IEEE Electrification - December 2021 - 31

Li-ion Safety Issues
All those early installations of Li-ion
ESSs were made in the absence of
safety standards relevant to the scale
of these systems. At that time, the
standards applicable to portable batteries
typically covered safety at the
level of individual cells and small
packs, while the issues facing larger
systems were more concerned with
the propagation of thermal runaway
(TR) between modules and racks,
effectiveness of fire suppression, and
management of explosive gas buildup.
It is not the purpose of this article to describe the
details of these safety issues but to show how current
codes and standards are influencing Li-ion system design
and providing both the operators and authorities having
jurisdiction (AHJ) with a greater sense of confidence in the
deployment of these systems.
Safety Standards
Development Timeline
In those earlier ESS deployments, it was left to the battery
manufacturers and system integrators to ensure an
adequate level of safety. Inevitably, there have been missteps
along the way, with system designers not always
considering all the ways in which cell failure can influence
system-level safety. While the industry understanding
of holistic safety has been advancing, codes and
standards have been playing catch-up. The standards
from Underwriters Laboratories started to move beyond
the portable battery world in 2013, but it was not until
the last few years that they comprehensively addressed
complete battery-based ESSs, with the second edition of
UL 1973, Standard for Batteries for Use in Stationary, Vehicle
Auxiliary Power and Light Electric Rail (LER) Applications
(2018); the second edition of UL 9540, Energy Storage Systems
and Equipment (2020); and the fourth edition of UL
9540 A, Standard Test Method for Evaluating Thermal Runaway
Fire Propagation in Battery Energy Storage Systems
(2019). Fire codes have similarly been evolving, introducing
stringent spacing and energy limitations for indoor
systems beginning in 2018. The code requirements for
ESSs have been further developed in NFPA 855, Standard
for the Installation of Stationary Energy Storage Systems
(2020), which is already in the process of being updated,
supported by more data from the field.
One of the challenges for manufacturers is product
development in the midst of an evolving landscape of
installation and product safety standards. The key
model fire codes in the United States, the International
Fire Code and NFPA1 Fire Code, are amended on a threeyear
cycle. The technology is evolving more rapidly than
the codes cannot keep up with evolution of the
While the industry
understanding of
holistic safety
has been advancing,
codes and standards
have been playing
catch-up.
technology, which may unintendedly
create barriers to solutions not
addressed or impose restrictions
with limited justification. The changes
in product safety standards can
require recertification, which may
impose significant time delays and
cost. It is imperative to have broad
stakeholder input into the development
of these consensus codes and
standards to ensure that the best
possible language is achieved with
the most current data available.
Mind the (Safety) Gap
An example of just one of the challenges fire codes
attempt to address is in the area of fire suppression and
explosion control. The current requirements call for fire
sprinklers for many systems installed indoors (which
includes walk-in enclosures) and outdoors near exposures.
Historically, sensitive electronics have been well
protected by clean-agent suppression systems, such as
Novec 1230, FM200, or carbon dioxide. As these agents are
designed to either displace oxygen or rapidly cool the
environment, they are quite effective for visible flames,
but for certain Li-ion chemistries are ineffective in suppressing
deep-seated fires in modules due to exothermic
and oxide-releasing reactions during TR.
When a clean-agent system is designed, one requirement
is to maintain a certain concentration and duration
in the space. This requires that the area be sealed from
exterior air, and as a result may allow continued TR and
the release of highly flammable gas constituents, such as
hydrogen, carbon monoxide, and hydrocarbons. So, in
essence, the attempt to suppress a fire will often create an
explosion hazard. Continued research is needed in effective
suppression systems designed to address fires at the
module level. Manufacturers must consider the explosion
hazard and respond with mitigation designs.
Similarly, it is possible for water-based fire-suppression
systems (FSSs) to cause more problems than they solve.
Water has much greater cooling efficiency than clean
agents, but it must be supplied in sufficient volume within
and around battery modules to arrest the propagation of
TR, thus limiting cell venting and the release of flammable
gases. Ceiling-level sprinklers may not be effective at delivering
sufficient water if modules are tightly packed, and
this may favor designs with more directed water flow
across the tops of modules. Another potential issue is with
water-mist systems, which, depending on the ingress protection
rating of the battery modules, may deliver enough
water flow to create short circuits between cells but not
enough to remove the heat generated by those shorts. The
need for, and effectiveness, of FSSs with Li-ion batteries
can be demonstrated most effectively by large-scale testing
according to UL 9540 A, as discussed later in this article.
IEEE Electrification Magazine / DECEMBER 2021
31
IEEE Electrification - December 2021

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