Abstract

Understanding the pulsed discharge behavior of low-rate lithium coin cells in wireless sensing systems is critical to prolong the operating life and/or reduce the size of battery-powered WSs. This dissertation presents the battery transient analysis for a sensor duty cycle, experimental studies for sustained pulse discharge cycling, and investigation on recharge strategies for a battery/power harvesting hybrid system for WSs. The transient behavior of the lithium coin cells during pulse discharge and subsequent relaxation was investigated with single-pulse experiments and theoretical analysis. The voltage response for a pulsed discharge had two parts: a region of rapid voltage change and a region of slower change. The magnitude of the rapid voltage losses was associated with ohmic and interfacial resistances. Solid phase diffusion in the cathode was found to be the major contributor to the "slow" transient voltage change that occurred during and after a pulse. An analytical model was developed to describe the time-dependent voltage and the corresponding non-uniform concentration distribution for the thick porous electrode. A fit of the analytical model to experimental data permitted an estimate of the solid phase diffusivity. Independent fitting of the pulse data and relaxation data both yielded a diffusivity of D ~ 4×10-11 cm2/s, which agreed well with measured values reported in literature. The interactive effect of battery characteristics and WS operating conditions was investigated during sustained pulsed-discharge cycling. At low standby currents (≤50 μA), the influence of the standby current on the operating voltage and battery capacity was negligible. The pulse current had a significant impact on the lower voltage and determined the maximum capacity that could be extracted from a battery regardless of the duty cycle factor. For each pulse length studied, the battery capacity increased as the standby time increased, until a maximum capacity was reached, which could not be increased by further increase in the standby time. The minimum standby time for full (or near full) relaxation for duty cycles with different pulse length was found to correlate well with ratio ts/tp2. Battery pulse discharge-recharge cycling as would occur in a hybrid power system was investigated, and the recharge strategies were evaluated in terms of capacity loss over cycling and energy efficiency. Results from the cycling tests suggested the importance of a rest period between the discharge and charge step of a cycle. PRCR cycling with a 2 s rest period could lower the capacity loss to 25% or less of that of PC cycling with no rest period over 10,000 cycles. Cycling the battery at 80% SOC rather than at 100% SOC (3.1 V) significantly reduced the capacity loss during cycling.

Degree

PhD

College and Department

Ira A. Fulton College of Engineering and Technology; Chemical Engineering

Rights

http://lib.byu.edu/about/copyright/

Date Submitted

2012-06-17

Document Type

Dissertation

Handle

http://hdl.lib.byu.edu/1877/etd5373

Keywords

lithium coin cell, pulsed discharge, wireless sensor, duty cycle, power supply

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