Underwriters Laboratories (UL), a world leader in safety testing and certification released the first in a series of white papers that review evolving battery technology.
This paper explores many of the issues and opportunities associated with the new technology as well as current and recommended safety standards to address changes in the technology and use.
Lithium-ion battery technologies have evolved over the last two decades, with batteries now offering longer cycle life and improved reliability for products in the areas of consumer electronics, medical devices, industrial equipment and automotive applications. In the white paper, UL explains the need for risk assessment as part of the product design and development process to identify and address root causes of safety issues.
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Safety Issues for Lithium-Ion Batteries
1. 01 For more information E:: ULUniversity@us.ul.com / T:: 888.503.5536
Safety Issues for Lithium-Ion Batteries
Lithium-ion batteries are widely used as a power source in
portable electrical and electronic products. While the rate
of failures associated with their use is small, several well-
publicized incidents related to lithium-ion batteries in actual
use (including fires and explosions) have raised concerns about
their overall safety. Test standards are in place that mandate a
number of individual tests designed to assess specific safety
risks associated with the use of lithium-ion batteries. However,
Underwriters Laboratories and other standards development
organizations are continuing to revise and update existing lithium
battery standards to reflect new knowledge regarding lithium ion
battery failures in the field. These organizations are contributing
to battery safety research with a focus on internal short circuit
failures in lithium ion batteries. The research is directed toward
improving safety standards for lithium ion batteries.
Overview
Over the past 20 years, rechargeable (also known as secondary)
lithium-ion battery technologies have evolved, providing increasingly
greater energy density, greater energy per volume, longer cycle
life, and improved reliability. Commercial lithium-ion batteries now
power a wide range of electrical and electronic devices, including the
following categories:
Consumer electrical and electronic devices• — Lithium-
ion batteries power consumer electrical and electronic devices
from mobile phones and digital cameras, to laptop computers.
Medical devices• — Lithium-ion batteries are also used in
medical diagnostic equipment, including patient monitors,
handheld surgical tools, and portable diagnostic equipment.
Industrial equipment• — Industrial equipment offers a
wide range of applications for lithium-ion batteries, including
cordless power tools, telecommunications systems, wireless
security systems, and outdoor portable electronic equipment.
Automotive applications• — A new generation of electric
vehicles is being powered by large format lithium-ion battery
packs, including battery-electric vehicles, hybrid-electric vehicles,
plug-in hybrid-electric vehicles, and light-electric vehicles.
WHITE PAPER ON:
1
“Lithium Batteries: Markets and Materials,” Report FCB028E, October 2009, www.bccresearch.com.
UL is involved in standards development worldwide and has technical staff
participating in leadership and expert roles on several national committees
and maintenance teams associated with battery and fuel cell technologies.
Ms. Laurie Florence is the convener (chair) of SC21A — Working Group 5.
She is a member of the SC21A US TAG & SC21A WGs 2, 3, 4, and 5. Florence
is a member of TC 35 US TAG & TC 35 MT 15 and a member of the ANSI
NEMA C18 committees. She participated in the IEEE 1625 and is participating
in IEEE 1725 revisions. Florence is also on the task group working on revising
UN T6 and participated in CTIA battery ad hoc committee.
Mr. Harry P. Jones is the convener (chair) of IEC TC105 — Working Group 8.
Florence and Jones participated in SAE J2464 and SAE J2929
standards development.
Florence and Mr. Alex Liang (UL Taiwan) and are on ETF 13 for batteries.
UL Taiwan was the first CBTL for IEC 62133, followed by UL Suzhou.
UL Japan is approved to provide PSE mark in Japan for lithium ion batteries
as part of the DENAN program.
UL is a CTIA CATL for the battery certification program.
2. 02 For more information E:: ULUniversity@us.ul.com / T:: 888.503.5536
The worldwide market for lithium batteries is projected to reach
nearly $10 billion (USD) in annual sales by 2014, with the market
for lithium-ion batteries representing almost 86-percent of those
sales ($8.6 billion)1
. However, as the use of lithium batteries is
growing globally and with the large number of batteries powering
a wide range of products in a variety of usage environments, there
have been several reported incidents raising safety concerns.
While the overall rate of failures associated with the use of lithium-
ion batteries is very low when compared with the total number of
batteries in use worldwide, several publicized examples involving
consumer electronics like laptop computers and electronic toys
have led to numerous product safety recalls by manufacturers, the
U.S. Consumer Product Safety Commission and others. Some of
these cases have been linked to overheating of lithium-ion batteries,
leading to possible fire or explosion.
