A battery pack is the complete, self-contained energy storage unit delivered to the application. It may contain a single cell or thousands of cells, but at its simplest it must do four things: store energy in cells, connect those cells electrically, keep them within their operating temperature range, and protect them, and the people around them, from the consequences of electrical or thermal faults.
Pack cost is dominated by cells. Cells typically represent 60–75% of total pack cost. This means that cell selection: chemistry, format, manufacturer, and purchasing volume has more leverage on pack economics than any other design decision. Understanding the cell market, the contract structures, and the relationship between volume and price is a commercial skill that battery engineers in OEM or tier-1 roles need to develop alongside the technical work.
In every aspect of operation, the pack’s capability is limited by its weakest cell. A single cell with lower capacity reduces the usable energy of its entire series string. A single cell running 5°C hotter than its neighbours ages faster and diverges from the group, accelerating imbalance. A single degraded cell that hits its voltage limit early curtails the performance of the whole pack. Pack design is therefore fundamentally the discipline of minimising variation and managing constraints — electrical, thermal, mechanical, and safety — so that the pack performs as close as possible to the sum of its parts, across its full lifetime.
Where to Start
New to battery packs? This is a suggested reading order through the core material. For the complete list of every page in this section, see the Full Index below.
- Battery Basics: Cells, Voltage & Building a Pack Explained – the whole picture: how a pack is built up from cells.
- Key Pack Metrics – the core numbers used to describe a pack.
- Battery Pack Sizing – translating application requirements into a sized pack.
- Pack Design – how the physical pack actually comes together.
- Pack Enclosure – housing and protecting it.
- Pack Mass – a key design constraint throughout.
- Swappable Battery Packs – an alternative pack architecture worth knowing about.
Full Index
A complete, always up to date index of every page in the Battery Pack section, organised by hierarchy. Click any heading to expand it.
What Goes Into a Battery Pack
A complete battery pack typically contains:
Cells — the electrochemical energy storage elements. In series and parallel to achieve the required voltage and capacity. The cell choice (chemistry, format, specification) determines the performance envelope of everything else.
Module or C2P structure — cells are either grouped into modules (a transportable sub-assembly with its own mechanical structure, interconnects, and sensing) or arranged directly cell-to-pack without the module layer. The module layer adds cost and mass but enables easier service, repair, and quality control.
Busbars — the primary electrical conductors connecting cells, modules, and the pack output terminal. Sized from the continuous current requirement; designed for low resistance, thermal management, and mechanical durability.
Battery Disconnect Unit (BDU) — houses the main contactors, fuses, pre-charge circuit, and current sensor. The control point of the high voltage system — all switching and protection of the HV circuit routes through the BDU.
Battery Management System (BMS) — the hardware and software control system. Measures every cell voltage and temperature, estimates SoC and SoH, controls contactors and thermal management, and communicates pack status externally.
Thermal management system — the system that maintains cell temperatures within the required window (typically ±2°C uniformity, absolute maximum depending on chemistry) across all operating and charging conditions.
Enclosure — the structural and protective housing for all of the above. Must satisfy IP rating, structural load, crash intrusion, sealing, and pressure management requirements simultaneously.
HV connectors and harness — the interface between the pack and the external HV circuit (motor, charger, auxiliaries). Must include HVIL contacts and meet IP67 or IP69K sealing.
→ Parts list for a battery pack — full component breakdown → Structure of a battery pack
Pack Design and Architecture
Pack Generations
Pack design has evolved through distinct generations, each driven by the pressure to reduce cost and increase energy density:
Gen 1 — Cell → Module → Pack (CMP): The original approach. Cells are assembled into modules; modules are assembled into the pack. Modules provide mechanical structure, thermal interface, and a replaceable service unit. High part count, high cost, but maximum repairability and design flexibility.
Gen 2 — Cell to Pack (C2P): The module layer is eliminated. Cells are integrated directly into the pack structure. BYD’s Blade cell was designed specifically for C2P — the long, thin cell spans the full pack width and provides structural rigidity. C2P increases pack-level energy density by 10–15% for the same cells, at lower part count and cost. The trade-off: no replaceable module; a cell failure requires pack-level intervention.
