TÜV SÜD

Designing Meaningful Lithium-ion Abuse TestsAn investigation into nail penetration abuse tests methodologies for consistent results

White paper

AbstractOver the past decade, regulatory authorities and standards development groups have worked to produce relevant standards for batteries and other electrification components used in alternative energy vehicles. However, the effort continues to identify the limits and shortcomings of certain testing approaches, as well as ways in which test methods can be improved to properly assess the safety of automotive power systems. Since 2010, TÜV SÜD has conducted testing on over a thousand battery samples, including abuse tests that establish the reaction of cells, modules or batteries under conditions that exceed those expected in normal vehicular use. The results of this testing provides important data that can help refine the recommended testing practices and procedures.

2 Designing Meaningful Lithium-ion Abuse Tests | TÜV SÜD

Erik J. SpekChief Engineer, TÜV SÜD CanadaErik J. Spek is Chief Engineer for TÜV SÜD Canada, a TÜV SÜD group company, and is responsible for battery verification services in North America. He holds degrees in mechanical engineering from the University of Waterloo in Waterloo, Ontario, Canada, including a Bachelor of Applied Science degree in mechanical engineering and a Master of Applied Science degree in metals fatigue testing.

Mr. Spek is a Professional Engineer in Ontario, Canada, a member of SAE since 1980, and a Certified Manufacturing Engineer in the Society of Manufacturing Engineers. He has authored and co-authored papers and articles on sodium sulfur battery development and on lithium-ion cell testing. Mr. Spek was a member of the ABB sodium sulfur battery team that provided 38 kWh battery packs for the Ford Ecostar program, and is also co-holder of a recent patent pending for bipolar sodium metal chloride batteries.

Prior to joining TÜV SÜD Canada, Mr. Spek was Director of Engineering for Innovative Testing Solutions which was acquired by TÜV SÜD in 2011. He has also served as Chief Engineer at Magna International, as Manager of Engineering and Operations at ABB Advanced Battery Systems, as Director of Engineering at Powerplex Technologies Inc., and as a Product Engineer at Black and Decker, General Electric and White Motor Corporation.

Erik J. Spek can be reached at 1-705-627-2466, or at espek@tuvcanada.com.

Contents

GAPS IN CURRENT BATTERY STANDARDS 3

KEY TO THE DEVELOPMENT OF SAFE PRODUCTS - ABUSE TESTING 3

APPROACH TO DESIGNING MEANINGFUL NAIL PENETRATION TEST METHODS 4

OTHER FINDINGS FROM ABUSE TESTS 8

CONCLUSION: THE NEED FOR MORE EXACT SPECIFICATIONS 10

About TÜV SÜD expert

3TÜV SÜD | Designing Meaningful Lithium-ion Abuse Tests

Gaps in current battery standards

Key to the development of safe products – abuse testing

Today’s consumer expects that the modern combustion engine automobile sold in dealers’ showrooms provides a level of safety consistent with applicable standards. That expectation is no less true when it comes to automobiles fueled by alternative technologies, including hybrid, plug-in hybrid and battery power. Standards for combustion engine vehicles have been developed over the past 100 years, thereby ensuring a reasonable level of safety for these vehicles. However, these standards must be revised and augmented with additional requirements to address the safety risks that accompany elevated voltage levels and significant amounts of stored electrical energy found in alternative energy vehicles. Appropriately updated standards ensure that these unique safety considerations can be addressed in the design phase of the automobile development process.

The task of writing and validating standards is a slow and laborious process, requiring all stakeholders to put aside individual interests and work together to develop requirements that effectively enhance overall safety. Over the past decade, regulatory authorities and standards development groups have worked to produce a considerable number of relevant standards for batteries and other vehicle electrification components. Despite these pioneering efforts, there remain a number of issues that must be addressed to ensure that the requirements in these standards are robust, consistent with other automotive requirements, and universally applicable regardless of the manufacturer.

In addition, the effectiveness of standards depends on the test methods used to verify that the end product meets the safety requirements as defined by the

Abuse tests are conducted to establish the reaction of cells, modules or batteries under conditions that exceed those expected in normal vehicular use. The information from these abuse tests can then be considered in the product design or design verification process. Abuse

testing is especially important for lithium-ion battery products, since they are still relatively new in automotive applications and do not yet have an established track record of safe operation over many years of service and under a variety of conditions.