Though global independent standards organizations, such as
the International Electrotechnical Commission and Underwriters
Laboratories, have developed a number of standards for electrical
and safety testing intended to address a range of possible abuses
of lithium-ion batteries, knowledge about potential failure modes
is still growing as this complex technology continues to evolve to
meet marketplace demands. Understanding and translating this
knowledge into effective safety standards is the key focus of UL
battery research activities and is intended to support the continual
safe public usage and handling of lithium-ion batteries.
Lithium-ion battery design and selection considerations
A lithium-ion battery is an energy storage device in which lithium ions
move through an electrolyte from the negative electrode (“anode”) to
the positive electrode (“cathode”) during battery discharge, and from
the positive electrode to the negative electrode during charging. The
electrochemically active materials in lithium-ion batteries are typically
a lithium metal oxide for the cathode and a lithiated carbon for the
anode. The electrolytes can be liquid, gel, polymer or ceramic. For
liquid electrolytes, a thin (on the order of microns) micro-porous film
provides electrical isolation between the cathode and anode, while still
allowing for ionic conductivity. Variations on the basic lithium chemistry
also exist to address various performance and safety issues.
The widespread commercial use of lithium-ion batteries began in
the 1990s. Since then, an assortment of lithium-ion designs has
been developed to meet the wide array of product demands. The
choice of battery in an application is usually driven by a number of
considerations, including the application requirements for power and
energy, the anticipated environment in which the battery-powered
product will be used and battery cost. Other considerations in
choosing a suitable battery may include:
Anticipated work cycle of the product (continual or intermittent)•
Battery life required by the application•
Battery’s physical characteristics (i.e., size, shape, weight, etc.)•
Maintenance and end-of-life considerations•
Lithium-ion batteries are generally more expensive than alternative
battery chemistries but they offer significant advantages, such as
high energy, density levels and low weight-to-volume ratios.
Causes of safety risk associated with lithium-ion batteries
Battery manufacturers and manufacturers of battery-powered products
design products to deliver specified performance characteristics in a
safe manner under anticipated usage conditions. As such, failure (in
either performance or safety) can be caused by poor execution of a
design, or an unanticipated use or abuse of a product.
Passive safeguards for single-cell batteries and active safeguards
for multi-cell batteries (such as those used in electric vehicles)
have been designed to mitigate or prevent some failures. However,
major challenges in performance and safety still exist, including
the thermal stability of active materials within the battery at high
temperatures and the occurrence of internal short circuits that may
lead to thermal runaway.
Photo 1: Examples of lithium-ion batteries for PCs and smart phones
2
“FTA/FMEA Safety Analysis Model for Lithium-Ion Batteries,” UL presentation at 2009 NASA Aerospace Battery Workshop.
3. 03 For more information E:: ULUniversity@us.ul.com / T:: 888.503.5536
As part of the product development process, manufacturers should
conduct a risk assessment that might involve tools such as failure
modes and effects analysis and fault tree analysis. UL employs these
tools to generate root cause analyses that lead to the definition of
safety tests for product safety standards2
.
Applicable product safety standards and testing protocols
To address some of the safety risks associated with the use of
lithium-ion batteries, a number of standards and testing protocols
have been developed to provide manufacturers with guidance on
how to more safely construct and use lithium-ion batteries.
Product safety standards are typically developed through a
consensus process, which relies on participation by representatives
from regulatory bodies, manufacturers, industry groups, consumer
advocacy organizations, insurance companies and other key safety
stakeholders. The technical committees developing requirements
for product safety standards rely less on prescriptive requirements
and more on performance tests simulating reasonable situations that
may cause a defective product to react.
The following standards and testing protocols are currently used
to assess some of the safety aspects of primary and secondary
lithium batteries:
Underwriters Laboratories
UL 1642: Lithium Batteries•
UL 1973: (Proposed) Batteries for Use in Light Electric Rail•
(LER) Applications and Stationary Applications
UL 2054: Household and Commercial Batteries•
UL Subject 2271: Batteries For Use in Light Electric•
Vehicle Applications
UL 2575: Lithium Ion Battery Systems for Use in Electric Power•
Tool and Motor Operated, Heating and Lighting Appliances
UL Subject 2580: Batteries For Use in Electric Vehicles•
Institute of Electrical and Electronics Engineers
IEEE 1625: Rechargeable Batteries for Multi-Cell Mobile•
Computing Devices
IEEE 1725: Rechargeable Batteries for Cellular Telephones•
National Electrical Manufacturers Association
C18.2M: Part 2, Portable Rechargeable Cells and Batteries —•
Safety Standard
Society of Automotive Engineers
J2464: Electric and Hybrid Electric Vehicle Rechargeable•
Energy Storage Systems (RESS), Safety and Abuse Testing
J2929: Electric and Hybrid Vehicle Propulsion Battery System•
Safety Standard — Lithium-based Rechargeable Cells
International Electrotechnical Commission
IEC 62133: Secondary Cells and Batteries Containing Alkaline•
or Other Non-acid Electrolytes — Safety Requirements for
Portable Sealed Secondary Cells, and for Batteries Made from
Them, for Use in Portable Applications
IEC 62281: Safety of Primary and Secondary Lithium Cells and•
Batteries During Transportation
United Nations (UN)
Recommendations on the Transport of Dangerous Goods,•
Manual of Tests and Criteria, Part III, Section 38.3
Japanese Standards Association
JIS C8714: Safety Tests for Portable Lithium Ion Secondary•
Cells and Batteries For Use In Portable Electronic Applications
Battery Safety Organisation
BATSO 01: (Proposed) Manual for Evaluation of Energy Systems•
for Light Electric Vehicle (LEV) — Secondary Lithium Batteries
Common product safety tests for lithium-ion batteries
The above standards and testing protocols incorporate a number of
product safety tests designed to assess a battery’s ability to withstand
certain types of abuse. Table 1 provides an overview of the various
abuse tests and illustrates the extent to which safety standards and
testing protocols for lithium-ion batteries have been harmonized.
