Conformity Magazine - November 2007 - (Page 16)

corrode metallic battery components. Ultimately, floods can deteriorate battery functional performance and safety. For this test, the battery’s flood resistance and ability to perform is evaluated when float charged per manufacturer’s specifications and partially immersed in 6 inches of salt water for 48 hours at 25°C (77°F) and then subsequently submerged in salt water for 7 days at 25°C. The immersion test is conditionally required. Batteries are required to not explode, ignite, vent, or leak during immersion, and when monitored for 24 hours following the removal of the salt water. For salt water, a solution of 1% by weight of sodium chloride (NaCl) and distilled or deionized water is used and maintained throughout the entire test. The immersion test setup is shown in Figures 3a and 3b. Section 4.5 Eletrical Criteria Section 4.5 of the safety section presents tests developed to determine the battery’s safety when subjected to common electrical disturbances, misuse and abuse. Electrical safety testing consists of overcharge, external short circuit, reverse polarity, and overdischarge. As mentioned in Part I of this series, lithium battery safety is dependent on the design and reliability of its manaagement system. With time, the management system protection circuits will most likely fail before the lithium cells. Knowing this, protection circuits are duplicated to improve system reliability and maintain designed system safety. Normally, only one level of redundancy is added. To assess safety during times of normal battery operation, battery management systems (BMSs) are assumed to have two levels of protection against excessive overcharge, external short circuit, reverse polarity, and overdischarge events. Overcharge, excessive short circuit, reverse polarity, and overdischarge safety tests are performed in Levels I, II, III. Descriptions of the overcharge, short circuit, reverse polarity, and overdischarge testing conditions are provided here. Section 4.5.1 – Overcharge Typically, abusive overcharge occurs when rectifiers lose regulation of current, and force excessive charge that can abuse batteries. Lithium batteries are typically outfitted with electronics that sense and stop overcharge currents from interacting with the electrochemistry. For overcharge testing, the battery under test is subjected to a constant potential of 60 V with current limited to 1C. Section 4.5.2 - Short Circuit A short circuit is a low impedance connection, internal or external, between the positive and negative of the battery, causing ultimately zero (0) voltage across the terminals and a very high current flowing through the short. Shorts can cause safety issues. Lithium batteries are typically designed with safety devices designed to lessen the effects of short circuits. For short circuit testing, the battery under test is subjected to a maximum short circuit resistance of 3.5 mΩ. Section 4.5.3 - Reverse Polarity An external reverse polarity event is defined as the reversal of voltage from a DC power source to the battery. External polarity reversal effects are similar to external short circuits as excessive currents are imposed to the battery and cause instability. For reverse polarity, the battery under test is subjected to voltage reversal at the manufacturer’s recommended constant potential with current limited to 2C. For this test, an extra battery is connected in parallel with a DC power source and the battery under test voltage is reversed. Section 4.5.4 - Overdischarge Excessive overdischarge is the continuation of discharge after the battery has provided 100% of its rated discharge energy. If overdischarged, battery electrodes can reverse in polarity and become unsafe. Figures 3a and 3b: The immersion test setup 16 Conformity noVEmBEr 2007

Table of Contents Feed for the Digital Edition of Conformity Magazine - November 2007

Conformity - November 2007 Contents Editor’s Note Commission Sets Date, Terms for 700 MHz Auction FCC Reaffirms Roaming Obligations Commission Moves to Improve USF Management FCC Proposes Fines for Violations of E911 Handset Requirements Commission Levies $1 Million Plus Fines for Unsolicited Fax Advertisements GR-3150 - Lithium Battery Certification Levels Based on Criteria for General Product, Safety, and Performance, Part 2 ESD Open Forum Machinery Field Evaluation for the United States Validation, Verification, and Immunity Testing Techniques for EMC for Functional Safety Thermal/EMC Simulation Addresses New Mechanical Design Challenges Product Safety Equipment Buyer’s Guide FCC Releases Report on Telephone Numbering Resource Utilization Canada Still Has PCS Spectrum to Assign Updated Standards List for the EU’s Medical Devices Directive Updated Standards List for EU Directive on Active Implantable Medical Devices Call for Papers for the 2008 EOS/ESD Symposium Announced Mobile Phone Use in Hospitals May Be Dangerous CPSC Actions in the News IEC Standards Update UL Standards Update Telcordia Standards Update From Our “You Can’t Make This Stuff Up” Department Product Reviews Looking Back: Items from Past Issues of Conformity Upcoming Events Advertisers

Conformity Magazine - November 2007

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