Lithium-ion Battery Pack Design and Process

In the modern lithium battery industry, a single cell is only the smallest unit of energy. To serve real-world applications, it must be scientifically assembled and managed into a complete battery pack (PACK). This process involves electrochemistry, structure, electronics, and safety engineering.

Step 1 – Defining Requirements

Battery pack design starts with application needs:

• Voltage and capacity requirements (e.g., 36V, 15Ah for e-bikes).

• Energy density, safety, and expected lifespan.

• Environmental factors such as waterproofing, vibration resistance, and temperature range.

Common Lithium Battery Cell Types

18650 Cells   Size: Ø18×65mm Capacity: 1800–3500mAh Mature technology, widely used in laptops, e-bikes, and early EVs (e.g., Tesla Model S).

26650 Cells   Size: Ø26×65mm Capacity: 4000–5000mAh Slightly lower energy density than 18650 but higher discharge rate and better cycle life. Ideal for power tools and high-power energy storage.

32700 Cells   Size: Ø32×70mm Capacity: 6000mAh+ Common in low-speed EVs, telecom energy storage, and home storage systems.

Cell choice directly impacts volume, weight, cost, and complexity of the PACK.

Step 2 – Cell Matching & Screening

Even cells from the same batch may differ in:

• Capacity

• Internal resistance

• Open-circuit voltage

To ensure consistency:

• Group cells with similar parameters.

• Example: In a 10s5p 18650 pack, the 5 cells in each parallel group must match closely.

Step 3 – Connection Methods

• Nickel strip spot welding: Common for small packs (laptops, power banks).

• Laser welding: Used in 26650 packs for high current and durability.

• Bolt & copper bar connections: Often used in 32700 energy storage cabinets for easy maintenance.

Step 4 – Battery Management System (BMS)

The BMS is the brain of the pack. Functions include:

• Monitoring voltage and temperature.

• Preventing overcharge, over-discharge, and overheating.

• Cell balancing to avoid uneven voltages.

• Advanced packs add SOC algorithms, remote monitoring, and communication.

Step 5 – Thermal Management

Heat control is crucial for safety and lifespan.

• 18650 packs: natural cooling or small fans.

• 26650 packs: often air cooling for tools and e-motorcycles.

• 32700 packs: large systems usually need liquid cooling for consistent temperature.

Step 6 – Mechanical Structure & Safety

• Cases: aluminum alloy or flame-retardant plastics.

• Protection: insulation sheets, gaskets, fuses, or breakers.

• Standards: e.g., IP65 waterproofing for e-bike packs, fireproofing for energy storage cabinets.

Step 7 – Testing & Verification

Before mass production, packs undergo:

• Electrical performance tests.

• Overcharge / overdischarge / short circuit tests.

• High & low temperature cycles.

• Vibration, drop, and shock tests.

Example: 26650 tool packs must withstand heavy vibration and drops at construction sites.

Conclusion

Designing a lithium-ion battery pack is a systematic engineering project.

• 18650: mature, portable devices, and small-to-mid power.

• 26650: excellent for high-current tools and equipment.

• 32700: best for energy storage and low-speed EVs.

A successful PACK requires demand-driven design, cell matching, reliable connections, intelligent BMS, robust thermal management, and strict safety testing. Only then can it operate stably in electric mobility, renewable energy storage, and daily electronics.