“Effectiveness of standards depends on the test methods used to verify that the end product meets the safety requirements as defined by the standards.”

standards. In this regard, third-party testing organizations like TÜV SÜD are on the front line of test method development. They are often the initial source of information about the limits and shortcomings of certain testing approaches, as well as ways in which test methods can be improved to properly assess the safety of automotive power systems.

4 Designing Meaningful Lithium-ion Abuse Tests | TÜV SÜD

Approach to designing meaningful nail penetration test methods

One of the common abuse tests conducted at TÜV SÜD on lithium-ion batteries used in automotive applications is the nail penetration test, which is described in detail in Section 4.3.3 of the standard SAE J2464, EV & HEV Rechargeable Energy Storage System (RESS) Safety and Abuse Testing Procedure. However, the nail penetration test specifications described in the standard leave the appropriate testing methodology open to interpretation, especially for large ampere-hour and pouch cells.

“The nail penetration test specifications described in the standard leave the appropriate testing methodology open to interpretation, especially for large ampere-hour and pouch cells.”

As part of the effort to actively support the electric vehicle industry, TÜV SÜD has worked continuously since 2010 to improve the cell penetration test through improvements in the test methodology. After conducting abuse tests on more than 680 large format lithium-ion cells using a standard methodology, TÜV SÜD now has a robust and repeatable process to support rapid cell development and verification testing.

Parameter Measured Value Value

Nail Nail material Mild steel

Nail diameter 3 mm (no tolerance applied)

Nail point taper No values for length, included angle or surface finish

Surface finish No value applied

Nail straightness No value applied

Nail orientation Perpendicular to cell electrodes

Penetration Rate of penetration ≥ 8 cm/second; measured during the test

Depth of penetration Through cell

Constraints Preload or holding fixture None specified

Supporting scheme None specified

Electrical Resistance of path from DUT to ground No value specified

Table 1 lists the parameters for the nail penetration test for battery cells, as described in SAE J2464. Other values are used for modules and packs.

TABLE 1: SAE J2464 PENETRATION TEST PARAMETERS, BATTERY CELLS

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In an effort to resolve the discrepancies between TÜV SÜD test data and that of the supplier, a number of the characteristics listed in Table 1 as “no value” were assigned values based on other successful tests. The so-called “no value” characteristics that were assigned initial values included 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 test method refinements, additional cells were tested from

selected anode and cathode chemistries and capacities. At the same time, the rate of penetration in open air was reduced from 100 cm/sec to the minimum 8 cm/sec for consistency with the cell supplier’s nail velocity. By implementing these test method changes at both the TÜV SÜD lab and at the selected cell supplier’s lab, greater consistency was achieved between the cell supplier’s data and that of TÜV SÜD.

TÜV SÜD | Designing Meaningful Lithium-ion Abuse Tests

Cell testing experiments

TÜV SÜD has primarily tested hard case and soft prismatic (pouch) cells for nail penetration. The first device used by TÜV SÜD for the penetration test was an air driven cylinder (see Figure 1) 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 the relatively low velocity value compared to actual road speeds is due to the fact that bare cells installed in a vehicle are enclosed in a pack structure that protects the cells from flying objects.)

The purpose of performing this test at the low velocity value on unprotected and vulnerable cells is to gauge the reactivity of the cell on a relative basis. With this test information, a designer can then assess the relative safety of the cell technology, and determine what needs to be considered in the battery pack design to protect the cell from the potentially higher abuse loads in a vehicle. Since the force produced by the air cylinder was significant at 1.5 kN, there was little chance that a nail penetrating a cell as thick as 12 mm would slow to less than 8 cm/sec. However, despite testing a substantial number of cells in this manner, the results did not correlate with penetration data provided by the cell supplier.

Figure 1: Pneumatic penetration table with open bottom  

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Module testing experiments

In planning for penetration testing on thicker and multiple stacked lithium-ion cells of the kind likely to found in modules, the air system for driving the nail was replaced with a hydraulically driven system (see

Figure 2). The hydraulic system was capable of delivering 45 kN of force, and provided precise control of speed, acceleration and depth of penetration. This advanced system was then used to test a large number of cells, but

discrepancies emerged in the results generated by the hydraulic system and the air system operating under apparently identical parameters.

