Tech Briefs Magazine - June 2021 - ST-14

TECHNOLOGY LEADERS Sensors
Piezoresistive Force Sensors
maintain over the charge and discharge
cycles.
One method to characterize Li-ion
Battery Housing
Figure 4. Representative design of a battery prototype. Two piezoresistive sensors placed on either end of the
battery cell measure force feedback from the battery while in use. (Image courtesy of Tekscan)
battery swelling in charge/discharge
cycle testing is with a fixture that incorporates
a load cell to detect swelling.
However, because load cells can only
collect average force feedback across a
battery assembly, and not insights on
the position of excessive pressure
occurring in the battery itself, battery
designers would be missing out on
important data.
More recently, battery designers have
turned to pressure mapping systems to
collect comprehensive data during Liion
battery testing. The thin and flexible
array of sensing elements can wrap
around the battery to provide a 360°
view within a testing fixture. These systems
can be used to evaluate pressure
profiles over thousands of charge and
discharge cycles, which can last several
months.
Accomodating Voltage Output Change
of a Relative Measurememt Application
T
T∝
Original
Calibration
Figure 3 shows an example of charge/
discharge characterization data from a
fixture incorporating a piezoresistive
pressure mapping system.
Force
Figure 5. Because this battery safety application only required measuring relative force - not absolute force -
the engineers could easily accommodate for changes in voltage output caused by sensor drift. (Image courtesy
of Tekscan)
effect on long-term cell performance,
with higher levels of stress leading to
higher rates of capacity fade. Over
time, significant internal pressures can
lead to plastic deformation and delamination.
The mechanical expansion and
contraction from temperature changes
causes the anode/cathode layers to separate
over time. When those layers sep14
Cov
arate,
the impedance of the battery
goes up, derating its capacity. On the
other hand, while lower internal pressures
may provide better long-term performance,
there may be too much
movement from thermal expansion
within the battery housing. Therefore,
battery designers must find a " sweetspot "
for housing pressure that can
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ToC
Insights from R&D Initiate an Embedded
Battery Safety Feature
Recently, a team of engineers designing
a new laptop with a high-efficiency
Li-ion battery found a unique application
inspired by their R&D efforts. After
characterizing the battery performance
using a fixture similar to the one shown
in Figure 3, the team had an idea to
embed the same piezoresistive sensing
technology into their laptop to serve as a
method to monitor abnormal battery
swelling while in use.
To begin, the design team purchased
sample piezoresistive force sensors,
specifically selecting a pressure-sensitive
ink variety formulated for high-temperature
and high-humidity operating environments.
Because piezoresistive sensors
are so thin - only 0.203 mm (0.008 in.)
- and can function with low-power electronics,
the team did not need to make
any significant adjustments to their
design.
For their prototype, the team positioned
two sensors on either end of
the battery compartment to detect
localized changes in force, as shown in
Figure 4. Based on the characterizaSensor
Technology, June 2021
Relative Voltage Output
The Pressure Within the Battery
Housing is the Same at Both Points

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Tech Briefs Magazine - June 2021

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