The rapid growth of electric vehicles (EVs) shows no signs of slowing. The Edison Electric Institute predicts that by 2030, there will be 18.7 million EVs on U.S. roads, a significant increase from just one million at the close of 2018. Additionally, the U.S. Department of Energy reported that the number of EV charging stations surged from fewer than 500 in 2008 to over 20,000 by 2019, with more than 68,800 connectors available.
This surge in EV adoption is driving increased demand for batteries, with forecasts from IDTechEx suggesting a 20% compound annual growth rate, potentially reaching a $15 billion market by 2026.
EV Battery Cell Types
Battery cells are the fundamental energy storage units for hybrid and electric vehicles (EVs). There are three main types of battery cells used in EVs: cylindrical, prismatic, and pouch cells. Each type consists of an anode and cathode, separated by an electrolyte, but they differ in how they are constructed and implemented.
Cylindrical cells are the most cost-effective to produce on a per kilowatt-hour (kWh) basis. However, their round shape means they don’t pack as efficiently, resulting in larger, heavier battery packs compared to other cell types.
Prismatic cells, with their rectangular shape, offer better space utilization, making them ideal for compact designs. While commonly found in electronics like smartphones and laptops, larger versions of prismatic cells are used in EVs and hybrid vehicles. Some expansion of prismatic cells is normal, so they require an allowance for this during operation.
Pouch cells are the most compact and lightweight option. Their soft, flexible casing allows for highly efficient packing, with packaging efficiency rates of 90% to 95%. However, pouch cells experience more swelling during use compared to prismatic cells, requiring special management.
Whether using a single pouch or prismatic cell, or a series of them, maintaining the proper pressure on all cells is crucial to minimize capacity loss. Battery compression pads are often used to ensure this optimal pressure is sustained.
Battery Cell Expansion and Contraction
Common EV battery chemistries experience two types of physical changes due to electrochemical reactions. The first type involves cyclical expansion and contraction during charging and discharging, causing reversible changes in cell thickness. The second, more gradual, change is an irreversible increase in thickness from the beginning of life (BoL) to the end of life (EoL) of the battery, occurring over many charge-discharge cycles.
For reversible changes, studies have shown that applying the right amount of pressure helps cells retain their capacity for longer. Excessive pressure can accelerate capacity fade and compromise cell stability, potentially leading to dangerous situations like thermal runaway. Conversely, insufficient pressure allows the cells to expand too much, which can affect their electrical connections and thermal management. The use of compression pads with optimal elasticity is key to maintaining proper pressure, enabling long-term capacity retention.
Thermal Management
Thermal management refers to controlling the temperature of a battery pack. Batteries generate heat, especially under high loads or during fast charging, making it essential to keep temperatures consistent—usually within a 3°C to 4°C range, even in extreme ambient conditions ranging from -35°C to 50°C.
There are two approaches to dealing with heat in a battery pack. One is heat evacuation, which involves cooling systems using plates and circulating fluids, often glycol-based, to dissipate heat. The other is insulation, where thermal materials, like elastomeric foams, prevent heat transfer between cells.
EV Battery Compression Pads
Compression pads help maintain consistent pressure on pouch or prismatic cells. If the compression pad lacks resilience, it allows excessive expansion, leading to faster capacity loss. These pads also act as insulators, channeling heat toward the cooling plate to maintain a uniform temperature across the cell stack.
Saint-Gobain offers silicone and micro-cellular polyurethane foam pads for battery compression. These materials are designed to offer consistent compression force deflection (CFD) over a wide range of compression and temperature conditions. The foam’s spring-like qualities ensure that optimal pressure is applied throughout the battery’s lifecycle, preventing excessive stress that could harm the cells. Furthermore, these pads are resistant to permanent deformation under extreme compression, helping maintain electrical insulation and preventing arcing within battery modules.
Saint-Gobain Norseal PF Series Compression Pads do more than accommodate the swelling of battery cells; they also offer mechanical stability by reducing cell movement during shocks or vibrations, preserving the integrity of the battery pack. With the rise of automation in manufacturing, reducing the number of steps and components is increasingly important. The PF Series Pads are designed with inherent tackiness, eliminating the need for additional adhesives or glue to attach them to cells during assembly, which streamlines the automation process. Additionally, the level of tackiness can be adjusted to suit different manufacturing requirements. These pads are customizable in terms of density, thickness, and tackiness, allowing for flexibility in various applications.
Norseal PF Series Compression Pads, including products from the PF27, PF47, and PF100 Series, offer an extensive range of thicknesses, even at high densities. Density is crucial for minimizing the overall weight of the module, battery pack, and vehicle. The flexibility of the Norseal PF Series, available in various densities and thicknesses, allows it to adapt to different cell chemistries and pack configurations.
Norseal PF27 is available in very thin options, aiding in achieving higher energy density within a pack. It also meets flame performance standards according to ASTM D4986, equivalent to UL94 HBF. The foam’s properties are stable over time and under varying environmental conditions.
Similar to the PF27, the Norseal PF47 is a line of micro-cellular polyurethane foams optimized for battery efficiency, but with thickness options starting from 2 mm.
The recently developed Norseal PF100 Series features premium micro-cellular polyurethane foam with the widest and flattest compression range in the PF Series. This is a crucial metric for today’s battery pack designers. The PF100 Series excels in “aged” compression set resistance under high temperatures and humidity, extending the lifespan of battery packs. Its low thickness-to-density ratio allows design engineers to maximize energy density and space while minimizing weight.
These consistent properties ensure the foam maintains its performance across a range of conditions, contributing to the long-term reliability of battery packs.