 Figure 2: Hydraulic penetration fixture with closed bottom

7TÜV SÜD | Designing Meaningful Lithium-ion Abuse Tests

Differences in methodologies

Efforts were then made to determine the key differences in the methodologies. The investigation included an analysis of nail geometry, ambient vibrations, test temperatures at the cell locale including supporting platen and chamber, electrical isolation and ground path of the device under test (DUT) to platen, DUT mounting features, path of the nail (misalignment and potential of dragging through the DUT), and test operator training. As a result of this investigation, the identifiable differences were either eliminated or accepted as having minimal influence. However, despite these test homologation efforts, results from cells penetrated with the hydraulic system were still markedly different from those generated by the air drive system.

A subsequent analysis was conducted to verify the velocities of the air system and the hydraulic system. The analysis included measurements of the integral linear variable differential transformer (LVDT) output of the hydraulic cylinder, a linear potentiometer on air and hydraulic systems, an independent LVDT and a light sensitive speed trap. These separate measurements produced consistent velocity results.

The linear string potentiometer was also used to assess any “slowing down” effect from the resistance of the cell on the air system. This assessment revealed that, from

the initial 8cm/sec setting, actual speed was reduced to 5 cm/sec as the nail partially penetrated the cell thickness. In order to elevate the actual value observed to the > 8 cm/sec specification, the initial speed of the air cylinder system would have to be raised in an iterative process on numerous real cells, which were unavailable in quantities sufficient to complete the assessment.

This finding validates the decision to switch from the air cylinder system to the hydraulic system, because of its negligible sensitivity to cell thickness compared with the “through the air” velocity of the air cylinder system. It demonstrates that a hydraulically-driven system will produce more consistent results for nail penetration tests on pouch cells. In addition,

this and other testing conducted by TÜV SÜD indicate that penetration testing performed at less than 8 cm/sec may produce misleading results. Therefore, it is critical for cell developers to evaluate test outcome sensitivity to nail speed.

It is worth noting that most professionally-designed battery packs will have little open volume inside, thus reacting more favorably to nail penetration testing. However, similar testing conducted by another TÜV SÜD battery testing lab in Garching, Germany has raised the possibility that the cell anode may release graphite or carbon powder under some conditions, and exacerbate the overall risk level.

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Other findings from abuse tests

Another important aspect of battery abuse testing is the actual response of a cell under test to nail penetration. Typically, cell response is assigned a hazard severity level (HSL) from 0 to 7, with 0 representing a battery

that was not affected by the test and which experienced no loss of functionality, and 7 representing a battery that exploded under test conditions, potentially resulting in flying debris and possible damage to

adjacent areas. An HSL level of less than 3 represents instances in which cell response is limited to loss of battery functionality but no leakage or venting. Table 2 provides a complete description of each of the 8 HSL levels.

HSL Description Classification criteria and effect

0 No effect No effect. No loss of functionality.

1 Passive Protection Activated No damage or hazard; reversible loss of function. Replacement or resetting of protection device is sufficient to restore normal functionality.

2 Defect/Damage No hazard but damage to RESS; irreversible loss of function. Replacement or repair needed.

3 Minor Leakage/Venting Evidence of cell leakage or venting with RESS weight loss <50% of electrolyte weight.

4 Major leakage/Venting Evidence of cell leakage or venting with RESS weight loss > 50% of electrolyte weight.

5 Rupture Loss of mechanical integrity of the RESS container, resulting in release of contents. The kinetic energy of released materials is not sufficient to cause physical damage external to the RESS.

6 Fire or Flame Ignition and sustained combustion of flammable gas or liquid (approximately more than one second). Sparks are not flames.

7 Explosion Very fast release of energy sufficient to cause pressure waves and/or projectiles that may cause considerable structural and/or bodily damage, depending on the size of the RESS. The kinetic energy of flying debris from the RESS may be sufficient to cause damage as well.

TABLE 2: EUCAR HAZARD SEVERITY LEVELS (HSL)

9TÜV SÜD | Designing Meaningful Lithium-ion Abuse Tests

FIGURE 3: HSL TREND OVER 3 YEARS

In addition to identifying improvements in the abuse testing method for lithium-ion batteries, testing at TÜV SÜD over the past three years has also resulted in the collection of important HSL data on batteries subject to abuse testing. An evaluation of this data reveals the following preliminary findings:

Lithium-ion battery cells are generally exhibiting increased resistance to abuse. This is evidenced by the increasing number of cells over the past three years that exhibit HSL scores of less than or equal to 3 when subjected to abuse testing.

