Battery Pack Design

Individual cells are the building blocks, but the battery pack is what powers an EV. Designing a pack involves choosing cell format, configuring cells in series and parallel, managing thermal distribution, and meeting vehicle integration constraints.

This lesson teaches you to calculate pack specifications and understand the tradeoffs in pack architecture.

From Cells to Packs

The hierarchy in most battery packs:

Cell → Module → Pack → Vehicle

• Cell: The fundamental electrochemical unit (3.2-4.2V, 3-300 Ah)
• Module: Group of cells with local BMS and thermal management
• Pack: All modules + master BMS + HV contactors + cooling system
• Vehicle integration: Pack mounted to chassis, crash protection, connectors

Modern designs are moving toward cell-to-pack (CTP) architecture, eliminating modules to improve energy density.

Series and Parallel Configurations

Series Connection (S)

• Increases voltage: V_pack = N_series × V_cell
• Current remains same as individual cell
• All cells must have same capacity for proper operation

Parallel Connection (P)

• Increases capacity: Ah_pack = N_parallel × Ah_cell
• Voltage remains same as individual cell
• Cells share current, improving power capability

Combined Configuration

Most packs use both: 96S4P means 96 cells in series with 4 parallel strings.

$$V_{pack} = N_S \times V_{cell}$$

$$Ah_{pack} = N_P \times Ah_{cell}$$

$$kWh_{pack} = \frac{V_{pack} \times Ah_{pack}}{1000}$$

$$N_{total} = N_S \times N_P$$

Example: Tata Nexon EV Pack

• Cell: LFP prismatic, 3.2V nominal, 100 Ah
• Configuration: 96S (estimated)
• Pack voltage: 96 × 3.2V = 307.2V nominal
• Pack capacity: 30.2 kWh (actual)
• Total cells: ~96 cells (single string, large prismatic)

Voltage Classes

EV packs operate at different voltage levels, each with tradeoffs:

Low Voltage: 48V (Mild Hybrid)

• Used in mild hybrids (Maruti Suzuki Smart Hybrid)
• Safer — no shock hazard below 60V DC
• Limited power capability (~15 kW max)
• Simple cabling and components

Standard: 400V (Most EVs)

• Industry standard for passenger EVs
• Good balance of power, cost, and complexity
• Charging limited to ~150 kW DC fast charging
• Examples: Nexon EV, Ather 450X, most EVs

High Voltage: 800V (Performance EVs)

• Half the current for same power = thinner cables, lighter
• Enables 350 kW+ ultra-fast charging
• More expensive switches and components
• Examples: Porsche Taycan, Hyundai Ioniq 5, Lucid Air

Pack Architecture

Traditional: Cell-to-Module (CTM)

• Modular — easy to replace individual modules
• Standard interfaces between OEM and cell supplier
• Established manufacturing processes

• Module casing adds 15-20% volume/weight overhead
• More connections = more potential failure points
• Lower volumetric energy density

Modern: Cell-to-Pack (CTP)

• Higher energy density (50% more cells in same volume)
• Fewer components, simpler assembly
• Better thermal uniformity

• Harder to service (replace entire pack)
• Requires structural cells
• Less standardization

• 50% higher volumetric energy density
• Structural pack (cells contribute to rigidity)
• Excellent thermal propagation resistance

Cell-to-Body (CTB)

Next evolution — the pack becomes part of the vehicle structure:

• Tesla structural battery (4680 cells with foam)
• Cells bonded directly to floor
• Maximum space efficiency
• Challenging repairs

Weight Breakdown

Understanding where weight goes in a pack:

• Cell energy density: 250 Wh/kg
• Pack energy density: 150-180 Wh/kg (60-70% of cell)

The industry targets:

• 2025: Pack-level 200 Wh/kg
• 2030: Pack-level 250 Wh/kg (solid-state batteries)

Thermal Considerations

Battery temperature affects everything:

Cooling Strategies

• Suitable for small packs (two-wheelers)
• Cannot handle fast charging heat

• Coolant channels between cells/modules
• Can reject 5-10 kW of heat
• Enables fast charging

• Cells submerged in dielectric fluid
• Best thermal uniformity
• Complex sealing requirements

Thermal Runaway Propagation

If one cell fails, heat can spread to neighbors:

• Traditional packs: 30-60 seconds between cells
• BYD Blade: >400 seconds (LFP + horizontal layout)
• Design goal: Enough time to evacuate vehicle

Indian Context: Pack Design Challenges

Designing for India requires special considerations:

• Outdoor parking at 45°C+ requires robust cooling
• Preconditioning before fast charging
• LFP preferred for thermal tolerance

• IP67 minimum for pack enclosure
• Conformal coating on BMS electronics
• Sealed connectors

• Ground clearance (potholes, speed breakers)
• Bottom protection plate
• Vibration resistance for rough roads

• LFP reduces cell cost by 30-40%
• Simpler cooling (no refrigerant loop)
• Local manufacturing under PLI scheme

Pack Specifications: Real Examples

Key Formulas Summary

$$V_{pack} = N_{series} \times V_{cell,nom}$$

$$Ah_{pack} = N_{parallel} \times Ah_{cell}$$

$$E_{pack} = V_{pack} \times Ah_{pack} / 1000 \text{ (kWh)}$$

$$m_{pack} = \frac{E_{pack}}{ρ_{pack}} \text{ where } ρ_{pack} \approx 150-180 \text{ Wh/kg}$$

$$N_{total} = N_{series} \times N_{parallel}$$

Key Takeaways

• Series increases voltage, parallel increases capacity
• Pack voltage classes: 48V (mild hybrid), 400V (standard), 800V (performance)
• CTP architecture improves energy density by eliminating module overhead
• Pack-level energy density is 60-70% of cell-level due to overhead
• Indian conditions require robust thermal management and IP67 sealing
• LFP chemistry is increasingly preferred for Indian EVs

What's Next

In the next lesson, we'll learn how to estimate the State of Charge (SOC) — the "fuel gauge" of an EV. Understanding Coulomb counting, OCV methods, and Kalman filters is essential for accurate range estimation.