Traction Battery Pack: Construction, BMS, and Thermal Management

Traction Battery Pack: Construction, BMS, and Thermal Management

How Traction Battery Packs Are Built

A traction battery pack is the high-voltage energy storage system that powers the electric drive motor in a hybrid or battery-electric vehicle. Understanding its construction is the first step toward working on it safely.

At the smallest level is the cell. Most OEM packs use one of three cell formats: cylindrical (like a large AA battery), prismatic (flat, rectangular), or pouch (thin, flexible laminate). Each cell is a self-contained electrochemical unit producing roughly 3.2–4.2 V depending on chemistry. Lithium-ion chemistries dominate current production; common variants include NMC (nickel-manganese-cobalt) and LFP (lithium iron phosphate).

Multiple cells are assembled into a module. Cells within a module are connected in series to raise voltage, in parallel to increase capacity, or in series-parallel combinations. Modules are then connected in series strings to build pack voltage—commonly 300–800 V or higher in production vehicles. Bus bars are the heavy copper or aluminum conductors that carry current between cells, between modules, and to the pack terminals.

Sense wiring (sometimes called cell voltage sensing harnesses) runs to every cell group within a module. These fine-gauge wires report individual cell-group voltages to the battery management system. Temperature sensors (typically NTC thermistors) are distributed throughout the pack and inside modules to give the BMS a thermal map of the pack.

The battery junction box (BJB), also called the battery disconnect unit or power distribution module depending on the OEM, contains the main contactors (high-current relays), the service disconnect, current sensors, pre-charge resistors, and the high-voltage fuse. The BJB is the electrical gateway between the battery cells and the rest of the vehicle's HV system.

Battery Management System Functions

The battery management system (BMS) is an electronic control module—or network of modules—that monitors, protects, and optimizes the traction battery. Its core functions include:

• Cell voltage monitoring: The BMS reads each cell group continuously and flags imbalances or out-of-range voltages as diagnostic trouble codes (DTCs).
• State of charge (SOC) estimation: The BMS calculates how much energy remains, typically expressed as a percentage, using algorithms that factor in voltage, current integration (coulomb counting), and temperature.
• State of health (SOH) estimation: Over time the BMS tracks capacity fade and internal resistance rise to estimate overall battery health.
• Cell balancing: To keep all cells at equal SOC, the BMS uses passive balancing (bleeding excess charge through resistors) or, less commonly, active balancing (transferring charge between cells).
• Thermal management control: The BMS commands the cooling or heating system based on temperature thresholds to keep cells in their optimal operating window.
• Contactor control: The BMS commands the main contactors open or closed. If a fault is detected—overvoltage, undervoltage, overcurrent, or over-temperature—the BMS opens the contactors to isolate the cells.
• Communication: The BMS communicates pack data over CAN or other vehicle networks to the power control module, the instrument cluster, and the charger.

Thermal Management Strategies

Lithium-ion cells are sensitive to temperature. Performance degrades below about 0 °C and above about 40–45 °C; sustained high temperatures cause permanent capacity loss and can lead to thermal runaway—a self-sustaining, uncontrollable exothermic reaction that can produce fire and toxic gases.

OEMs use three main thermal management strategies:

1. Liquid cooling/heating: A dedicated coolant loop circulates through channels in cooling plates beneath or between modules. A refrigerant-based chiller can actively cool the loop in hot conditions; a PTC heater or heat-pump circuit warms the pack in cold conditions. This is the most effective method and is used in most modern BEVs.

2. Air cooling: Cabin or external air is ducted through the pack. Simpler and lighter than liquid systems but less effective at high loads or extreme ambient temperatures.

3. Refrigerant direct cooling: Some designs route refrigerant directly through plates in the pack, eliminating the intermediate coolant loop for faster response.

The BMS continuously reads pack and module temperatures and commands the coolant pump speed and chiller or heater operation to maintain cells within the OEM target range. Technicians must understand this loop to diagnose cooling faults that appear as BMS over-temperature DTCs or reduced power modes.

Safety Requirements Before This Lesson

Traction battery packs operate at high voltage—commonly several hundred volts DC. That level of voltage is immediately life-threatening. Before any hands-on activity:

• Verify you have completed HV safety qualification for this program and are authorized to work on HV training equipment.
• Wear Class 0 or Class 00 electrical insulating gloves (rated for the voltage class of the training unit) with leather protectors and inspect them for pinholes before each use.
• Wear safety glasses or a face shield, flame-resistant (FR) clothing, and insulating footwear.
• Follow the vehicle/display unit-specific lockout/tagout (LOTO) procedure and the service information for removing or defeating the service disconnect. Confirm HV absence with a CAT III/IV rated digital multimeter before touching any HV conductor.
• Never work alone on HV systems. A trained second person must be present.
• Know the location of the nearest Class D or appropriate dry-chemical extinguisher and the facility's emergency plan for a thermal runaway event.
• Handle the cutaway or display unit only as directed by the instructor; even decommissioned cells can retain dangerous voltages.