)—the electronic brain that monitors and controls the pack
* [Thermal Modelling](/guide/batteries-101/thermal-modelling)—managing heat at the pack level
* [State of Charge](/guide/batteries-101/state-of-charge)—estimating charge across cells in a pack
* [Internal Resistance](/guide/batteries-101/internal-resistance)—how cell resistance affects pack performance