2. DESIGN CONSTRAINTS The production of a new battery charging system for hybrid vehicles can greatly increase the longevity of battery packs in vehicles. Monitoring temperature and voltage levels will provide appropriate fast and trickle charging and ensure the best possible life expectancy of the batteries. Also, by creating an entirely modular array of batteries, replacement of only a single dead battery will be much less expensive. Typical hybrid battery systems are extremely specific to one application. With a modular design, the new battery system will be more versatile and more easily changed to fit different needs. To help meet the above expectations, design constraints must be placed on the battery system. 2.1 Technical Design Constraints The technical design constraints outlined in Table 1 below give an overview of the detailed parameters of the battery system. Not only will these be used in the design of the system, but will be used as a reference point when 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 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 in order to assure the drive train has access to battery power for as much time as possible. However, charging too quickly could potentially damage the batteries and reduce overall lifetime. The system will fast-charge at a rate between 4/3C and 2C (where C is the cell capacity). In other words, since the cells have a capacity of 3600mAh, the batteries will fast-charge using current between 4800mA and 7200mA. Charge time can be calculated using C ÷ I (where I is the charging current). For the two boundary currents above, the associated charging times are: 3600mAh ÷ 4800mA = 0.75h = 45min 3600mAh ÷ 7200mA = 0.5h = 30min According to these figures, the batteries can be fast-charged in a minimum of 30 minutes when using a current of 7200mA. Ideally, the batteries should be able to fast-charge to full capacity. However, using this technique with real-world batteries is dangerous. Because slightly over-charging a battery could damage or destroy it, the timing would need to be exact. Also, because cell capacity varies from battery to battery, a single time will not work for every battery. To get around these variances, batteries are charged in multiple stages. Three stages of charging will be employed in this system: fast-charge, top-off charge, and trickle-charge. The system will fast-charge the batteries to 3/4 capacity, then top-off charge and trickle-charge the batteries until they are full. Despite the differences, the time added to charge is negligible. Overall, the charging process will still take between 30 and 45 minutes. 2.1.2 Voltage Output Voltage output of the battery system is a key factor when addressing concerns of convenience, cost, and safety. A nominal output voltage of 276 volts was chosen to accommodate these concerns. The exact value of voltage was chosen because the battery cells used are grouped in 12-volt packs. However, reasons for the high voltage need more explaining. Using the basic definition of electrical power,P V I, a larger voltage (V) will lower the amount of current (I) needed to obtain a fixed value of power (P). If a lower voltage were used, large wires would be needed to handle the amounts of current produced to meet power requirements. Wires of this size are both bulky and expensive. Another concern with higher current is safety. A common standard of caution states, “The damage caused by electric shock depends on the current flowing through the body -- 1mA can be felt; 5mA is painful. Above 15mA, a person loses muscle control, and 70mA can be fatal. [4]” Therefore, lower current reduces risk of electric shock. 2.1.3 Power Output The battery system’s main function is to provide power to the electric motor of the HEV. Power requirement of the electric motor used in the ChallengeX project is 100kW. Therefore, the battery system will be designed to output 100kW. 2.1.4 Operational Temperature It is imperative to continuously monitor the temperature when dealing with NiMH batteries. After a NiMH battery is fully charged, the temperature rapidly increases. 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 is set for each battery pack. Thermal sensors will alert if this threshold is being approached. Also, the thermal sensors will help monitor the state of charge of each battery pack. 2.1.5 Monitoring Frequency The batteries’ State-of-Charge can vary greatly over just a small amount of time. This is especially true when the batteries are being charged very rapidly. There is a critical point at which the rapid charging current needs to be pulled back to just a trickle. Supplying the batteries with a rapid charging current past this point can be very degrading to their overall health. Because of this, the monitoring circuit will cycle through the array every 120 seconds. Based on 23 packs in the array, each pack will be checked at least once every six seconds. 2.2 Practical Design Constraints The design of this product is also subject to practical constraints. These constraints, shown in Table 2 below, outline important regulations for the product, including its cost, reliability, and safety. Type Name Description Economic Cost Maximum of $3000 Sustainability Reliability maximum of 100,000 miles Manufacturability Dimensions 5” l x 7”w x 7” 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 Constraints 2.2.1 Cost To create a successful product, the cost must be kept low for all parts of the system. The most expensive part of the hybrid battery system is the batteries. Some current hybrid battery systems contain battery packs that cost approximately $6800 to replace [1]. The batteries for the new hybrid battery system will cost $96 for a 12-volt pack. Since 276 volts will be provided, the totally battery cost should not exceed $2500. Once the batteries are purchased, the cost of the microcontroller, PLDs