advanced simulation technologies
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Different types of battery cell abuse
can lead to a condition known as
‘thermal runaway’. This is where
the temperature increase within a
battery cell triggers a chain of self-accelerating
exothermic chemical reactions,
leading to a sudden release
of energy and the venting of highly
toxic and flammable gas. When the
thermal runaway propagates to the
neighboring cells and modules, the
entire energy stored in the battery
pack can be released, which can
lead to fire and explosions. That’s
why it is fundamental that thermal
runaway is contained to prevent
propagation from one cell to the
next – and new safety requirements
demand this level of protection in
batteries in electric vehicles.
Currently almost all OEMs and
battery manufacturers are facing
the same problem: how to ensure
they meet and surpass the new regulations
regarding protection and
safety measures in case of thermal
runaway. From this year onwards
Global Transportation Regulations
will become mandatory in almost all
countries around the globe. OEMs
will need to prove that passengers
will have enough time to safely escape
from a vehicle which is experiencing
thermal runaway in a single
battery cell. In numbers, that means
that there must be at least a safe time
period of five minutes between
warning the driver (and therefore
the detection of a safety critical
event) and visible flames and fire
outside the battery.
To realize this goal, different countermeasure
concepts must be investigated
to determine which are the
most sensitive and effective. However,
exploring any new technical
concept can be costly and time
consuming. And so, to cut time and
costs in development, at AVL we
have developed our own methodology
for the simulation of thermal
runaway events and venting behavior
in a pack, allowing fast and easy
analysis of these phenomena.
The AVL approach conducts tests
to characterize a single cell in our
self-developed test chamber. The
cell’s behavior is then transferred
to and integrated into module and
pack-level simulation models for
the assessment of:
• Propagation time of thermal
runaway, between first cell and
neighboring cells
• Venting flow distribution and
thermal risk to other modules or
the pack sealing
• Melting of housing, cover or high
voltage isolations, which would
lead to electrical failures
• Distribution of toxic or flammable
gases within the pack
To achieve these simulations, we use our thermodynamic modelling tool
AVL FIRE™ M. This dynamic tool offers several unique features which
allow the accurate modelling of those harmful and extreme conditions.
Occurrences such as the melting of thin aluminum covers or plastic parts
can be investigated, and even the influence on the gas flow distribution
before and after the melting of the pack cover can be assessed.
This kind of simulation offers a wide range of benefits, some of which would
not be possible any other way. For example, insight into internal phenomena
during fire in a battery pack can be gained. This type of knowledge
cannot be obtained via post-mortem analysis as the pack is normally completely
destroyed. Additionally, this approach allows sensitivity analysis
of different countermeasure possibilities to be carried out, since experience
has shown that even small changes can have a significant impact on the
overall behavior in a case of thermal runaway. Furthermore, the simulation
allows the effectiveness of fire-retardant materials to be explored quickly
and cost-effectively.
Simulation allows development teams to investigate how thermal propagation
behavior changes depending on which cells are triggered inside the
pack. These specific kinds of explorations are otherwise only possible
with very expensive destructive tests. Thus simulation offers
benefits in time and costs for development engineers, and
increased safety for the passengers of electrified vehicles.
WHEN THE THERMAL
RUNAWAY PROPAGATES
TO THE NEIGHBORING
CELLS AND MODULES,
THE ENTIRE ENERGY
STORED IN THE BATTERY
PACK CAN BE RELEASED,
WHICH CAN LEAD TO
FIRE AND EXPLOSIONS.