Battery Squeezing under Low-Power Pulsed Loads

Autonomous sensors are wireless measurement systems that in order to achieve longer runtimes remain inactive most of the time and wake up only to acquire and send data. As a result, autonomous sensors can be modeled as low-power pulsed loads. Power is provided by batteries, either primary or secondary. Whenever the autonomous sensor wakes up, a significant voltage drop can appear between the battery terminals, thus preventing to squeeze all the available energy from the battery and leading to a reduction of the load runtime. This work proposes the use of hybrid-storage units, formed by the parallel combination of a battery and a (super)capacitor, in order to extend the runtime of low-power pulsed loads. We show analytical expressions of the transient voltage drop at the terminals of the hybrid-storage unit and deduce the minimal value of the capacitor to be used for a given voltage drop. Experimental results support the theoretical analysis. We tested several combinations of batteries and capacitors as the hybrid-storage unit under pulsed loads of tens of milliamps. Finally, we assessed the performance of the hybrid-storage unit when using as a load a commercial transceiver intended for wireless sensor networks applications.