IEEE Electrification - December 2021 - 34

consume vented flammable gases,
thus reducing a buildup of these
gases within the enclosure. In the
field, however, TR is more likely to be
initiated by an internal cell short,
where the cell's energy rapidly reduces
as the temperature rises and venting
occurs. This difference in cell
status at the point of venting raises
the question of whether there will
still be sparking and flames, and if
not, the resulting gas buildup will be
quite different. Furthermore, if flames
are suppressed by the discharge of a clean agent but TR is
allowed to propagate, there could be a considerable
increase in flammable gas concentration before a water
FSS is actuated. This situation should be seen in the UL
9540 A testing, but all possible conditions leading up to
such an event should be considered in a failure-modesand-effects
analysis.
On the other hand, LFP cells are more stable at higher
Managing vented
gases has become
one of the thorniest
issues in Li-ion
battery-enclosure
design.
hydrogen concentration would not
rise in those few seconds to multiples
of the LFL.
Additional challenges are presented
in gas detection due to the many
variables of a TR event. Examples of
these variables include the rate of gas
evolution, variability of gas species,
mixing within the enclosure, cross
contamination of sensors, and fouling
due to the by-products of combustion.
With the seeming impossibility of
NFPA 69 compliance in preventing
temperatures and less likely to exhibit propagating TR.
When venting, they may not produce the sparking and
flaming normally seen in NMC products, and individual
cells can produce very high amounts of hydrogen and
other flammable gases per cell. This is particularly the case
as LFP cells are often manufactured with higher amperehour
capacities because of their lower risk. The designs of
enclosures to address this explosion hazard must base
their solutions on accurate gas generation data from testing
to the most current edition of UL 9540 A. The resulting
enclosure designs will incorporate systems to either
address any explosion pressure waves, or ideally, prevent
an explosion in the first place.
Managing vented gases has become one of the thorniest
issues in Li-ion battery-enclosure design. The two
National Fire Protection Association documents that apply
here are NFPA 68, Standard on Explosion Protection by Deflagration
Venting and NFPA 69, Standard on Explosion Prevention
Systems. These documents are called out in NFPA 855
as a binary choice, but the situation is not that simple.
Obviously, it would be preferable to prevent an explosion
in the first place rather than having an explosion occur
and managing it with deflagration venting. However, it
would be virtually impossible for an enclosure designer to
achieve compliance under NFPA 69 because that standard
does not allow for flammable gas accumulation above 25%
of the lower flammable limit (LFL) (60% of the LFL is also
allowed under certain conditions). The LFL for hydrogen is
4%, so NFPA 69 compliance would require that the concentration
of that gas never exceed 1% (or 2.4%). Cell venting
is quite violent, with the possibility of hundreds of
liters of gas (depending on the cell size) being released in
just a few seconds. With the need to design for the highest-possible
energy density, there is little free space in
today's battery enclosures, and it is inconceivable that the
34
IEEE Electrification Magazine / DECEMBER 2021
explosions, does this mean that system designers should
ignore prevention and simply plan on managing explosions?
It is our opinion that this is a false choice and that
system designers should do what they can to meet the
intent of NFPA 69 by providing a means for rapid dilution
and exhaust of vented gas and, where space allows, to
also meet the letter of NFPA 68 by installing an appropriate
level of deflagration venting. An amendment to NFPA
855 addressed this issue, including correcting a gap for
ESS cabinets (where all the components are accessed from
exterior doors). The amendment provides for explosioncontrol
solutions in lieu of NFPA 68 or 69, in ESS cabinets
where fire testing demonstrates an absence of shrapnel,
projectiles, or pressure waves.
Beyond Li-ion
Battery technologies for ESS are not limited to Li-ion, particularly
considering the future need for technologies
capable of providing low-cost storage for discharges of
longer than 4 h. Many new technologies have been proposed,
each promising lower cost at scale than Li-ion,
often with better cycle life or some other advantage.
Although there may indeed be a " Li-ion killer " out there, it
can be extremely difficult to determine which claims are
real and which are projected from early test results. It is
not unusual for a new technology to show very promising
results with small button cells in a lab, only to find that
the technology cannot be made to work in larger formats,
that manufacturing cannot be scaled up as expected, or
that unexpected failure modes show up, causing the new
batteries to fail prematurely.
Li-based batteries are far from immune from this sort of
hype. New Li-ion active materials are proposed with some
frequency, promising to be safer, cheaper, more energy
dense, or able to be recharged in scant minutes-and often
with all four features at the same time. Furthermore, there
is a new class of batteries using solid-state electrolyte and
metallic Li negatives, which promises all of these improvements.
Technologies of this type have been around since
the 1990s, although not in a form acceptable for mass-produced
electric vehicles, and now enormous sums are being
spent by auto companies and venture capitalists to accelerate
their development and industrialization. Although it
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