Practical Battery Pack
Design Workflow
The complete end-to-end battery pack design process — from capturing application requirements through chemistry selection, electrical and thermal design, safety integration, BMS coordination, prototype development, and validation. This chapter ties together everything from Chapters 1–9 into a single coherent workflow.
The battery pack design workflow is an iterative, multi-disciplinary engineering process. Each phase produces defined outputs that feed the next phase, and test results from later phases frequently drive refinements to earlier decisions. The pipeline below shows the logical sequence — not a rigid waterfall.
Every battery pack design begins not with cells, not with calculations, and not with CAD — it begins with a thorough, rigorous understanding of what the application actually requires. Every decision made in subsequent phases flows from the requirements established here. A design built on a poorly defined set of requirements will fail to meet the application's needs regardless of how competent the subsequent engineering work is.
- Total usable energy at end of life (not beginning of life)
- Peak power output and sustained duration
- Peak charge power and charging time constraints
- Min / max operating voltage for load electronics
- Energy requirements by operating mode
- Calendar life (e.g., 10 years) AND cycle count (e.g., 1,500)
- Expected usage profile — depth of discharge, C-rates
- Operating temperature range during service
- Whichever of calendar/cycle life is reached first = end of life
- Operating and storage temperature range
- Humidity, condensation, and IP rating requirement
- Altitude, vibration, and shock environments
- EMC requirements and certifications
- Available volume and shape (battery design space)
- Maximum mass (vehicle dynamics / structural limits)
- Interface requirements — attachment, cooling, connectors
- Centre of gravity constraints
- Applicable standards by market: ECE R100, FMVSS 305, GB 38031, AIS 038
- Identify at project start — not at end — some standards drive fundamental design decisions
- Thermal propagation time requirements (UN ECE R100: 5 min)
- Target cost per kWh or per pack
- Target manufacturing volume and available processes
- Supply chain constraints and preferred suppliers
- Serviceability and repairability requirements
The BRS is the formal document that captures all requirements, their sources, priorities, and any conflicts between them. It is the contract between the battery design team and the rest of the project. Every significant design decision should be traceable back to a requirement in the BRS. No BRS = no coherent design.
With requirements clearly defined, the first major design decision is chemistry and cell format selection. This decision is foundational — it determines energy density, safety characteristics, cycle life potential, cost profile, and the thermal and electrical architecture required.
With chemistry and cell format selected, the designer defines the precise S×P configuration that achieves the required voltage, capacity, and energy within physical constraints. This phase applies the calculation methodology from Chapter 4.
One of the most common pack sizing errors is designing to meet the energy requirement at beginning of life with no margin for degradation. By end of life (80% SOH), a pack with no capacity margin will deliver only 80% of its nominal energy — potentially falling below the minimum acceptable performance. Always size to the end-of-life energy requirement, then add the capacity margin on top.
With the S×P configuration defined, the electrical layout design specifies how cells are physically interconnected — the geometry of busbars, location of the main fuse and contactors, routing of the HV wiring harness, and the electrical interface to the rest of the system.
| Output Document | What It Defines | Used By |
|---|---|---|
| Cell Interconnection Diagram | S×P connection geometry, busbar geometry, cell-level fuse locations, BMS sense connection routing | Module assembly, BMS team |
| Pack Electrical Schematic | Complete circuit from cells through contactors, fuses, pre-charge to HV connector. Reference for BMS protection logic design. | BMS engineer, safety engineer |
| Busbar Design Sheet | Cross-section for every current-carrying conductor. Cu thickness, width, routing. Sized to carry max continuous + peak current within allowable ΔT. | Mechanical design, manufacturing |
| HV Architecture Drawing | Contactor locations, MSD position, HV connector orientation, pre-charge circuit routing, accessibility compliance verification | Systems engineer, certification team |
All electrical layout decisions must account for the accessibility requirements in the BRS. The MSD must be accessible from outside the pack without tools. BMS connectors must be reachable without full pack disassembly. HV connectors must be positionable for interlock verification before engagement. Layout decisions that feel convenient for assembly often compromise serviceability — check both simultaneously.
The thermal design phase develops the thermal management system from concept to detailed design, using the heat generation estimates from the electrical design and the temperature requirements from the BRS as inputs.
The structural packaging design phase creates the physical architecture of the module and pack — the mechanical structures that hold cells in position, maintain compression, provide structural load-bearing capability, protect against mechanical damage, and integrate the thermal management system.
Safety integration is the formal checkpoint at which all safety-relevant design decisions are reviewed against the complete safety architecture. Every failure mode must have a designed protection mechanism, and every protection mechanism must be present in the design.
BMS coordination formally aligns the pack design team and the BMS engineering team on the requirements the physical design must meet and the physical design parameters the BMS algorithm must accommodate.
| Coordination Output | Owner | Content |
|---|---|---|
| Cell Model Parameter Set | BMS + Cell Team | ECM or physics-based model parameters derived from cell characterisation tests. Used to calibrate SOC/SOH estimation algorithms. |
| Protection Threshold Definitions | BMS + Safety Team | Specific overvoltage, undervoltage, overcurrent, and overtemperature thresholds with appropriate margins and hysteresis for the selected cell chemistry. |
| Sensor Placement Specification | Pack Design + BMS | Temperature sensor locations derived from thermal simulation, ensuring BMS has accurate knowledge of hottest cells under all operating conditions. |
| Balancing Strategy | BMS Team | Target maximum cell voltage imbalance, balancing current level, and operating conditions under which balancing is performed (typically during CV charging phase). |
| SOC Window Definition | Systems Engineer | Maximum and minimum SOC at which the pack will be operated. Defines usable energy window and maps to BMS overvoltage/undervoltage protection thresholds. |
Prototype development builds the first physical embodiment of the design. Prototypes validate that the design can actually be assembled as intended, reveal manufacturing issues invisible in CAD, provide first measured data for simulation correlation, and enable initial safety and performance testing.
Testing and validation is the systematic process of demonstrating, through physical measurements, that the battery pack meets all requirements in the BRS. Testing covers electrical performance, thermal performance, mechanical integrity, safety, and regulatory compliance.
The DVR documents every test performed, the results obtained, any failures encountered, and the corrective actions taken. It is the primary evidence that the design is fit for production release. No DVR = no production release. DVR sign-off by engineering management and the safety team is required before production tooling investment is authorised.
The design workflow described in sections 10.1–10.10 is not a linear sequence executed once — it is an iterative process in which design decisions made in later phases feed back into earlier phases, and test results from prototype testing drive design changes that must be revalidated.
Every design change in a battery system, however small it appears, must be evaluated for its impact on safety, performance, and regulatory compliance. A change to the thermal interface material specification — perhaps switching to a different TIM supplier offering the same nominal specifications — can change the module's thermal resistance, affecting cell temperatures, which affects degradation rates, which affects the pack's ability to meet its life requirement. A change that appears purely commercial in nature can have safety implications requiring re-evaluation and potentially re-testing.
Battery pack design optimisation typically targets simultaneous improvement in energy density, cost, cycle life, and safety. These objectives are generally in tension: the most energy-dense cells are the least safe; the safest cells are the least energy dense; the most cycle-life-optimised operating strategy uses only a fraction of total energy. The designer's skill in navigating these trade-offs — and communicating them clearly to engineering and business stakeholders — is one of the most valuable capabilities in the discipline.
