The eMobility sector is going through its transformation phase. With
the increasing focus on electric vehicle from the public and private sector, every
player in the eMobility industry is working relentlessly on increasing the
performance of
the electric vehicle with
higher efficiency, larger capacity and reduced size. The only objective of all this research and development
is to make electric vehicle at par or even better than IC engines
at a lower cost.
High Voltage EV Batteries, being the most critical component of the electric
vehicle are the ones that are focused most for capacity enhancement,
performance optimization and Cost/size reduction. Researchers in the entire
value chain of EV Battery, Cell, Module and Pack level are constantly working
on fast charging and capacity enhancement projects.
Introduction:
Battery thermal management is one of the most critical aspect in the design
and development of EV Batteries for fast charging and capacity enhancement
projects. The crucial steps involved in battery thermal
management are first identifying the source of the heating, second localizing
the weak points in the design and then finally managing thermal issues either
with design changes or with better cooling mechanism. This article is mainly
focused on Benefits
of fiber optic sensor in core temperature monitoring of cylindrical cell.
Why Temperature
Monitoring:
Under the fast charging (3C, 4C and more) and discharging (6C, 8C or
more) cycles cylindrical cells face tremendous electrochemical and mechanical
stress. As a result of these continuous stress, Cells heat up internally and
heat gets transmitted to the outer surface in radial and axial directions. It
becomes very crucial to understand the stress handling capability of cells
under different operating conditions. The EV battery cells must be designed for
a wide range of ambient and automotive operating conditions. Identifying thermal
issues accurately, during the product development stage and mitigating them
effectively is the key for avoiding the huge cost of product recall.
Benefits of
Fiber Optic Temperature Sensors:
Engineers have been using very small thermocouples to measure the
thermal profiling of cylindrical cell core. In order to avoid damage to the
cell chemical due to the thermocouples, the sensors are coated with a
complex and expensive chemical isolation. The isolation process is complex and
still not a full proof solution for safe and accurate temperature measurement
of cell core. Therefore, Fiber
Optic Temperature Sensors are the most suitable alternative to
the thermocouples due to the following features of fiber optic sensors:
- Ultra-Small
footprint (0.4mm) to fit into Cell Core. This
will ensure minimal damage to the mechanical structure of the
cylindrical cell.
- Safety: Fiber optic sensors are made of silica, Polyimide, Gallium
Arsenide (GaAs) crystal and very small Epoxy. Any of the constituents of the complete
sensor does not pose any risk to the cell chemical.
- Accurate
and Noise Free readings – The sensors have an accuracy of ±0.2⁰C (relative) with 100% repeatability. And this
accuracy is not impacted by Strain / Pressure inside Cell.
- Wide
Measurement Range: The rage of measurement is
-269⁰C to +300⁰C.
- Higher
Response Time: The sensors are capable of measuring with 5Hz to 30Hz sampling rate.
- Sensors
Stability: These sensors are very stable under
high electrical, Magnetic and Chemical fields.
- Lower
cost of installation – The sensors do not need
any expensive Isolation / coating. The sensors are also adjustable to fit
at different locations inside the cell core.
How to install a
Fiber Optic Temperature Sensor in Cylindrical cell Core?
Fiber Optic Temperature sensors can be installed either
at the cell formation stage or afterwards. It is simple to fit the sensors
during the cell formation stage compared to the fitting sensor on a
manufactured cell core. Figure 1 below shows the detailed view of how the Fiber
Optic Temperature Sensor installed into Cylindrical Cell Core.
a)
Bare Fiber Optic Sensor b) Fiber Optic Sensor with Disposable
Tip
Figure.
1 Fiber Optic Sensor installed inside the EV Battery Cell
This is a simple approach where
Fiber Optic Temperature Sensor can be placed at the core of the cylindrical
cell while the assembly process of the cell, before the formation stage. This
will require to drill a hole in the cathode cap of the cell. After the formation
process, the cathode cap opening must be sealed with silicone sealant, epoxy or
Kapton tape.
2) Into Manufactured Cell
To install a Fiber Optic Temperature
sensor on the readymade Battery cell, requires the use of a drill and glove box. Firstly, it
will require to disassemble approximately 8 to 10 cells to find out the
internal structure of the cell type. Once the internal structure of the cell is
determined a new cell can be placed on the Glove Box for drilling hole. The Glove
box is used to prevent the exposure of cell internals to oxygen (O2)
and moisture (H2O). A sharp and high precision drill is used to
drill a small hole into the cell core. The hole must be as small as possible so that it does not
impact the cell electrochemical behavior. Care must be taken while drilling the
hole to avoid short circuit and protect
the electrode jellyroll.
The easiest option for drilling hole
is, open the cathode and drill hole on the plastic protection and insert the fiber optic sensor inside the
core. The opening must be sealed with special glue and tape to make it
hermetically sealed without damaging the Fiber Optic Temperature Sensor.
Disposable Caps (made of Polyimide
material) can also be used to fit into the hole first and then insert the fiber
optic temperature sensor, as shown in Figure 1 (b) above.
The fiber optic temperature sensors than
can be connected to the monitor for temperature measurement and trending. The
monitor has flexibility to record the temperature data, display time-stamped
trending and export data to third party systems. The monitor supports industry-standard
protocols i.e. High-Speed CANBUS, Modbus, DNP3.0 and comes with drivers for
major development environments i.e. Matlab, LabView and python.
Conclusion:
The Fiber Optic Temperature Sensor is the most suitable sensors to use inside the battery cell for Cell
Core temperature monitoring. The process of installing the fiber optic
temperature sensors is easier than the one used for traditional sensors because
fiber optic temperature sensors do not require any isolation. With higher
accuracy, repeatability and response of fiber optic temperature sensors, it has
become possible to understand better the chemical process inside and identify
the real causes of temperature increase. It was found from multiple experiments
that the Cell Core temperature is mostly higher than the cell
body temperature and the difference is not constant but varies with the
charging and discharging rate. The core temperature is maximum during the end
of charging and discharging. The difference between the core and cell body
temperature could be anywhere from 1⁰C up to 8⁰C.
Temperature monitoring of
the core cell becomes very critical for fast charging
applications. The outcomes of the Cell Core temperature monitoring are being
used for battery modelling, Battery Management System and thermal protection of
battery cell, module and the entire pack. The accurate Core Cell temperature
ensures that thermal safety limits are set correctly to avoid thermal runaway
issues.
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ReplyDeleteI found your blog very interesting to read. Thank you very much for sharing this information with us. Your blog helps me get information about the temperature sensor .
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