It is important to note that similarly named test procedures in various
documents might not be executed in a strictly identical manner. For
example, there may be variations between documents regarding the
number of samples required for a specific test, or the state of sample
charge prior to testing.
The most common product safety tests for lithium-ion batteries are
typically intended to assess specific risk from electrical, mechanical
and environmental conditions. With minor exceptions, all of the
above mentioned standards and testing protocols incorporate these
common abuse tests. The following sections describe individual
common tests in greater detail.
4. 04 For more information E:: ULUniversity@us.ul.com / T:: 888.503.5536
UL IEC NEMA SAE UN IEEE JIS BATSO
TestCriteria/Standard UL1642 UL2054 UL
Subject
2271
UL
Subject
2580
UL2575 IEC
62133
IEC
62281
C18.2M,
Pt2
J2464 Pt.III,S
38.3
IEEE
1625
IEEE
1725
JIS
C8714
BATSO
01
External short circuit • • • • • • • • • • • • • •
Abnormal charge • • • • • • • • • • • • • •
Forced discharge • • • • • • • • • • • • •
Crush • • • • • • • • • • • •
Impact • • • • • • • • •
Shock • • • • • • • • • • • • • •
Vibration • • • • • • • • • • • • • •
Heating • • • • • • • • • • •
Temperature cycling • • • • • • • • • • • • • •
Low pressure (altitude) • • • • • • • • • • • •
Projectile • • • • • •
Drop • • • • • • •
Continuous low rate
charging
• •
Molded casing heating test •
Open circuit voltage •
Insulation resistance • •
Reverse charge • •
Penetration • • •
Internal short circuit test • • •
Safety standards and testing protocols for lithium-ion cells
Table 1: Summary of abuse tests found in international safety standards and testing protocols for lithium-ion batteries3
3
Jones, H., et al., “Critical Review of Commercial Secondary Lithium-Ion Battery Safety Standards,” UL presentation at 4th IAASS Conference,
Making Safety Matter, May 2010.
5. 05 For more information E:: ULUniversity@us.ul.com / T:: 888.503.5536
Electrical tests
External short circuit test• — The external short circuit test
creates a direct connection between the anode and cathode
terminals of a cell to determine its ability to withstand a
maximum current flow condition without causing an explosion
or fire.
Abnormal charging test• — The abnormal charging test
applies an over-charging current rate and charging time to
determine whether a sample cell can withstand the condition
without causing an explosion or fire.
Forced discharge test• — The forced discharge test
determines a battery’s behavior when a discharged cell is
connected in series with a specified number of charged cells
of the same type. The goal is to create an imbalanced series
connected pack, which is then short-circuited. To pass this
test, no cell may explode or catch fire. (This test is not required
under BATSO 01.)
Mechanical tests
Crush test• — The crush test determines a cell’s ability to
withstand a specified crushing force (typically 12 kN) applied by
two flat plates (typically although some crush methods such as
SAE J2464 include a steel rod crush for cells and ribbed platen
for batteries). To pass this test, a cell may not explode or ignite.
(This test is not required under IEC 62281 or UN 38.3.)
Impact test• — The impact test determines a cell’s ability
to withstand a specified impact applied to a cylindrical steel
rod placed across the cell under test. To pass this test, a cell
may not explode or ignite. (This test is not required under SAE
J2464, JIS C8714, or BATSO 01.)
Shock test• — The shock test is conducted by securing a
cell under test to a testing machine that has been calibrated
to apply a specified average and peak acceleration for the
specified duration of the test. To pass this test, a cell may not
explode, ignite, leak or vent.
