Abstract—This paper presents a distributed energy storage device (DESD) based on a novel isolated bidirectional DC-DC converter with 650V GaN transistors. The device integrates a low-voltage (13.2V) Li-ion battery pack, an embedded bidirectional DC-DC converter and wireless communication system. The three parts are packaged together, thus it can be directly connected to high-voltage (380V) DC grid, enabling a modular approach for battery energy storage systems. Two 650V enhancement mode GaN transistors are used at the high voltage side. Compared with Si device, three improvements can be achieved in the application: expanding the operation range to light load, reducing switching loss and EMI, increasing the total efficiency of charging and discharging operation. The power stage design as well as a loss analysis of GaN is based on a steady state analysis and PSpice simulation. A 400V to 12V DC, 1kW converter for 1kWh DESD prototype is designed, fabricated, and tested. Experimental results verify the validity of the proposed DESD and the performance improved by using GaN transistors. Index Terms—Distributed energy storage device, batteries, GaN, isolated bidirectional converter, dc microgrid. I. INTRODUCTION Energy storage systems are enabling technologies for well established and new applications such as power peak shaving, electric vehicles, integration of renewable energies, etc. It is said the grid-connected energy storage market will increase from 340 MW in 2013 to more than 6 GW in 2017 [1] . With the promising growth driven by legislation and the increasing need for flexible capacity as a result of growing levels of renewable penetration, the research of advanced energy storage device is becoming more and more popular and pressing. Compared with centralized energy storage device, the distributed energy storage device (DESD) has the merits of high flexibility, reliability, sustainability, efficiency and cost effective [2] . The advantages of Li-ion battery technology are high energy-to-weight ratios, no memory effect, and a low self-discharge. However, the price of lithium-ion (Li-ion) batteries is still high for many applications. Besides, from safety point of view, to ensure safe operation, it is mandatory to use a battery management system (BMS) to at least provide overvoltage, undervoltage, overtemperature, and overcurrent protection [3] . Thus the main hurdle for making Li-ion batteries based energy storage device is the high cost and design of battery management system (BMS). Generally, the smaller the number of batteries, the lower cost and complexity of BMS will be. So our approach is to use a low-voltage (only 13.2V) battery pack as the energy storage component and a high step up/down bidirectional dc-dc converter as the interface to connect the 400V DC grid. Our proposed DESD integrates a bidirectional DC-DC converter, a Li-ion battery pack, BMS and communication system. Among the four parts mentioned above, the bidirectional DC-DC converter is the essential of the DESD to implement bi-directional power transferring functionality. In my application, high power conversion efficiency and high power density are required for all operating conditions. Since GaN transistor has emerged as a promising device for high frequency, high efficiency power conversion due to a better figure of merit than comparable Si transistors. Compared with Si device with the same voltage parameter, the GaN device has low switching loss, low output capacitance, ultra-fast freewheeling diode and competitive drain-source on-state resistance [4]. With the commercial production of 650V GaN device, its application in our 400V DC microgrid becomes feasible. Fig. 1 380V DC microgrid system The objective of this paper is to investigate the feasibility and dispatchability of a compact, low cost, autonomous distributed battery energy storage device based on high efficiency, high Distributed Energy Storage Device Based On A Novel Bidirectional DC-DC Converter With 650V GaN Transistors Fei Xue, Ruiyang Yu, Wensong Yu, Alex Q. Huang FREEDM Systems Center, North Carolina State University Raleigh, NC 27606, USA {fxue, ryu3, wyu2, aqhuang}@ncsu.edu 978-1-4799-8586-9/15/$31.00 (c)2015 IEEE