An Advanced Battery Management System for Lithium Ion Batteries

This paper describes the development of a Battery Management System (BMS) State of Charge/Health (SOC/SOH) algorithm that was developed and proven for three different lithium ion based cell chemistries (nanophosphate, lithium manganese oxide, lithium iron phosphate). In addition, a universal BMS architecture based on this algorithm was developed that can support other chemistries, capacities, and formats. Algorithm performance was compared to actual data in the laboratory environment and also to data from a lithium iron phosphate hybrid electric vehicle pack that was integrated with an XM1124 hybrid electric HMMWV operating in a vehicle environment under realistic conditions. The system demonstrated accuracy within 5% in a software upgradeable, low cost package. INTRODUCTION Lithium-based batteries promise excellent performance, although they require careful management to avoid personnel injury and equipment damage. Consequently, there is extreme interest in developing an accurate Battery Management System (BMS) to take advantage of the positive attributes of lithium-based chemistries without sacrificing flexibility and safety. However, the lack of adequate BMS standardization and inaccurate state estimation algorithms have hampered the widespread adoption of lithium chemistries in spite of their advantages. In this paper we describe our BMS and present results that show that we can provide State of Charge (SOC) estimation accuracy to better than 5%; that we can utilize existing life cycle data from battery manufacturers to estimate State of Health (SOH) and State of Life (SOL); and that we have a universal architecture that is adaptable to other chemistries, capacities, and formats. The primary application that we describe is for Silent Watch, but the BMS is also adaptable for other Hybrid Electric Vehicle (HEV) applications. SYSTEM DESCRIPTION Figure 1 illustrates our concept for a BMS that provides health monitoring for a typical 24 VDC Silent Watch pack [1]. In most lithium-based battery pack applications, the pack is comprised of a number of seriesand parallelconnected cells to achieve the required voltage, current, and energy/power capacity. As one example, a typical Silent Watch configuration could consist of eight submodules of six parallel-connected, prismatic, 3.3 V, 20 Ah cells (8S6P). In total, there are 48 cells in this configuration, the nominal pack voltage is 26.4 V, and the capacity is 120 Ah. The parallel-connected cells are referred to as “super-cells” and require relatively little oversight compared to the seriesconnected cells, which pose the most challenges because of the need for cell balancing. The BMS monitors the voltage and temperature of each super-cell, and the series current of the overall pack via individual super-cell sense modules. Although the Silent Watch battery pack described above uses eight series-connected super-cells, this Universal BMS architecture is expandable to any number of super-cells, Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 09 AUG 2011 2. REPORT TYPE N/A 3. DATES COVERED 4. TITLE AND SUBTITLE An Advanced Battery Management System for Lithium Ion Batteries 5a. CONTRACT NUMBER