Gen 3 — Cell to Body (C2B) / Cell to Chassis (C2C): The pack enclosure is eliminated entirely or integrated into the vehicle structure. BYD Seal uses C2B; the Tesla Model Y structural battery uses a variant in which the pack floor is a stressed structural member of the vehicle. Maximum integration and mass efficiency; maximum complexity for manufacturing, repair, and end-of-life disassembly.
Gen 4 — Electrode to Pack (E2P): The cell casing is eliminated. Electrode stacks are assembled directly into the pack. Very early stage; demonstrated by 24M Technologies. Potential for significant cost reduction but significant manufacturing challenges.
→ Pack generations in depth → Cell to Pack → Cell to Body → Electrode to Pack
Modular vs Dedicated BEV Platform
OEMs building both ICE and BEV vehicles face a fundamental architecture decision: build the EV on a modified ICE platform (modular approach, lower investment, compromised battery packaging) or invest in a ground-up BEV platform (optimised battery integration, superior performance, higher capital cost).
Dedicated BEV platforms — Volkswagen MEB, GM Ultium, Hyundai E-GMP, Rivian skateboard — allow the battery to span the full floor area of the vehicle, maximising capacity while keeping the centre of gravity low. Modified ICE platforms constrain the battery to the tunnel, under-seat, or other available spaces, significantly limiting energy density and forcing complex, non-rectangular pack shapes.
→ Modular vs dedicated BEV platform
Pack Sizing
Pack sizing translates the application requirements — energy, power, lifetime, weight, volume — into a cell configuration (S×P) and a set of design constraints. It is the first quantitative step in any battery pack development programme.
The Sizing Process
1. Define the application requirements
Before any cell can be selected or any S×P calculated, the full set of requirements must be established:
- Usable energy (kWh) — the energy the application must access, which is less than the total pack energy by the usable SoC window
- Peak power (kW) and duration
- Continuous power (kW)
- Voltage range (V_min, V_max at the pack terminals)
- Charge rate and charge time target
- Operating temperature range
- Lifetime requirement (years, cycles, km)
- Mass budget (kg)
- Volume envelope (L, with dimensional constraints)
- Cost target ($/kWh or absolute)
→ Data required to size a pack
2. Establish the usable SoC window
The usable SoC window is the fraction of cell capacity that the BMS allows to be accessed. It is always less than 100%:
- A long-range BEV might use 90–95% of total capacity
- A PHEV balancing charge acceptance and longevity might use 70%
- An HEV operating in a narrow power buffer window might use 30%
The narrower the SoC window, the longer the cycle life — cells aged at partial depth of discharge cycle longer than cells cycled fully. The usable window is therefore a lifetime-energy trade-off set by the application requirements.
3. Select the cell and calculate S×P
The number of cells in series (S) is determined by the required pack voltage divided by nominal cell voltage. The number in parallel (P) is determined by the required capacity divided by single-cell capacity.
Pack voltage = S × V_nom_cell Pack capacity = P × Ah_nom_cell Pack energy = S × V_nom_cell × P × Ah_nom_cell
Iteration is required — the cell choice affects the achievable voltage (integer multiples of cell voltage), the thermal management requirement (DCIR drives I²R heat), and the mechanical packaging (cell dimensions constrain the enclosure).
4. Verify power capability
The S×P configuration must deliver the required peak and continuous power within the cell’s DCIR and thermal limits. Peak discharge current = Peak power / Pack voltage. This current must not cause the weakest cell in the string to drop below its minimum voltage or exceed its maximum temperature.
5. Thermal sizing
If the application duty cycle generates more heat than passive cooling can reject, active cooling is required. The thermal sizing determines whether air, liquid cooling plate, or immersion cooling is needed and establishes the cooling circuit requirements.
