Battery cell separating layer with integrated dilation sensors

As part of the SPARTACUS project, dielectric elastomer sensors from Fraunhofer ISC were adapted to the specific requirements for detecting volume changes in battery cells. The elastomer sensors perform several essential functions in one material:

1. Detection of cell expansion: As capacitive deformation sensors, they measure very accurately the dynamic volume changes of the cells during charging and discharging. In addition, they also detect the irreversible and permanent volume change due to cell aging.
2. Mechanical decoupling: As elastomers with readily adjustable compression modulus, they separate the cells in a battery module from one another and, as a continous damping layer, ensure a more uniform pressure distribution to the individual cells. Pressure peaks that accelerate cell aging can thus be avoided. They also provide thermal shielding, which can be further optimized depending on the application.

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Specifications deformation sensor

Detection of mechanical cell properties by ultrasound

Ultrasound is used for structural monitoring and thickness measurement of components. For continuous monitoring of the state of charge and the chemical (ageing) processes inside a battery, the interaction of the ultrasonic waves with differently dense and hard individual layers (lithium de-/intercalation in electrodes) can be used.
The principle requirements for the ultrasonic transducer on the measuring foils within a battery pack are:

• Small size (max. 20 mm in diameter, small thickness (max. 1-2 mm))
• Simple and inexpensive components and construction
• Compatible with processes for mounting additional sensors
• Wide frequency range (100 kHz – 10 MHz)

Specifications ultra sonic sensors (piezo)

Advanced battery modeling

In the context of ultrasound, the Time of Flight (ToF) and maximum amplitude undergo significant changes during battery charge and discharge processes, potentially coming from:

• Cell thickness change and
• Stiffness change of electrodes.

The similar effects can be witnessed when speaking of acoustic measurements, measurements of the cell expansions during the operation. These changes correlate with the State of Charge (SoC) of the battery thus allowing for the extra layer of the supervision of the battery states beyound the conventional current and voltage measurement.

ORP-EIS sensor

For the SPARTACUS project, the ORP-EIS technique was implemented in a standalone sensor to measure the impedance of the battery cells. The impedance provides information on the state of charge and the state of health of the cell. ORP-EIS stands for “Odd Random Phase Electrochemical Impedance Spectroscopy”, this is a multisine technique that measures all excited frequencies simultateously, providing a shorter measurement time. The measurement validity can by analysed by looking at the different noise levels of the measurement.

Specifications ultra sonic sensors (piezo)