Battery Power - March/April 2012 - (Page 15)

Making Lithium-Ion Abuse Tests Meaningful E.J. Spek, Chief Engineer TÜV SÜD Canada, Inc. Customers of car companies have come to expect vehicles that are considered safe, according to a catalog of standards. Hybrid, plug-in hybrid and battery electric vehicles are and will be subject to the same expectations. The standards that have been developed for more than 100 years for combustion-engine vehicles have been effective in ensuring that the vehicles are indeed considered safe. These same standards need to be augmented to account for the electrification component that comes with high voltage and significant amounts of stored electrical energy. The risks that have been introduced in these electrified vehicles need to be considered in the design and development phase of the automotive product development process. The task of writing and then proving out these standards is a slow and laborious process. It has taken more than 10 years of work by a number of regulatory and certification bodies to produce a considerable number of relevant standards documents. Notwithstanding the pioneering yet thorough effort, there are a number of issues that need to be addressed to make the final standards robust, consistent and universal. Central to the effectiveness of the standards are the test methods used to verify that the product satisfies the requirements as defined by the standards. Some of the first organizations to implement the test methods are third party test agencies. These organizations can be very helpful in identifying test method shortcomings and opportunities for improvement. TÜV SÜD is one of those organizations. TÜV SÜD is a third-party ISO 17025-certified global testing company that performs battery testing for several organizations in the mobile, stationary and consumer products fields. Abuse tests are conducted to establish the reaction of cells, modules or batteries to conditions exceeding those expected to be encountered in normal vehicular use. This information can then be used to make decisions in the design or design verification process. This is especially important for lithium-ion products that do not yet have an established track record of safe operation over many years of service and abuse conditions. One of the common abuse tests conducted at TÜV SÜD is the nail penetration test as described in SAE J2464 section 4.3.3. The short history of lithium-ion battery development, especially in large ampere-hour and pouch cells, is one reason that the current state of cell nail penetration test specifications is substantially open to interpretation in test methodology. TÜV SÜD, in actively supporting the electric vehicle industry, has pursued a path of continuous test methodology improvement for the cell penetration test over the last two years. After having abuse-tested hundreds of large format lithium-ion cells, TÜV SÜD has a robust and repeatable process to support rapid cell development and verification testing. The hurdles that TÜV SÜD has encountered in making this test effective are described qualitatively below. It is hoped that others engaged in a similar improvement process can benefit from the knowledge gained. Recommended Practice SAE (Society of Automotive Engineers) has a recommended practice in SAE J2464 that describes a nail penetration test in section 4.3.3. The description includes the parameters as listed in Table 1 for cells. Other values are used for modules and packs. Test Methodology History TÜV SÜD has primarily tested both hard case and soft prismatic (pouch) cells for nail penetration. The first TÜV SÜD test device for the penetration test was an air-driven cylinder set at a nail velocity of 100 cm/sec. This high value was used to ensure that the minimum nail velocity through the actual penetration of cells as thick as 12 mm would be above the minimum specified 8 cm/sec. Note that in a vehicle, the bare cells are protected from flying objects by the pack structure, this explains the relatively low velocity value compared to road speed. Since the force capable of being produced by the air cylinder was substantial at 1.5 kN, there was no concern for a slowing down of the nail through the cell thickness to less than 8 cm/sec. A substantial number of cells were tested in this manner but results at EUCAR 5 and 6 could not be correlated to cell suppliers’ similar penetration test data. In a concerted effort to resolve the discrepancies, many of the characteristics listed in Table 1 as ‘no value’ were given values based on successful tests given an arbitrary assignment of EUCAR Hazard Severity levels of less than five as ‘acceptable’. The following were given initial values: nail surface finish, point included angle and surface finish, material selection within steel, nail straightness and perpendicularity, cell preload and cell restraint method. With these refinements, more cells were tested from selected anode and cathode chemistries as well as capacities. Concurrently, the rate of penetration in open air was reduced from the 100 cm/sec to the minimum 8 cm/sec to be aligned with the cell supplier’s nail velocity. With the TUV SUD continued on page 16 Table 1. SAE J2464 Penetration Test Parameters www.BatteryPowerOnline.com March/April 2012 • Battery Power 15 http://www.BatteryPowerOnline.com

Table of Contents for the Digital Edition of Battery Power - March/April 2012

Battery Power - March/April 2012
Nissan and ABB Link to Evaluate Second-Life Storage Applications for Nissan LEAF Battery Packs
Nichicon Develops 50 kW Quick Charger for Evs
Lithium-Ion Battery-Emulation Circuit Using a Microcontroller
High-Efficiency Switching-Mode Charger System Design Considerations with Dynamic Power Path Management
Making Lithium-Ion Abuse Tests Meaningful
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Battery Power - March/April 2012