→ Pack sizing in depth → Battery calculators — S×P, energy, power → Road vehicle power demand
Key Pack Metrics
When specifying or benchmarking a battery pack, the following metrics are universally used:
| Metric | Unit | What it measures |
|---|---|---|
| Gravimetric energy density | Wh/kg | Energy stored per unit mass — the primary range metric |
| Volumetric energy density | Wh/L | Energy stored per unit volume |
| Gravimetric power density | W/kg | Peak power per unit mass |
| Cell-to-pack mass ratio | % | Cell mass as a fraction of total pack mass — efficiency of packaging |
| Cell-to-pack volume ratio | % | Cell volume as a fraction of total pack volume |
| Usable energy | kWh | Energy the BMS makes available to the application |
| Peak discharge power | kW | Maximum power for a defined duration (typically 10s) |
| Continuous discharge power | kW | Sustained power within thermal limits |
| $/kWh | Cost per kWh | Pack-level economics |
| Cycle life | cycles to 80% SoH | Lifetime at defined conditions |
The cell-to-pack ratio is a particularly informative metric for comparing pack architectures. A CMP pack might achieve 60–65% cell-to-pack mass ratio; a well-designed C2P pack can reach 75–80%, reflecting the mass saving from eliminating module housings and inter-module busbars.
→ Key pack metrics in depth → Pack benchmarking database — real production data
Pack Enclosure
The pack enclosure is far more than a box. It must simultaneously satisfy structural, electrical, thermal, chemical, and safety requirements — often in direct tension with each other.
Mechanical requirements — the enclosure must carry the static and dynamic loads imposed by the cells it contains (a 75kWh NMC pack with cells may weigh 400–600kg), survive crash loads and road vibration, resist intrusion in a side impact, and maintain structural integrity if the pack internally generates heat and pressure during a thermal runaway event.
Electrical requirements — the enclosure must electrically isolate the HV system from the vehicle chassis. Any conductive enclosure (aluminium or steel) must be isolated from the HV busbars and cell terminals throughout the pack interior.
Sealing — IP67 (protection against temporary immersion) is the automotive standard minimum. IP6K9K (high-pressure, high-temperature wash) is required where underbody wash-down is a design case. Sealing must be maintained over a decade of thermal cycling and vibration.
Pressure management — cells vent gas during thermal events. The enclosure must either contain this gas safely (directing it to a controlled vent path) or accommodate the pressure rise without catastrophic failure. Burst discs and pressure equalisation vents balance these requirements.
Thermal interface — the enclosure lower case typically incorporates or directly mounts the cooling plate. The thermal path from cells through compression pads or TIM to the cooling plate must be continuous and consistent across the full cell population.
Enclosure materials — the main choices are aluminium (dominant in automotive — good thermal conductivity, low mass, weldable, crashworthy), steel (heavier but lower cost, used in commercial vehicles and some BESS), carbon fibre (very low mass, aerospace and performance applications, high cost), and engineering plastics (consumer and portable applications).
Joining techniques — aluminium pack enclosures are typically assembled by friction stir welding (FSW), GMAW (MIG) welding, hybrid laser welding, or structural adhesive bonding, depending on the section geometry, production volume, and required joint strength. Each technique has different implications for joint quality, production rate, and repairability.
→ Pack enclosure in depth → Joining techniques for pack enclosures → Battery to vehicle mechanical integration → Pack mass
Voltage Variants
12V Battery
The 12V battery is the auxiliary power system present in almost every road vehicle — ICE, HEV, PHEV, and BEV. In ICE vehicles it starts the engine and powers all low-voltage accessories. In EVs it powers the BMS, contactors, lighting, HVAC controls, and safety systems independently of the HV traction pack.
The traditional chemistry is lead-acid (VRLA). The shift to lithium iron phosphate (LFP) for 12V batteries has accelerated since 2020, driven by mass reduction, improved charge acceptance, and better performance at low temperatures.
48V Battery
The 48V system (nominal 48V, actual operating range ~42–52V) is the enabling technology for mild hybrid electric vehicles (MHEV). It falls below the 60V safety threshold, avoiding the full HV design requirements, while delivering enough power (~15kW peak) to provide meaningful fuel economy improvements through regenerative braking, engine-off coasting, and electric boost.
48V systems are also used in e-compressors, electric power steering, electric superchargers, and 48V auxiliary networks in larger vehicles.
400V and 800V Architectures
Most current production EVs use a 400V nominal architecture (300–450V actual range). 800V architecture (600–900V actual range) is increasingly adopted for premium and performance vehicles and is becoming the standard for new platforms targeting ultra-fast charging.
At 800V, the same power level is delivered at half the current, halving I²R losses in pack wiring and reducing conductor mass. A 350kW charger at 800V delivers ~437A; at 400V it requires ~875A — with significant implications for cable sizing, connector rating, and charging infrastructure.