Vibration test• — The vibration test applies a simple harmonic
motion at specified amplitude, with variable frequency and time
to each cell sample. To pass this test, the cell may not explode,
ignite, leak or vent.
Environmental tests
Heating test• — The heating test evaluates a cell’s ability to
withstand a specified application of an elevated temperature for a
period of time. To pass this test, the cell may not explode or ignite.
(This test is not required under IEC 62281, UN 38.3 or BATSO 01.)
Temperature cycling test• — The temperature cycling test
subjects each cell sample to specified temperature ranges
above and below room temperature for a specified number of
cycles. To pass this test, the cell may not explode, ignite, vent
or leak.
Low pressure (altitude) test• — The low-pressure test
evaluates a cell sample for its ability to withstand exposure
to less than standard atmospheric pressure (such that in an
aircraft cabin that experiences sudden loss of pressure). To
pass this test, the cell may not explode, ignite, vent or leak.
(This test is not required under UL 2054.)
Photo 2: Lithium-ion batteries under test
6. 06 For more information E:: ULUniversity@us.ul.com / T:: 888.503.5536
Additional specialized tests
In addition to the common abuse tests discussed above, certain
product safety standards and testing protocols for lithium-ion
batteries require additional specialized testing. These specialized
tests address specific uses and conditions in which the batteries
might be expected to operate.
Projectile (fire) test• — The projectile test is required under UL
1642. UL 2054, UL Subject 2271, IEEE 1625 and IEEE 1725. The
test subjects a cell sample to a flame from a test burner, while
positioned within a specified enclosure composed of wire mesh
and structural support. If the application of the flame results in
an explosion or ignition of the cell, no part of the cell sample may
penetrate or protrude through the wire mesh enclosure.
Drop test• — The drop test is required under UL Subject 2271,
UL Subject 2580, IEC 62133, IEC 62281, NEMA C18.2M Part 2,
JIS C8714 and BATSO 01. The test subjects each cell or battery
sample to a specified number of free falls to a hard surface. The
cell sample is examined after a time following each drop. To
pass this test, the cell may not explode or ignite.
Continuous Low Rate Charging Test• — The continuous
low rate charging test is required under IEC 62133. This
test subjects fully-charged cell samples to a long-term,
uninterrupted charge at a rate specified by a manufacturer. To
pass this test, the cell may not explode, ignite, vent or leak.
Mold stress test• — The molded casing-heating test is
required under NEMA C18.2M Part 2, UL 2054, IEC 62133 and
UL Subject 2271. The test exposes plastic-encased batteries
to a specified elevated temperature and for a specified time.
Once the battery has cooled to room temperature the cell is
examined. To pass this test, the internal cells may not show any
evidence of mechanical damage.
Insulation resistance test• — The insulation resistance test is
required under UL Subject 2580, IEC 62133 and NEMA C18.2M
Part 2 (conducted as a pretest condition only). The test subjects
a cell sample to a resistance measurement between each
battery terminal and the accessible metal parts of a battery
pack. To pass this test, the measured resistance must exceed
the specified minimum value.
Reverse charge test• — The reverse charge test is required
under IEC 62133, UL Subject 2271 and UL Subject 2580.
This test determines a discharged cell sample’s response
to a specified charging current applied in a reverse polarity
condition for a defined period of time. To pass this test, the cell
may not explode or ignite.
Penetration test• — The penetration test is required under
UL Subject 2271, UL Subject 2580 and SAE J2464. The test
uses a pointed metal rod to penetrate a cell and simultaneously
measures rod acceleration, cell deformation, cell temperature,
cell terminal voltage and resistance.
Internal short circuits: potential causes and testing issues
A review of lithium-ion battery safety research shows a strong focus
on internal short circuits. Some field failures resulting in fires or
explosions, leading to product damage or personnel injury, have
been linked to an internal short circuit within the lithium-ion battery.
However, as shown in Table 1, most lithium-ion battery safety
standards and testing protocols do not specifically include testing
for internal short circuits. In recent years, UL has partnered with key
battery research facilities such as Argonne National Laboratories
and the National Aeronautic and Space Administration to better
understand the root causes of internal short circuits. The focus of
our research has been on defining and developing safety tests that
assess the propensity of a battery to experience a short circuit under
certain abuse conditions.
Potential causes of internal short circuits
Although an internal short circuit may have many causes, it is
basically a pathway between the cathode and anode that allows for
efficient but unintended charge flow. This highly localized charge flow
results in joule heating due to internal resistance, with subsequent
heating of the active materials within the lithium-ion battery. The
increased heat may destabilize the active materials, in turn starting
a self-sustaining exothermic reaction. The subsequent heat and
pressure build-up within the cell may lead to catastrophic structural
failure of the battery casing and the risk of additional combustion as
a result of exposure to outside air.