3.2.2 Voltage Measurement Circuit3.2.3 Temperature Measurement CircuitTwo main classifications of thermistors exist. The first class of thermistors exhibits large negative temperature coefficients (NTC), and the other class of thermistors exhibits large positive temperature coefficients (PTC). Since the NTC thermistor best relates to theory and offers a high level of accuracy over a wide temperature band, this type of thermistor was chosen to construct the temperature monitoring circuit [16]. Also, the particular shape of thermistor chosen is the disc shape because this shape offers the most surface area contact with the batteries when compared to the bead shape thermistor. Figure 5 contains the shape and type of thermistor used in the monitoring circuit.3.3 Software3.1 Slave PIC3.3.2 Master PIC3.6 Prototype and Test Apparatus3.7 Diagnostic Monitoring Software4.3 Test Specification – Softwaredesign document forSelective Battery Chargersubmitted to:Professor Joseph PiconeECE 4532 : Senior Design IDepartment of Electrical and Computer Engineering413 Hardy Road, Box 9571Mississippi State UniversityMississippi State, Mississippi 39762November 30, 2004prepared by:B. Gore, B. Holbrook, H. Massey, D. MayFaculty Advisor: Professor Marshall MolenDepartment of Electrical and Computer EngineeringMississippi State University413 Hardy Road, Box 9571Mississippi State, Mississippi 39762email: {btg8, bwh34, hjm2, dom5}@ece.msstate.eduDEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERINGExecutive SummaryDuring recent years, the popularity of Hybrid Electric Vehicles (HEVs) has increased. Their high fuelefficiency and overall competitive performance has drawn the attention of many consumers. However,before these vehicles can truly become a success, their price needs to be decreased. In order for thesevehicles to become cheaper, the manufacturing price needs to be reduced. A significant portion of theHEVs’ high cost is due to the battery-charging systems and the non-recurring engineering costs (NRE)associated with developing them. Today, HEV manufacturers are still in the early stages of designingthese battery systems. A different system must be designed for every model and each system is usuallyincompatible with different models. Since most of today’s other automobile systems have becomestandard, companies face very little NRE costs to adapt these systems to a new model. The reason this isdifficult to do with battery-charging systems is that HEV models of different sizes and shapes have verydifferent power requirements. A city bus, for example, needs many more batteries than a small car. Auniversally compatible and expandable battery-charging system would dramatically reduce themanufacturers’ cost to produce HEVs and thus reduce the price for consumers.The purpose of this project is to design a universal and modular battery-charging system that answers thepower needs of various HEVs. Our design is based on a master/slave configuration. A mastermicrocontroller communicates with multiple slave monitoring circuits, each associated with a 12-voltbattery module. The slave circuits are responsible for measuring four characteristics of the individualbattery modules. These characteristics are voltage, current, temperature, and time. Voltage is used toestablish a base estimate of the module’s state-of-charge (SOC). By measuring current and time, thesystem is able to refine this estimate to a more accurate value. Since the batteries are discharged at highcurrents, the temperature is monitored and controlled to ensure that the batteries do not overheat anddamage the rest of the system. To monitor the rapid changes in SOC, the master controller periodicallyobtains these measurements from the slave circuits. This data is used to estimate the overall SOC of theentire 276-volt battery array. A battery-charging system that dynamically monitors SOC assures longevityof the battery packs, and a modular design allows for a wide variety of applications and low replacementcosts.To confirm the operation of the battery-charging system, each of the system’s modules is subjected toextensive testing. The modules are split into software and hardware categories. The hardware consists ofthe 23 12-volt battery modules, the slave circuits, and the power supply. The master microcontroller andslave PICs (Programmable Interconnect Chips) make up the software category. The battery system istested by charging and discharging the batteries repeatedly. The communication between the master andslave circuits is manually monitored through a serial connection from the system to a PC. All modulesoperate according to design constraints, and the communication data indicates that the system charges anddischarges the batteries when SOC is between 20% and 80%. In order to further test the system, it needsto be operated in an environment that simulates operation in a vehicle. Also, the current system does notconsider behavioral changes in the battery modules as they age. Plans for implementing this type ofsupport are under development. As HEV technology improves, the need for advanced vehicle systems increases. While this battery-charging system answers the needs of HEV manufacturers, it also provides a solution for HEV researchand prototyping. The modular design of this system will allow it to be used for many differentapplications, and this system is cheaper than other systems currently on the market since it can be scaledto meet the needs of the manufacturer or designer. As manufacturing and development costs decrease, theconsumer costs will also decrease. This will encourage HEV use, and ultimately benefit both the user andthe environment. Overall, the system performs to specifications, and with further improvements, it has thepotential to revolutionize the HEV market.2TABLE OF CONTENTS1. PROBLEM................................................................................................................................... 42. DESIGN CONSTRAINTS........................................................................................……………62.1. Technical Design Constraints ………... ...............................................................................62.2. Practical Design Constraints ………... .................................................................................73.