A Fuel-Cell-Battery Hybrid Platform for Portable Embedded Systems

A portable proton exchange membrane (PEM) fuel cell operates at room temperature, has an energy density which is 4 to 6 times larger than that of a Li-ion battery, and thus is a promising next generation power source for power-hungry portable devices. However, in spite of the high energy density, fuel cells have limited power capacity and cannot respond to sudden changes in the load. Thus, a system powered solely by the fuel cell should be over-designed and not economical. Thus, fuel cells are commonly used with a battery to utilize both the high energy density of the fuel cell and the high power capacity of the battery. Since power consumption of embedded systems dynamically changes over time, the control of a fuel cell and the charge management in the battery is sophisticated. Ultimate fuel-optimal configuration is achievable only when the control of the fuel cell, charge management of the battery, and the power management of the embedded system are jointly optimized. Since this is certainly a new problem, we first need an evaluation platform for the measurement and characterization. Nevertheless, there has been no existing platform to tackle such a problem. In this paper, we demonstrate the design and implementation of a 20W average, 80W peak power fuel cell and dual Li-ion battery hybrid power source platform. This platform is designed to characterize the performance of hybrid power source, and also to explore new energy management strategies for embedded systems powered by a hybrid source. We devise a new hybrid power supply scheme named DTC (Dynamic duTy cycle Control) so that we may fully control the proportion of the fuel cell current and the battery current.