Managing Visited Network NAS Signaling Overrides during Cellular IoT Roaming
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Lithium Thionyl Chloride Degradation
Meaning
Electrochemical degradation in primary cell batteries results in decreased voltage and capacity during high-current demands. Analyzing lithium thionyl chloride degradation helps estimate battery life under intermittent heavy load profiles. This process involves the growth of a passivation layer on the lithium anode that increases internal resistance.
It impacts devices designed for long-term unattended operation in the field.
Chemical Passivation
Storage of these cells at elevated temperatures accelerates the growth of lithium chloride crystals on the anode surface. While lithium thionyl chloride degradation protects the cell from self-discharge, the thick crystal layer blocks the movement of ions when the device wakes up to transmit data. This block causes a temporary drop in output voltage known as voltage delay.
It can trigger unexpected low-voltage resets in the device.
Performance Impact
High internal resistance reduces the capacity of the cell to deliver short current pulses. The battery may fail to power the transceiver even if it contains ample charge.
Mitigation Strategy
Circuit designers place large supercapacitors in parallel with the battery to handle pulse currents. This design bypasses lithium thionyl chloride degradation effects by using stored energy to power the transmitter while the battery voltage recovers. However, this strategy does not prevent the underlying chemical degradation over multi-year periods.