Automotive Battery Pack Design

Key Takeaways

• The structure of the wet cells that form the battery's energy storage.

• The components of battery packs are where form meets function.

• Power and system control allocate voltage outputs with the electric distribution system.

Automotive battery pack design requires multiple fail-safes and control systems to leverage performance safely.

The electric vehicle is nothing new, but its design and technology are rapidly maturing. Compared to internal combustion, the electric engine offers many advantages: electric drivetrains are far more effective at energy conversion than their combustion counterparts, and energy costs of electricity versus fuel on a per kWh basis range from equivalent to several factors cheaper for electricity. 

The major drawback for electric vehicles remains the battery, which hinders total energy storage capacity, cost to manufacture/replace, and recharge time. Research and development efforts continue to advance battery capabilities, yet many electromechanical optimizations are ongoing in the automotive battery pack design. 

Building a Battery Pack

Automotive Battery Pack Design: Cells to Systems

Battery packs leverage many individual cells to the meet a car’s power and fuel needs.

Current automotive battery pack designs build around wet battery cells, which immerse the electrodes (anode and cathode) in an electrolyte solution with a semi-permeable separator between them. The anode and cathode are the primary points of interest in the battery; the former will oxidize molecules to gain their electrons.  At the same time, the cathode reduces molecules to return electrons, completing the electrical circuit. The electrolyte and separator facilitate this process by transporting the carrier ions (in this case, lithium) and selectively allowing the passage of said ions to prevent a short circuit, respectively. 

For safety purposes, automotive batteries also contain a current interrupt device (CID) which actively senses unsafe current, voltage, or pressure that might result in damage or injury. Once detected, the CID disables the cell in the hopes of preservation or arresting a thermal runaway event.

The cells form the fundamental unit of the battery, but similar to a PCB, they require housing for mechanical support and protection while still maintaining acceptable ambient temperatures for operation. These modules integrate into the overall battery pack system:

• Case—the case is multifunctional: it mechanically supports the weight of the battery, prevents the ingress of dirt and water that could detrimentally affect the operation, and protects maintenance technicians from high-voltage components.

• Battery module—each battery comprises multiple cells housed within a module for structure and electric terminal provision. Individual modules connect through the bus bars to the contactors.

• Contactors—a switch operated by the control system for electrical isolation between the battery pack and the vehicle. Closes the circuit after passing safety checks and opens the circuit in the event of a crash or detected battery malfunction.

• Fusing and disconnect—break the circuit; the former prevents damage to expensive components when power spikes, while the latter electrically isolates the pack from the vehicle during maintenance.

• Cooling—battery packs require extensive air or liquid cooling to achieve the dense cell packing necessary for volumetric energy storage.

• Battery management system (BMS)—the system and battery pack must exist in active communication for safe operating conditions. The BMS tracks the status of the battery’s charge, monitors for faults, and verifies the pack’s connection and isolation before closing the contactors.

The BMS is integral to the dynamic optimization of the battery’s performance. A current sense circuit connected to the CAN bus measures the line between the battery pack and the traction inverter (responsible for converting the battery output to AC). Meanwhile, the inverter communicates with the vehicle controller and battery charger to return charge to the batteries for efficiency and to balance energy storage among the cells when instantaneous power demands fall. 

This cycle continues during runtime: the BMS oversees the circuit parameters, cell condition, and usage statistics to capitalize on the battery more effectively.

Controlling Battery Output with an EDS

The sophistication of modern automotive batteries requires an electrical distribution system (EDS) to map out the conduction pathway between the modules, BMS, and control lines for various system functions. The need for redundancy is palpable as the system operates bidirectionally and is responsible for powering the vehicle and supporting features. 

The EDS uses multiple sensors to rapidly respond to the needs of the car, including the I/O of the BMS: depending on the configuration of the battery pack, the BMS may either receive signals that monitor the circuit state or drive other system components like battery cooling (fans, pumps, etc.) 

Additionally, the EDS assesses the electrical isolation of the battery and the isolation of any conduction paths for leakage current. The two primary outputs of the EDS are the high and low-voltage lines:

High Voltage (LV)

Given the danger associated with the HV line, the EDS has numerous checks to ensure safe operation. The system checks the instantaneous current and voltage and can modify the conduction path when measurements exceed safety ratings. While the car is under maintenance, the EDS also manually disconnects the HV line to prevent shock or other battery-related injuries to technicians.

Low Voltage (LV)

The LV line powers the control systems of the battery and enables communications between the battery and the vehicle through the CAN bus. This control includes a signal confirming all interfacing connectors are completely engaged so that no external contact points exist for HV conductors. 

Powering the Future with Cadence Solutions

Automotive battery pack design marries the latest material research into a highly responsive system that can quickly adjust to changing internal and external demands. The battery—a passive electrochemical device—can only operate to its full potential with an extensive network of control circuitry. High-speed communication between and within automotive systems builds on excellent signal and power integrity characteristics which start at the PCB system level. 

Cadence’s PCB Design and Analysis Software suite allows design teams to simulate electronic systems thoroughly to reduce time spent on debugging, revisions, and other corrective actions. After completing the schematic, effortlessly move to layout: OrCAD PCB Designer offers a complete toolset of constraint-driven functionality in a fast and user-friendly interface.

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