Battery Pack & Configuration

An EV battery pack combines hundreds or thousands of cells into a complete energy system. How those cells are connected and integrated affects voltage, capacity, charging performance, power output, weight, and pack efficiency.

This article explains how battery cells are connected in series and parallel, how pack voltage affects EV design, and how different battery-pack architectures are built.

Battery configuration

In an EV, battery configuration describes how individual cells are connected within the battery pack. The two basic connection methods are series and parallel.

• Series connection increases voltage.
• Parallel connection increases capacity.

Most EV battery packs use a combination of both. Cells are connected in series to reach the required pack voltage, while parallel connections increase the total capacity and energy content.

To estimate the gross energy content of a battery pack, multiply pack voltage by pack capacity:

Energy (Wh) = Voltage (V) × Capacity (Ah)

This gives the battery's gross energy content in watt-hours.

Why series and parallel matter

A higher number of cells in series increases the pack voltage, which is important for delivering power efficiently. A higher number of cells in parallel increases the amount of energy the pack can store and how much current it can deliver.

This means battery configuration directly affects:

• Pack voltage
• Energy capacity
• Power delivery
• Charging behavior
• Vehicle performance

Example: Audi Q8 e-tron 55

The diagram below shows the configuration of one battery module in the Audi Q8 e-tron 55. This module contains 12 cells arranged as 3 groups in series and 4 cells in parallel, often written as 3s4p.

• Cell voltage: 3.6667 volts nominal
• Cell capacity: 72 Ah
• Module voltage: 3 × 3.6667 V = 11 V
• Module capacity: 4 × 72 Ah = 288 Ah

The full battery pack contains 36 such modules connected in series:

• Pack voltage: 36 × 11 V = 396 V
• Gross capacity: 396 V × 288 Ah = 114,048 Wh, or about 114 kWh

Example: Tesla Model Y Long Range

The Tesla Model Y Long Range uses 4,416 cylindrical cells arranged as 96s46p.

• Cell voltage: 3.7 volts nominal
• Cell capacity: 4.8 Ah
• Parallel capacity: 46 × 4.8 Ah = 220.8 Ah
• Pack voltage: 96 × 3.7 V = 355.2 V
• Gross capacity: 355.2 V × 220.8 Ah = about 78.4 kWh

This shows how a pack with many small cylindrical cells can reach the same overall function as a pack using fewer, larger cells.

More battery pack examples

The specific configuration used in an EV depends on the vehicle's targets for range, performance, charging speed, packaging, and cost.

400-volt and 800-volt battery systems

Most EVs are built around battery systems with a nominal voltage of roughly 400 volts or 800 volts.

Higher voltage does not automatically make an EV better, but it can make it easier to deliver high power and high charging performance with lower current. Lower current can reduce heat generation and allow thinner cables, lower losses, and more efficient high-power operation.

Advantages of 400-volt systems

• Mature and widely used: 400-volt systems are common across the EV market.
• Lower cost: Components are often less expensive.
• Broad charging compatibility: They work easily with the large installed base of DC fast chargers.
• Well suited to many vehicle classes: 400-volt systems are sufficient for many mainstream EVs.

Disadvantages of 400-volt systems

• Higher current for the same power: This can increase cable thickness, losses, and heat.
• More limited peak charging potential: Reaching very high charging power is more difficult.
• Less efficient for very high-performance applications: High power demand puts greater stress on the electrical system.

Advantages of 800-volt systems

• Higher charging potential: 800-volt systems can support very high charging power when the battery, charger, and thermal system also allow it.
• Lower current for the same power: This can reduce heat and electrical losses.
• Potential weight savings: Lower current can reduce cable size and supporting hardware.
• Strong fit for high-performance EVs: Especially useful in vehicles with high charging and power demands.

Disadvantages of 800-volt systems

• Higher system cost: Components such as inverters, power electronics, insulation, and charging hardware can be more expensive.
• More complex integration: The vehicle's full electrical system must be designed around the higher voltage.
• Charging performance still depends on more than voltage: Cell chemistry, thermal management, and charger capability remain critical.

