Considerations for intrinsically safe batteries

Considerations for intrinsically safe batteries

Intrinsically-safe technical option for batteries meets ATEX safety standards for explosive conditions

BY NICOLAS PAQUIN
Saft Portable Battery Div., Valdese, NC
http://www.saftbatteries.com

Batteries perform countless functions ranging from the main power supply for high-tech instrumentation to emergency back-up for critical process control equipment and production systems. A battery is usually one of many components in a larger system and must comply with the safety standards of the end-use application. In all cases, it is critical that the battery operate safely and reliably, especially when it is within close proximity to a human operator, such as a handheld radio.

Manufacturers must design their batteries to meet a range of safety standards. (Shown here are Saft’s MP Series lithium-ion batteries.)

People who work in flammable and potentially explosive conditions must take extra safety precautions. The electronic equipment they use, such as lamps, gas detectors, and air-monitoring devices, is held to the highest safety standards.

ATEX Directive 94/9/EC is a common safety directive that applies to devices used under these conditions. It identifies electronic components with built-in safety features designed to safeguard users who work under hazardous conditions. Equipment manufacturers bear the responsibility for designing and testing their equipment to ensure that it incorporates all of the required safety features.

When designing equipment, several different technical options are available to comply with the ATEX directive, including explosion-proof enclosures, specific protection devices and encapsulation. Among these options, the use of precertified “intrinsically safe” (IS) components is one of the most efficient, as it avoids the use of additional protection features a process that is often costly and time-consuming. Extra protection features also take up additional volume within the device and can result in a heavier, more bulky unit.

The International Electrotechnical Commission (IEC) standard 60079-11 dictates that to be considered “intrinsically safe” such components, including batteries, should not produce sparks and should not release electrical or thermal energy that could cause ignition of flammable or combustible atmospheres, even under abusive conditions. For instance, the IEC standard mandates that cells be subjected to an external short-circuit test, with all protection devices removed.

To pass the test, the cells must not vent or leak and can display only a limited skin temperature increase during the short. The ability to meet such highly demanding requirements lies in critical choices made by the cell manufacturer about key materials such as the can type or electrochemical compounds.

Since 2003, all new equipment intended for use in potentially explosive atmospheres must comply with ATEX, the most recognized directive. In order to ensure that a product meets essential safety requirements, manufacturers typically work with a third-party certification group, known as a Notified Body (for example, Baseefa, Sira, Lloyd’s, or TUV). Once certified, the equipment is marked with “CE” (meaning it complies with ATEX and all other relevant directives) and “Ex” symbols to identify that it is approved under the ATEX Directive.

IS batteries are commonly used for mining, pipeline, drilling, gas detection, communications and underground applications; however, there are above-ground uses as well, such as gas metering and two-way radios.

Multiple levels of safety

The first step to incorporating IS batteries into a design is choosing an experienced design partner who is capable of meeting specific product specifications and global safety standards. The battery design should provide several levels of protection to ensure intrinsic safety.

A thermal fuse and electronics to monitor the individual cells within the battery pack, are two common battery-level safety features. In addition, it is essential that the electrochemical cells continue to operate safely even if the safety devices within the battery fail.

The way a battery and its cells are designed plays a critical role in how safe it is during use. In order to ensure the highest degree of battery safety, highly safe components and concepts must be selected during the design process.

For example, Saft’s high safety standards and performance quality help provide cells that withstand common field abuses, such as mechanical shock and vibration, as well as external short-circuit test, crushing, nail piercing and more. Because safety standards vary among countries, it is best to meet the highest standard to ensure compatibility with customer requirements around the world.

The MINER Act

In the United States, large-scale deployment of intrinsically safe systems are expected to occur in compliance with the 2006 Mine Improvement and New Emergency Response (MINER) Act, which requires all underground coal mines to submit a plan for a miner tracking system by June 15, 2009. Congress passed the MINER Act shortly after the mine disaster in Sago, WV, where a coal mine explosion killed 12 underground workers.

Already, we are seeing an influx of new designs to support the MINER Act. Saft IS batteries have been approved by the U.S. Mine Safety and Health Administration to power a number of emergency response electronics. For example, the MineTracer, a wireless miner-tracking and communication system, can transmit information on the location of trapped miners for at least 48 hours. During an emergency, power to coal mines is typically shut off to reduce the risk of methane gas ignition and the obvious danger to the miners underground. In this situation, the system is completely reliant on power from Saft single-cell MP Series batteries.

Alion Science and Technology developed an IS handheld radio for underground coal mines. Also powered by Saft’s lithium-ion (Li-ion) IS batteries, the system provides a redundant and survivable voice and data network that allows peer-to-peer communication among miners underground and among below- and above-ground personnel. It also offers a real-time situational awareness capability and the ability to integrate with existing wired mine communications systems.

Technology

IS batteries come in a variety of chemistries, including rechargeable Li-ion, NiCd, and lithium-thionyl chloride (Li-SOCl2 ). The design of the battery pack is very important and many factors must be considered in addition to environmental conditions, such as performance, life cycle, and weight.

Li-ion chemistries offer unrivaled nominal capacities and long life cycles; the most innovative feature is the ability to be charged and discharged over a wide temperature range, especially at low temperatures. Additional safety design features could include stainless-steel or aluminum containers, sturdy glass-to-metal sealing, and easy state-of-charge detection.

The necessity for IS batteries is clear, and many manufacturers are stepping up their product offerings to meet the demands of high-risk applications. Events like the 2006 coal mine explosion in West Virginia remind designers about the importance of maintaining communication during a catastrophic event. ■

Related Products: Batteries