2. DESIGN CONSTRAINTS The production of an efficient battery-charging system for hybrid-electric vehicles (HEVs) will significantly increase the lifetime of the battery packs that these vehicles use. By monitoring changes in temperature and voltage levels, the system ensures that the batteries are charged and discharged in a safe and efficient manner. Also, by using a modular array of batteries, replacement costs are lower since the cost of replacing a single dead battery pack is cheaper than replacing the entire array. Typical HEV battery systems are usually specific to the application for which they were designed. A modular design will be versatile and can be easily modified for different needs. The design constraints described below outline the quantitative technical aspects of the battery-charging system that will be built for ChallengeX. 2.1 Technical Design Constraints The technical design constraints outlined below in Table 1 give an overview of the detailed parameters of the battery-charging system. These constraints will be used as a reference while designing and testing the final product. Name Description Charge Time between 30 and 45 minutes Voltage Output nominal voltage of 276 volts Power Output minimum of 100kW Operational Temperature maximum of 50º C Monitoring Frequency once every six seconds Table 1: Technical Design Constraints 2.1.1 Charge Time Since battery power is an essential element in an HEV, the batteries need to charge quickly so that the drive train has access to this power as much as possible. However, charging too quickly could potentially damage the batteries and reduce their overall lifetime. The system will fast-charge at a rate between 4/3C and 2C (where C is the cell charge capacity). In other words, since the cells will have a capacity of 3600mAh, the batteries will be fast-charged between the boundary currents of 4800mA and 7200mA. Charge time can be calculated using C ÷ I (where I is the charging current). For the two boundary currents, the associated charging times are: 3600mAh ÷ 4800mA = 0.75h = 45min 3600mAh ÷ 7200mA = 0.5h = 30min According to these calculations, the batteries can be fast-charged in a minimum of 30 minutes when using a current of 7200mA. Ideally, the batteries would be able to fast-charge to full capacity. However, using this technique with real-world batteries is dangerous since slightly over-charging a battery could damage or destroy it. Additionally, charge capacity varies between batteries, and a constant charge time could possibly over-charge a battery that has a slightly lower charge capacity. To avoid problems caused by these variances, the batteries are charged in multiple stages. Three stages of charging will be used by this system: fast-charge, top-off charge, and trickle-charge. The system will fast-charge the batteries to 3/4 of full capacity, then top-off charge and trickle-charge the batteries until they are full. Overall, the charging process will take between 30 and 45 minutes.2.1.2 Voltage Output Choosing an appropriate output voltage is a key factor when addressing concerns of convenience, cost, and safety. A nominal voltage of 276 volts will accommodate these concerns. The exact value of the voltage is based on the battery cells being grouped in 12-volt packs. A total of 23 battery packs will be used in this system. According to the basic definition of electrical power, P V I, a larger voltage (V) will lower the amount of current (I) needed to attain a fixed quantity of power (P). If a lower voltage is used, high-gauge wires will be needed to handle the amount of current produced to meet power requirements. Thick wires are both bulky and expensive. Another concern with higher current is safety. Lower current will reduce the risk of electric shock to a curious owner or a vehicle mechanic. 2.1.3 Power Output The batteries’ main function is to provide power to the electric motor of the HEV. The power requirement for the electric motor used for the ChallengeX project is 100kW. Therefore, the battery system will be designed to provide at least 100kW. The amount of power required by the charging system itself and the other subsystems of the vehicle is minimal. 2.1.4 Operational Temperature It is imperative that the system continuously monitors the battery temperature. As a NiMH battery approaches full charge, the temperature will rapidly increase. Risk of reduced battery life and possible explosion can stem from improper battery temperature control while charging. For these reasons, a maximum temperature of 50 degrees Celsius will be allowed for each battery pack. Thermal sensors will alert if this threshold is crossed. The thermal sensors will also help monitor the state-of-charge of each battery pack and will warn if one battery pack begins heating up much faster than the others. A sudden rise in temperature of a battery pack could indicate that it is damaged. 2.1.5 Monitoring Frequency The batteries’ state-of-charge can vary significantly over a small period of time. This is especially true when the batteries are being charged rapidly. There is a critical point at which the fast-charge current needs to be pulled back to a trickle. Supplying the batteries with a fast-charging current past this point can degrade their overall health. Because of this, the monitoring circuit will cycle through the battery array once every 120 seconds. With 23 packs in the array, each pack will be checked for approximately six seconds at a time. 2.2 Practical Design Constraints The design of this product is also subject to practical constraints. The constraints listed in Table 2 outline important regulations for this product. Type Name Description Economic Cost maximum of $3000 Sustainability Reliability minimum of 100,000 miles Manufacturability Dimensions 20.5” l x 8”w x 4” h Health and Safety Safety complies with UL specs 2231-1 and 2231-2 Environmental Power Consumption monitoring system will consume a maximum of 5W Table 2: Practical Design Constraints2.2.1 Cost To build a successful product, the cost must be kept as low as possible. The most expensive part of the battery system will be the batteries themselves. Some HEV battery systems contain battery packs that cost