The Porsche Taycan, Hyundai Ioniq 6/EV6, Kia EV6, Audi e-tron GT, and Lucid Air all use 800V architecture in production. Many new platform announcements for 2025–2028 are designed around 800V from the outset.
→ Higher voltage packs → Battery vs system voltage → Split pack / bank charging
Pack Benchmarking
The pack benchmarking section assembles architecture and performance data from real production packs — cell-to-pack ratios, gravimetric and volumetric energy density, voltage, cell chemistry and format, module count, thermal management approach, and structural architecture. It is one of the most widely used resources on the site.
Production packs covered include Tesla (Model 3, Y, S, Cybertruck), BYD (Han, Seal, Atto 3, Tang), BMW (iX, i4, i3), Porsche (Taycan), Audi (e-tron GT), GM (Ultium / Hummer EV), Lucid Air, Rivian R1T/R1S, Nissan Leaf, Toyota Prius, Formula E, and many more.
→ Pack benchmarking — full database → Module benchmarking → Sodium-ion pack benchmarking
Pack Testing
Testing a battery pack is a multi-stage programme covering performance, electrical, mechanical, thermal, safety, and legislative requirements. Packs are designed to survive a decade or more in harsh environments, and the test programme must validate that they will.
Performance testing — capacity, energy, peak and continuous power, efficiency, all vs temperature and SoC. Establishes that the pack meets its energy and power targets across its operating envelope.
Electrical testing — isolation resistance, HV continuity, BMS communication and control, contactor function, HVIL integrity, pre-charge behaviour. Verifies the HV system functions correctly in all operating states.
Mechanical testing — vibration (road-spectrum PSD profiles), mechanical shock, crush, drop, immersion. Verifies that the pack maintains structural and electrical integrity across its mechanical life.
Thermal testing — thermal performance at temperature extremes, thermal uniformity across cell population, thermal runaway propagation containment. Verifies the thermal management system keeps cells within limits and that a single-cell thermal runaway event does not cause uncontrolled propagation.
Safety and legislative testing — UN 38.3 (transport), IEC 62619 (industrial), ISO 12405, GB 38031 (China), and application-specific standards. Required for market access in relevant regions.
→ Pack testing in depth → Abuse testing → UN 38.3 transport test
Swappable Batteries
Battery swapping — replacing a depleted pack with a charged one at a swap station rather than waiting for in-vehicle charging — is an alternative to fast charging that is commercially deployed in China by NIO and CATL’s EVOGO system, and in two- and three-wheelers across South and Southeast Asia.
Unlike fixed batteries, swappable batteries inherit their outer dimensions, connectors, and communication protocols from their predecessors. This constrains the design freedom of subsequent generations and creates a long-term compatibility obligation that fixed batteries do not have.
The CATL Choco-SEB (Swapping Electric Blocks) and NIO’s 75kWh/100kWh swap battery standardise the interface within their respective ecosystems.
→ Designing swappable batteries
Related Sections
The battery pack integrates every discipline in battery engineering. All sections of this site connect to pack design:
- Battery Cell — cell specification, format, and performance define the pack’s capability
- Module — the intermediate assembly between cell and pack in CMP architectures
- BMS — controls, protects, and reports the state of the pack
- High Voltage System — the HV electrical architecture within and at the interface of the pack
- Electrical — series/parallel configuration, current rating, voltage architecture
- Thermal — cooling architecture, TIM selection, thermal runaway containment
- Mechanical — enclosure structure, cell mounting, vibration and crash loads
- Safety — thermal runaway, propagation, functional safety, emergency shutdown
- Manufacturing — pack assembly, quality control, end-of-line testing
- Testing — performance, abuse, legislative, and safety test programmes
- Costs — cell cost dominance, pack-level cost drivers, $/kWh targets
- Lifetime — how operating strategy, thermal management, and SoC window affect pack longevity
- 4R’s — Recycling — end-of-life options and the implications for pack design (design for disassembly)
- Legislation, Rules and Regulations — the standards and regulations the pack must comply with for market access
- Benchmark — real-world production pack data for calibrating design targets
See also: OLA 2-Wheeler Battery Pack Teardown, Pack Testing, Electric Vehicle Range Calculator, Module Design, Pack Size versus Application