Batteries with a state of charge (SOC) of less than 70% are more

likely to exhibit HSL scores of less than 3 under abuse test conditions. This finding suggests an opportunity for battery manufacturers to reduce risk from penetration with batteries with high (i.e., 90-100%) SOC percentages.

Under abuse testing conditions, nail velocity of 100 cm/sec tends to result in HSL level 3 or higher cell reactions. This finding points to the continued importance of battery pack design sufficient to protect the cell from high velocity projectiles under actual operating conditions.

These findings point to significant recent improvements in the safety performance of lithium-ion batteries under actual use conditions. At the

same time, they identify specific areas where manufacturers can conduct further research that will lead to future improvements in the overall safety of lithium-ion batteries.

“These findings point to significant recent improvements in the safety performance of lithium-ion batteries under actual use conditions.””

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Chart shows the trend based on 182 penetration test cells with HSL ≤ 3

15-40 Ah, restrained, standard test specification.

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Conclusion: The need for more exact specifications

If each and every cell and battery manufacturer and testing provider developed their own abuse testing methodology, the cost and extended development timelines would be onerous. That’s why the testing experiments described in this white paper underscore the importance of evaluating and refining as necessary the recommended testing practices

in a given specification. Hopefully, the testing undertaken by TÜV SÜD and presented in this white paper will benefit those engaged in similar test process improvements.

In the meantime, the question of why cells react as they do to this kind of abuse test remains unanswered. TÜV SÜD’s experienced materials science

professionals in Canada, Germany and China are continuing their research in this area, and are available to assist cell component manufacturers, cell manufacturers, battery assemblers, original equipment manufacturers and others in battery engineering and post mortem analysis.

COPYRIGHT NOTICE

The information contained in this document represents the current view of TÜV SÜD on the issues discussed as of the date of publication. Because TÜV SÜD must respond to changing market conditions, it should not be interpreted to be a commitment on the part of TÜV SÜD, and TÜV SÜD cannot guarantee the accuracy of any information presented after the date of publication. This White Paper is for informational purposes only. TÜV SÜD makes no warranties, express, implied or statutory, as to the information in this document. Complying with all applicable copyright laws is the responsibility of the user. Without limiting the rights under copyright, no part of this document may be reproduced, stored in or introduced into a retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise), or for any purpose, without the express written permission of TÜV SÜD. TÜV SÜD may have patents, patent applications, trademarks, copyrights, or other intellectual property rights covering subject matter in this document. Except as expressly provided in any written license agreement from TÜV SÜD, the furnishing of this document does not give you any license to these patents, trademarks, copyrights, or other intellectual property. ANY REPRODUCTION, ADAPTATION OR TRANSLATION OF THIS DOCUMENT WITHOUT PRIOR WRITTEN PERMISSION IS PROHIBITED, EXCEPT AS ALLOWED UNDER THE COPYRIGHT LAWS. © TÜV SÜD Group – 2013 – All rights reserved - TÜV SÜD is a registered trademark of TÜV SÜD Group.

DISCLAIMER

All reasonable measures have been taken to ensure the quality, reliability, and accuracy of the information in the content. However, TÜV SÜD is not responsible for the third-party content contained in this newsletter. TÜV SÜD makes no warranties or representations, expressed or implied, as to the accuracy or completeness of information contained in this newsletter. This newsletter is intended to provide general information on a particular subject or subjects and is not an exhaustive treatment of such subject(s). Accordingly, the information in this newsletter is not intended to constitute consulting or professional advice or services. If you are seeking advice on any matters relating to information in this newsletter, you should – where appropriate – contact us directly with your specific query or seek advice from qualified professional people. The information contained in this newsletter may not be copied, quoted, or referred to in any other publication or materials without the prior written consent of TÜV SÜD. All rights reserved © 2013 TÜV SÜD.

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GLOSSARY OF ACRONYMS DUT – Device under test LVDT – Linear variable differential transformer HSL – Hazard severity level RESS – Rechargeable energy storage system SOC – State of charge

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