97486 LOAD LEVELED BATTERY SYSTEM CHARACTERISTICS USING SEALED LEAD-ACID BATTERIES John T. Guerin Andrew F. Burke Institute of Transportation Studies University of California, Davis Davis, California 95616 Phone (916) 752-9812 Fax (916) 752-6572 ABSTRACT zyxwvutsrqpo The characteristics of a load leveled battery system were studied experimentally using a zyxwvutsrqp 36 V pack of sealed lead-acid batteries (38 Ah, 12 V modules) as the energy source and a 24 V pack of the same batteries as the pulse power unit. The control strategy for the load leveling was implemented such that the results could be easily interpreted relative to the use of ultracapacitors as the pulse power unit. The system response and test results are in good agreement with previously published SIMPLEV simulations of battery load leveling with pulse power batteries and ultracapacitors in electric vehicles and show the importance of high efficiency control electronics and the expected high charge/discharge efficiency of ultracapacitors to optimize system performance. INTRODUCTION Batteries for electric vehicles must be designed to provide the peak power required by the vehicle during accelerations and to accept high charging power from regenerative braking during rapid vehicle decelerations. These requirements mean that the batteries must have a high maximum power density (W/kg) along with high energy density (Wh/kg), long cycle life, and relatively low cost ($/kWh) if they are to be attractive for use in electric vehicles. The requirement t o design for high power density (200-400 Wkg) for most battery types results in compromises in energy density, cycle life, and cost. One approach to decoupling the power requirement from the other requirements is to load level the energy storage battery using a pulse power device that is especially designed for high power in both discharge and charge. In this way, the energy storage battery can be discharged at the average power required by the vehicle and it never experiences the transient power pulses that it must the vehicle driveline. There have been numerous "paper" studies (References 1-3) of load leveled battery operation, but few, if any, experimental studies or vehicle demonstrations of a battery system that consists of an energy battery and a pulse endure if it must meet the instantaneous power excursions of power unit. This paper, which is based on the M.S. Thesis of J.T. Guerin (Reference 4) at UC Davis, is concerned with such an experimental study directed to evaluating in the laboratory a system in which one battery pack is used to load level another with electronics to control the discharge of the energy battery based on the state-of-charge of the pulse power battery. Ultracapacitors are a near optimum technical alternative for the pulse power unit as they have very high power density (>lo00 W/kg) for both charge and discharge at all states-of- charge and very long cycle life (>200,000 cycles). The development of ultracapacitors for vehicle applications is in a relatively early stage (Reference zyxw 5,6) and units storing even 50-100 Wh are not readily available and if available, are very expensive. For these reasons, batteries rather than ultracapacitors were used as the pulse power unit in the present study of battery load leveling, but the useable Wh capacity of the pulse power battery was limited to that expected to be available from an ultracapacitor zyx - that is 300- zy 500 Wh in a full-scale vehicle driveline system. The response of the battery-battery and battery-ultracapacitor systems would be expected to be quite similar so the results of the present study of the battery-battery system should be relevant to the operation of future systems consisting of batteries and ultracapacitors. A second special feature of the present study is that the control electronics (a DC/DC converter chopper) is between the energy storage batteries and the load and not between the pulse battery and the load as battery- ultracapacitor systems are usually configured (Reference zyx 7). In order to utilize this configuration, the voltage of the energy storage battery must always be above that of the pulse power unit, because the DC/DC converter can not boost the voltage if the voltage of the energy storage battery falls below that of the pulse power unit. In that case, either the battery or the pulse power unit would be forced to provide the required power to /from the load and the system will return to normal operation when the voltage of the energy battery becomes higher than that of the pulse power unit.