Battery Power - Fall 2016 - 9

Feature Rupture Disks for Venting Cells, Packs Although battery cells and packs typically feature active and passive safety devices and protection circuitry to prevent or mitigate potential cell failures, the last resort is typically a mechanical safety vent that is intended to release the internal pressure of the cell or pack when a specified pressure is reached. One of the most popular pressure relief devices for lower-pressure applications such as lithium batteries is the rupture disk. Also known as a pressure safety disk or burst diaphragm, the rupture disk is a passive pressure relief device long adopted by the oil, gas and chemical process industries to protect pressure vessels, tanks and other equipment from overpressurization. Rupture disks are available in various designs, sizes, shapes and set pressures and can be installed on cylindrical, button, prismatic, or pouch cell designs. However, as battery cells become increasingly smaller, so must the rupture disks that protect them. This is driving the need for miniaturized rupture disks as small as 1/8-inch and challenging an industry in which products for many decades have been measured in inches. "As pressure relief devices become increasingly miniaturized, the rupture disk industry is running squarely into design and raw material challenges that often require reengineering the product itself," said Geof Brazier, managing director, BS&B Safety Systems, Custom Engineered Products Division, a major supplier of rupture disks that has been involved in the lithium battery industry for more than 30 years. According to Brazier, this is due to a delicate balancing act between the shrinking diameters of the burst area, the limitations of the specific raw materials utilized for the membrane, and the variations in designs required at low, medium and high pressures. Fortunately, the rupture disk manufacturer is embracing this challenge with novel structures and design elements that have led to a new category of miniaturized options from 1/8 inch to 1 inch at all ranges of pressure, with larger nominal sizes suited to battery pack applications that require a greater vent area. "In many ways it can be like trying to fit a camel through the eye of a needle." To resolve this issue, BS&B has created novel structures that control the reversal of the rupture disk to always collapse in a predictable manner. This includes, for example, a hybrid shape that combines reverse buckling and forward bulging characteristics that are pre-collapsed. In this type of design, a line of weakness is typically placed into the rupture disk structure to define a specific opening flow area when the reverse type disk activates. Small, nominal size rupture disks are sensitive to the detailed characteristics of the orifice through which they burst, which requires close cooperation between the rupture disk manufacturer and the user to achieve the optimum mounting and installation arrangement. "With small size pressure relief devices, the influence of every feature of both the rupture disk and its holder is amplified," said Brazier. For miniaturized products, BS&B manufactures the rupture disk from Stainless Steel, Aluminum and Nickel alloys to achieve compatibility with lithium battery operating conditions. For more information, visit BS&B Safety Systems at www.bsbsystems.com. The Miniaturization Challenge Miniaturization of rupture disks presents unique challenges, best met utilizing reverse buckling technology, said Brazier. Unlike traditional forward-acting disks where the load is applied to the concave side of a dome, in a reverse buckling design, the dome is inverted toward the source of the load. Reverse buckling disks are typically sturdier than forward-acting disks, which are thin and difficult to handle, and as a result have greater longevity, accuracy and reliability over time. "As burst diameters decrease dramatically it becomes challenging to design a reverse buckling disk that will reliably collapse through such small orifice sizes," said Brazier. www.BatteryPowerOnline.com Fall 2016 * Battery Power 9 http://www.bsbsystems.com http://www.BatteryPowerOnline.com

Table of Contents for the Digital Edition of Battery Power - Fall 2016