Power management solutions for cellular IoT systems utilizing power constrained energy sources

The number of cellular Internet of Things (IoT) devices is constantly rising, which sets challenges to powering all of them. A typical solution has been the use of onboard batteries. However, the fundamental problem with traditional batteries is the limited battery life as well as challenges with the disposal process. The challenges can potentially be answered with energy harvesting systems and alternative battery chemistries. This thesis focuses on providing powering options for two common cellular IoT use cases: an indoor temperature sensor and an asset tracker. The requirements of the devices are significantly different from operating life and size perspective.

The temperature sensor application is powered by an indoor photovoltaic panel which charges a lithium-ion capacitor with a power management circuit which implements a maximum power point tracking algorithm and steps up the voltage. With this system energy autonomy of the system is achieved with data transmission interval of 30 minutes, without a significant difference between the output power classes 3 and 5, which are supported by the device. The use of a lithium-ion capacitor as the energy source instead of a traditional rechargeable lithium-ion battery offered improvements in the cycle life of the energy storage as well as in sustainability of the overall design.

The energy source of the asset tracker is selected to be a zinc-based printed battery. The limited output power due to the high internal resistance was addressed with a supercapacitor that was charged by a buck-boost converter before data transmissions. With the proposed power management system design the useful voltage range of the battery was increased and operating ranges of 5 to 85 days were achieved, with data transmission intervals ranging from 30 minutes to 6 hours. The use of assisted global navigation satellite system decreased the energy consumption of the device significantly. The advantages of the design used in this work include lower leakage currents in power management systems compared to traditional designs, where the boost converter is always on and connected to the secondary energy storage. The reduction in leakage currents is primarily achieved by disconnecting the secondary energy storage when high power pulses are not needed. The leakage current of the power management system is a major contributor to overall energy consumption during the low power sleep modes in which IoT devices spend most of their operating lives. Therefore, reducing leakage currents can significantly extend the operating lifespans of IoT devices.