1. Problem The idea that fossil fuels will not always be available has caused concern within the automobile industry. Research is currently underway to develop an alternate energy resource for vehicles. Hybrid-electric vehicles (HEVs) use both fossil fuel and electrical energy sources and are much more efficient than conventional vehicles that depend only on fossil fuels. When first conceived, HEVs were only expected to be a temporary fix for the shortcomings of batteries used in electric vehicles. Now, after years of technological advances, hybrid vehicles seem to have more potential than full electric vehicles. Pressure applied by the U.S. Department of Energy has helped to popularize HEVs in the United States. In 1993, an HEV program was introduced to create a cost-shared partnership between General Motors, Ford, and DaimlerChrysler. With more money being spent on research directed toward HEV development, legislation was introduced to give tax payers an incentive to purchase an HEV. A tax cut of $2,000 for hybrid owners started in 2003[1]. All of this motivation through legislation and government programs, along with obvious potential of HEV technology, opened the eyes of consumers. The first HEV, the Honda Insight, went on sale in the United States in 1999. Hybrid vehicle sales reached 38,000 by 2002. By the year 2005, HEVs are expected to make up 1% automobile sales. By the year 2008 annual HEV sales are expected to reach 500,000[2]. One of the largest differences between a full electric vehicle and an HEV is the procedure used to recharge the batteries. A fully electric vehicle requires an electrical outlet at a service station or in the owner’s home. The batteries can only provide power for short trips, and relatively large amounts of time are required to recharge the system. HEVs, on the other hand, use a method that continuously recharges the batteries while the vehicle is in motion. An internal combustion engine and an electric generator are used to recharge the batteries while the vehicle is in operation. The ability to recharge the batteries of an HEV while the vehicle is in use is one key advantage that HEVs have over their fully electric predecessor. Complex systems are continuously being developed to provide the best charging method. Because HEVs are still relatively new and few in number, demand for these products is low. Thus, most charging systems are developed and manufactured by the automobile manufacturers themselves. Currently, there is not much of a commercial market for these products, or even a common standard by which to build these systems. Automobile manufacturers design these systems specific to the HEVs they produce. In order to jumpstart the market, a battery system must be designed that will be general and flexible enough to be applied to any HEV. Designing such a general system will be difficult since all HEVs operate according to different specifications and requirements. These differences include the power required to drive the electric motor, space required to store the battery system, and the different data communication protocols used between the systems in the HEV. Since power requirements differ among different HEVs, the system must be able to alter power output relatively easily. The system must be capable of accepting different numbers of battery packs and adapting to the changes as dynamically as possible. Ideally, the system should require little or no changes to be manually applied by the user. Since most current systems are application dependent, this feature will be very attractive to potential consumers. The system must also be compact and as self-contained as possible. This way, incorporating the system into different HEVs with different space constraints will be relatively simple. Finally, the system must be able to adapt to different communication protocols. These protocols differ between various HEVs, and in order to make this system appealing to potential consumers, the system cannot depend on a specific protocol. The purpose of our project is to develop a system to charge the batteries for the HEV that will be designed for ChallengeX, a GM sponsored contest between seventeen universities to convert a 2005 GM Equinox into an HEV. The system will utilize a series of complex algorithms to monitor the state-of-health (SoH)and state-of-charge (SoC) of each battery pack. The system will use this data to selectively charge the individual battery packs and automatically diagnose problems that may occur. Battery charging systems for modern HEVs are much more complex than one might think. Most modern HEVs use Nickel-Metal Hydride (NiMH) rechargeable batteries since they require only a short amount of time to charge, weigh less than most other battery types, and are more efficient at storing energy[3]. NiMH batteries also provide an excellent balance between cost and cycle life[4]. In order to ensure that maximum cycle life will be achieved, the system must charge and discharge the batteries in a very specific manner. The techniques used to accomplish this are still being researched. Modern systems use a microprocessor and a complex series of algorithms to monitor charging and discharging. These algorithms may not be as efficient as they have potential to be. As HEV technology advances, these algorithms will be revised and refined. Our system will use the latest research and technology to ensure that the batteries are used as efficiently as possible. Modern battery charging systems face several problems. One of these problems, as mentioned above, is that most systems today are specific to a certain vehicle. Such systems must be redesigned for use in any other application. Second, the battery life of current systems is cut short by improper and inefficient charging and discharging methods. Replacing a battery system is expensive, and should not have to be done often. In most modern battery charging systems, there are two key aspects of a battery that must be monitored while charging and discharging: SoC and SoH. The SoC is usually determined by measuring battery characteristics such as voltage and temperature. For