Cell Selection: Vehicle OEMs Must Select Appropriate Cells at the First Stage
Designing a power battery system starts with cell selection. What key parameters should be evaluated when choosing cells? From a vehicle-level perspective, the core parameters are summarized as follows:
| Parameter Category | Key Parameters | Impacts on the Whole Vehicle |
| Energy Parameters | Capacity (Ah), Energy Density (Wh/kg) | Directly determine vehicle driving range |
| Electrical Parameters | Nominal Voltage (V), Internal Resistance (mΩ), Continuous Discharge Rate (C) | Determine maximum output power, affecting acceleration performance and heat generation |
| Safety Parameters | Thermal Stability, Thermal Runaway Temperature | Set the safety threshold of the battery system |
| Cycle Life | Number of cycles (80% capacity retention) | Determine the service life of the battery |
| Cost Parameters | Cell Price (CNY/kWh) | Directly affect the overall vehicle cost |
The mainstream cell technical routes currently are Lithium Ternary (NCM/NCA) and Lithium Iron Phosphate (LFP). How do vehicle OEMs make the selection?
· For high-end long-range vehicles: Choose ternary lithium cells. They feature high energy density, long driving range and excellent low-temperature performance.
· For mid-range cost-effective vehicles: Choose LFP cells with low cost, superior safety and long service life.
· For performance vehicles requiring high-rate discharge: Ternary lithium cells can more easily meet high power output demands.
Notably, rapid technological advancement blurs this boundary: LFP cells are seeing continuous improvements in energy density, while ternary lithium cells are also achieving better safety performance. Therefore, the selection is not absolute.
2.2 Cell Series & Parallel Configuration Design: Meeting Vehicle Voltage and Energy Requirements
After cell selection, series-parallel matching design is carried out to meet vehicle specifications. A power battery pack has two core performance requirements: total voltage and total energy.
Basic Series & Parallel Rules
· Series connection: Voltage accumulates, capacity remains unchanged → To meet total voltage demand
· Parallel connection: Capacity accumulates, voltage remains unchanged → To meet total capacity (total energy) demand
Calculation Formulas
· Total Voltage = Number of series cells × Nominal voltage of single cell
· Total Capacity = Number of parallel branches × Single cell capacity
· Total Energy = Total Voltage × Total Capacity (unit: kWh)
Practical Example
· Vehicle requirements: 500 km driving range, total energy ≈ 60 kWh, motor operating voltage range: 250 V–400 V
· Selected cell: LFP cell, nominal voltage 3.2 V, capacity 100 Ah
· Target total voltage: 320 V
Number of series cells = 320 V ÷ 3.2 V = 100 pcs in series
· Required total capacity = Total energy ÷ Total voltage = 60000 Wh ÷ 320 V ≈ 187.5 Ah
· Configure 2 parallel branches: Total capacity = 2 × 100 Ah = 200 Ah
· Actual total energy = 320 V × 200 Ah = 64 kWh, which satisfies the energy demand
· Final configuration: 100S2P, 200 cells in total
Engineering Considerations for Series & Parallel Design
1. Strict consistency control: The deviation of capacity, internal resistance and open-circuit voltage among parallel cells must be limited to a narrow range. Cells with poor consistency will age prematurely and degrade the whole pack.
2. Thermal design consideration: Uneven current distribution among parallel cells leads to inconsistent heat generation. Impedance matching shall be considered when designing busbars.
3. Fault tolerance design: If one cell suffers short circuit, structural design shall prevent fault propagation to the entire module or battery pack.
2.3 Module Design: Why Modules Are Needed, and Why the Industry Is Moving to Module-Free Designs
The traditional industrial chain follows the path: Cell → Module → Battery Pack. What is the value of modules?
Functions of Battery Modules
1. Mechanical fixation: Bind cells together to provide mechanical support and resist vibration and shock during vehicle operation.
2. Consistency management: Group cells with close consistency into one module for easier monitoring and control.
3. Simplified thermal dissipation: Unified heat dissipation channels can be designed for each module.
4. Modular production: Adjust the number of modules to match different driving range requirements for various vehicle models, lowering development costs.
5. Convenient maintenance: Only the faulty module needs replacement instead of disassembling the whole battery pack.
However, the industry has widely adopted CTC (Cell to Chassis) and CTB (Cell to Body) technologies in recent years, which eliminate independent modules and install cells directly into the pack. The driving factors are as follows:
Advantages of Module-Free Designs
1. Higher space utilization: Remove module end plates, side plates and other structural components, allowing 5–10% more cells to be arranged and effectively extending driving range.
2. Weight reduction: Cut numerous structural parts to lower vehicle curb weight and further improve cruising mileage.
3. Cost reduction: Fewer components and assembly procedures reduce manufacturing costs.
Does module-free design raise requirements for cells? Yes. Higher standards are required for cell consistency, production precision and thermal management. Nevertheless, continuous technological breakthroughs have addressed these challenges, making this route popular among more vehicle manufacturers.
