An electric vehicle (EV) battery pack consists of several individual cells connected in series or parallel. This article highlights using interconnects to bind the individual cells and provide electrical, mechanical, and thermal support to the battery pack.
EVs use various types of battery packs to power their electric motors. The primary types of battery packs used in EVs are lithium-ion, nickel-metal hydride, lead-acid, and ultracapacitors.
Lithium-ion batteries are the most widely used due to their efficiency and stability, while nickel-metal hydride batteries are used in hybrid vehicles. Lead-acid batteries are used for auxiliary purposes, and ultracapacitors provide additional power during acceleration.
Within the lithium-ion battery family, there are three main types of cells: cylindrical, prismatic, and pouch. Each type has distinct characteristics, such as chemical composition, size, capacity, and lifespan, which make them more or less suitable for different applications.
Cylindrical batteries are the classic version, known for their high capacity density and low cost. They come in various sizes and have a long lifespan. Prismatic batteries have a moderate size and capacity, with a lifespan similar to pouch batteries. Pouch batteries are small, lightweight, and flexible but have high production costs and are prone to overheating.
An EV battery pack consists of several such cells in a designed combination depending on the battery type and the application. Interconnects are primarily used to join the individual battery cells. However, interconnects provide more than just a joining force in EV battery packs.
Interconnects for building battery packs
The primary purpose of interconnects in EV battery systems is to connect numerous batteries in series/parallel to form an EV battery module. In the next step, several battery modules are connected in series to make an EV battery pack.
The battery pack design may involve high-capacity individual battery cells connected in series. Another possible way to arrange the battery cells is to use a parallel connection of several small-capacity cells.
The parallel connection is generally preferred for the highest capacity and amperage requirements. The series connection is good if an enhanced power supply is preferred. Figure 1 shows how battery cells are connected through bus bar as interconnects.
Figure 1. Assembling EV batteries into a battery pack using interconnects in multiple ways. (Image: Electronic Products)
Interconnects as a battery management system (BMS)
Interconnects facilitate data transmission between individual cells and the BMS (Figure 2). This includes critical information such as voltage, current, and temperature readings. Reliable interconnects ensure accurate and timely data flow, essential for the BMS to make informed decisions.
Interconnects can include embedded sensors or connections to external sensors that monitor cell voltages and temperatures. These sensors provide the BMS with real-time data, allowing it to manage the charging and discharging processes effectively.
Figure 2. An EV battery interconnect system holding all the individual battery cells with a common current collector for BMS. (Image: Interplex)
Interconnects provide the pathways for balancing currents. The BMS uses these pathways to equalize the charge among cells, preventing overcharging or deep discharging individual cells. Effective balancing extends the overall battery life and improves performance.
Some interconnects are designed to act as heat sinks or conductive paths that aid thermal management. By helping dissipate heat, they prevent hotspots and ensure that the BMS can maintain an optimal temperature range for the battery pack.
Interconnects for mechanical stability
Mechanical aspects of battery packs are important for the safety of the EV. Interconnects help mitigate mechanical instability associated with dynamic loading and random vibrations, as seen in Figure 3. Failure to achieve mechanical stability can lead to short circuits and fire accidents.
Interconnects are especially useful in thin prismatic or soft polymer cells with notable changes in the dimension of the battery cell. Such dimensional changes can be in the form of expansion during charging and contraction during discharging.
Figure 3. EV battery interconnects used for the mechanical stability of the battery. (Image: Elsevier)
Interconnects, such as busbars and wiring harnesses, can be designed to distribute mechanical loads evenly across the battery pack. This helps prevent stress concentration on individual cells, reducing the risk of damage and ensuring longevity.
Interconnects can reinforce the overall rigidity of the battery pack by acting as a structural element. Materials like copper or aluminum can support the pack structure.
Summary
Interconnects are not to be taken as a mere connection between the battery cells of an EV battery pack. Their role has evolved to act in multiple roles.
From an electrical perspective, interconnects offer continuity in current flow from one cell to another. From a thermal point of view, they can also dissipate heat and help in the battery pack’s BMS. From a mechanical perspective, they hold the battery cells together and protect against shocks and vibrations when the EV is running.
References
- Automotive battery pack manufacturing –a review of battery to tab joining, Elsevier
- Joining Technologies for Automotive Battery Systems Manufacturing, MDPI
- Electrification of EV Automobiles and Trucks Shifts into High Gear, Interplex
- Building a Better Battery Module Interconnect, Aptiv
- Design considerations for interconnect technology used in EV powertrains, Charged
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