In practice, the choice between 400 volts and 800 volts is a system-level design decision. It affects not only charging speed, but also efficiency, packaging, cost, and performance.

Battery pack designs

Battery packs are not defined only by cell chemistry and voltage. The way cells are grouped and integrated also has a major effect on energy density, serviceability, safety, and manufacturing complexity.

There are several common approaches to battery-pack design.

Cell-to-Module (C2M)

In a Cell-to-Module design, battery cells are grouped into modules, and those modules are then assembled into the full battery pack.

Modules usually include structural elements, electrical connections, and often cooling components. This has been the traditional design approach for many EV battery packs.

Advantages of Cell-to-Module (C2M)

• Modularity: Individual modules can be replaced or serviced more easily.
• Thermal integration: Modules provide space for cooling plates and thermal interfaces.
• Scalability: Manufacturers can build different pack sizes using similar module designs.
• Safety and isolation: Modules can help contain faults and separate groups of cells.
• Manufacturing flexibility: Modules can simplify assembly and quality control.

Disadvantages of Cell-to-Module (C2M)

• More inactive material: Module housings and extra connections add weight and reduce pack-level energy density.
• Lower packaging efficiency: More structure is required between the cells and the final pack.
• Higher part count: More components can increase cost and assembly complexity.

Cell-to-Pack (CTP)

Cell-to-Pack designs remove or reduce the intermediate module level and integrate cells more directly into the battery pack.

This can improve packaging efficiency by reducing the amount of inactive material between cells. It can also reduce part count, weight, and cost.

BYD Blade and CATL Qilin are examples of battery designs that push cell integration further than traditional modular packs, although they do so in different ways.

Advantages of Cell-to-Pack (CTP)

• Higher packaging efficiency: More of the pack volume can be used for active cell material.
• Lower weight: Fewer intermediate structures are needed.
• Lower part count: Reduced complexity can support lower manufacturing cost.
• Potentially higher pack energy density: Less inactive material improves pack-level efficiency.

Disadvantages of Cell-to-Pack (CTP)

• Reduced modular serviceability: Repairs and replacements can be more difficult.
• Greater dependence on pack-level design: Cooling, structure, and safety must be solved at the full-pack level.
• More integrated manufacturing: Production and assembly can become less flexible.

Structural battery pack

A structural battery pack goes one step further by making the battery pack part of the vehicle's structure.

Instead of only carrying energy, the pack also contributes to the strength and stiffness of the vehicle body. This can reduce duplicate structures, lower weight, and improve packaging efficiency.

Structural packs are attractive because they can improve overall vehicle efficiency and reduce part count, but they also increase integration complexity. The battery pack is no longer just a component mounted into the car; it becomes part of the vehicle platform itself.

Tesla is one example of a structural battery approach. Porsche uses a different solution in the new Cayenne Electric, where the battery is directly integrated into the vehicle structure while still using six interchangeable modules. This makes it a good example of a structurally integrated modular pack rather than a traditional self-contained battery enclosure.

Advantages of structural battery packs

• Lower vehicle weight: Structural integration can remove redundant parts.
• Higher system efficiency: Better integration can improve packaging and performance.
• Reduced part count: The battery pack can replace some traditional body structures.
• Potential stiffness benefits: The pack can contribute to chassis rigidity.

Disadvantages of structural battery packs

• Higher design complexity: Vehicle and battery engineering become more tightly linked.
• Repair challenges: Damage to the pack can have broader structural implications.
• Less flexibility: A highly integrated platform can be harder to modify across different vehicle versions.

The video below shows a detailed analysis of the pack by Munro & Associates.

Energy density at the battery pack level

Cell-level energy density does not directly translate to pack-level energy density. At pack level, the final result also depends on cooling, structure, wiring, crash protection, electronics, and how efficiently the cells are integrated.

The table below shows how pack-level density has varied between some example battery packs.

For more details about battery packs, we recommend visiting BatteryDesign.net.