Traction Battery

1. Product Definition

A traction battery, formally known as an electric vehicle traction battery, refers to the rechargeable battery pack engineered to supply electrical energy to an EV's powertrain. Regarded as the "heart" and primary power source of a new energy vehicle (NEV), it directly determines the vehicle's driving performance, operating range, and overall safety. As the cornerstone of the electric vehicle industry chain, the traction battery serves as the EV equivalent of a conventional vehicle's fuel tank, with its cost typically accounting for 30% to 50% of the total vehicle manufacturing cost.

2. Working Principle

Currently, mainstream electric vehicles predominantly utilise lithium-ion batteries, which are rechargeable secondary cells. Their operation relies on the shuttling of lithium ions (Li+) between the positive and negative electrodes, commonly termed the "rocking-chair" charge-discharge mechanism.

During Charging: External electrical energy forces lithium ions to de-intercalate from the positive electrode (cathode), migrate across the electrolyte, and intercalate into the layered structure of the negative electrode (anode).
During Discharging: Lithium ions de-intercalate from the anode and return to the cathode via the electrolyte, while the released electrons flow through the external circuit to generate electric current, thereby driving the electric motor.

Lithium ions essentially act as the "energy carriers" throughout this cycle. A fundamental rule is that the greater the number of lithium ions intercalating and de-intercalating between the electrodes, the higher the battery's charge and discharge capacity.

3. Main Classifications

Traction batteries are primarily categorised by their active chemical media into lithium-ion, nickel-metal hydride (NiMH), and lead-acid batteries. Modern electric vehicles almost exclusively rely on lithium-ion chemistry, which is divided into two mainstream technological pathways:

Lithium Iron Phosphate (LFP) Battery: Utilises lithium iron phosphate as the cathode material. Its standout advantages include exceptional safety, high thermal stability, lower production cost, and a long cycle life (exceeding 1,500 full cycles). Its primary drawbacks are relatively lower energy density (gravimetric energy density around 140–180 Wh/kg) and compromised discharge performance in cold weather.
Ternary Lithium Battery: Uses Nickel Cobalt Manganese (NCM) or Nickel Cobalt Aluminium (NCA) as the cathode material. Its key strengths lie in high energy density (gravimetric energy density reaching 200–300 Wh/kg, with volumetric energy density exceeding 600 Wh/L) and robust power output. The downsides are higher manufacturing costs and greater chemical reactivity, which demand more sophisticated safety engineering and thermal management systems.

At present, both battery chemistries co-exist in the market to cater to different vehicle segments, pricing strategies, and consumer demands.

4. Basic Structure

A traction battery system is a sophisticated multi-tiered assembly, typically engineered across three hierarchical levels: cells, modules, and the battery pack.

Cell: The fundamental electrochemical unit that stores energy. Its core architecture consists of the positive electrode, negative electrode, separator, and electrolyte. The electrodes facilitate chemical reactions, the separator prevents internal short circuits while allowing ion transfer, and the electrolyte provides the conductive medium. Based on packaging form factors, cells are broadly classified into cylindrical, prismatic , and pouch types.
Module: An intermediate sub-assembly created by grouping multiple cells in series and parallel configurations, integrated with monitoring and protection hardware. The module provides structural rigidity, mechanical support, and physical protection for the cells, while streamlining control for the Battery Management System (BMS).
Battery Pack: The complete, vehicle-ready assembly that integrates multiple battery modules alongside the Battery Management System (BMS), thermal management system, high-voltage wiring harnesses, and structural casing. This is the final high-voltage component mounted onto the vehicle chassis.

5. Battery Management System (BMS)

The BMS serves as the "brain and nervous system" of the traction battery pack. Its core responsibilities include:

Real-time Monitoring: Continuously monitors critical parameters such as voltage, temperature, and current across every individual cell.
State Estimation: Accurately calculates the battery's State of Charge (SOC, representing remaining battery level) and State of Health (SOH, indicating overall battery degradation and lifespan) .
Intelligent Management: Regulates charging and discharging rates and executes cell balancing to prevent overcharging or deep discharging.
Safety Protection: Instantly isolates the high-voltage circuit within milliseconds upon detecting anomalies such as short circuits or excessive temperatures, mitigating hazardous risks.

6. Daily Care and Extending Battery Life

Traction battery lifespan is heavily influenced by everyday driving and charging patterns. Adopting sound maintenance practices can significantly prolong its operating life:

Optimise Charging Habits: Follow the "shallow charge and shallow discharge" rule. It is recommended to plug in when the battery level drops to 20%–30% , avoiding running the pack completely flat. For daily commuting, prioritise AC slow charging to minimise reliance on DC fast charging. Furthermore, avoid running heavy electrical loads like the air conditioning while charging, as this causes frequent switching between charge and discharge states, accelerating cell ageing.
Manage Operating Temperature: Avoid parking the vehicle under scorching direct sunlight or in freezing conditions for extended periods, as extreme ambient temperatures can adversely affect battery chemistry and service life.
Adopt a Smooth Driving Style: Avoid aggressive driving habits such as sudden, hard acceleration and harsh braking, as high-current discharge draws place undue stress on cell health.
Conduct Regular Inspections: Keep an eye on any battery warning indicators on the instrument cluster. If an unusual, significant drop in driving range occurs, visit an authorised service centre promptly for diagnostics and maintenance.

7. Safety and Recycling

Safety Hazards: The most critical hazard associated with traction batteries is "thermal runaway". This refers to an uncontrollable chain reaction of exothermic events within the battery that causes temperatures to spike rapidly, potentially resulting in fire or explosion. Primary triggers include internal short circuits (caused by manufacturing defects or structural damage from collisions), overcharging, and extreme external heat. Selecting vehicles with proven safety credentials, using certified charging equipment, and taking care to avoid underbody scrapes are essential preventive measures.
Recycling and Repurposing: When a traction battery's capacity degrades below 80% of its original rating, it is generally retired from automotive service. However, these decommissioned packs are not scrap, but valuable "urban minerals". They first undergo second-life repurposing, serving in stationary energy storage systems (BESS) or low-speed utility vehicles. Once fully depleted, they proceed to material recycling, where precious metals like lithium, cobalt, and nickel are extracted through specialised dismantling processes. Currently, China has enacted multiple national standards to regulate and standardise traction battery recycling.

8. Summary

The traction battery represents the single most crucial and highest-value component in any new energy vehicle, with its technical maturity directly shaping the development of the entire EV industry. Understanding its working principles, chemistry characteristics, system architecture, and best maintenance practices is essential for every EV owner. By exercising mindful daily usage and routine upkeep, drivers can substantially extend battery longevity, ensure driving safety, and maximise the environmental benefits of green mobility.