2. DESIGN CONSTRAINTSThe production of an efficient battery-charging system for hybrid-electric vehicles (HEVs) willsignificantly increase the lifetime of the battery packs that these vehicles use. By monitoring changes intemperature and voltage levels, the system ensures that the batteries are charged and discharged in a safeand efficient manner. Also, by using a modular array of batteries, replacement costs are lower since thecost of replacing a single dead battery pack is cheaper than replacing the entire array. Typical HEV batterysystems are usually specific to the application for which they were designed. A modular design will beversatile and can be easily modified for different needs. The design constraints described below outlinethe quantitative technical aspects of the battery-charging system that will be built for ChallengeX.2.1 Technical Design ConstraintsThe 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 DescriptionCharge Time between 30 and 45 minutesVoltage Output nominal voltage of 276 voltsPower Output minimum of 100kWOperational Temperature maximum of 50º CMonitoring Frequency once every six secondsTable 1: Technical Design Constraints2.1.1 Charge TimeSince battery power is an essential element in an HEV, the batteries need to charge quickly so that thedrive train has access to this power as much as possible. However, charging too quickly could potentiallydamage the batteries and reduce their overall lifetime. The system will fast-charge at a rate between 4/3Cand 2C (where C is the cell charge capacity). In other words, since the cells will have a capacity of3600mAh, 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 boundarycurrents, the associated charging times are:3600mAh  4800mA = 0.75h = 45min3600mAh  7200mA = 0.5h = 30minAccording to these calculations, the batteries can be fast-charged in a minimum of 30 minutes when usinga current of 7200mA. Ideally, the batteries would be able to fast-charge to full capacity. However, usingthis technique with real-world batteries is dangerous since slightly over-charging a battery could damageor destroy it. Additionally, charge capacity varies between batteries, and a constant charge time couldpossibly over-charge a battery that has a slightly lower charge capacity. To avoid problems caused bythese variances, the batteries are charged in multiple stages. Three stages of charging will be used by thissystem: fast-charge, top-off charge, and trickle-charge. The system will fast-charge the batteries to 3/4 offull capacity, then top-off charge and trickle-charge the batteries until they are full. Overall, the chargingprocess will take between 30 and 45 minutes.2.1.2 Voltage OutputChoosing 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 thevoltage is based on the battery cells being grouped in 12-volt packs. A total of 23 battery packs will beused 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 isused, high-gauge wires will be needed to handle the amount of current produced to meet powerrequirements. 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 OutputThe batteries’ main function is to provide power to the electric motor of the HEV. The power requirementfor the electric motor used for the ChallengeX project is 100kW. Therefore, the battery system will bedesigned to provide at least 100kW. The amount of power required by the charging system itself and theother subsystems of the vehicle is minimal. 2.1.4 Operational TemperatureIt is imperative that the system continuously monitors the battery temperature. As a NiMH batteryapproaches full charge, the temperature will rapidly increase. Risk of reduced battery life and possibleexplosion can stem from improper battery temperature control while charging. For these reasons, amaximum temperature of 50 degrees Celsius will be allowed for each battery pack. Thermal sensors willalert if this threshold is crossed. The thermal sensors will also help monitor the state-of-charge of eachbattery pack and will warn if one battery pack begins heating up much faster than the others. A suddenrise in temperature of a battery pack could indicate that it is damaged.2.1.5 Monitoring FrequencyThe batteries’ state-of-charge can vary significantly over a small period of time. This is especially truewhen the batteries are being charged rapidly. There is a critical point at which the fast-charge currentneeds to be pulled back to a trickle. Supplying the batteries with a fast-charging current past this point candegrade their overall health. Because of this, the monitoring circuit will cycle through the battery arrayonce every 120 seconds. With 23 packs in the array, each pack will be checked for approximately sixseconds at a time.2.2 Practical Design ConstraintsThe design of this product is also subject to practical constraints. The constraints listed in Table 2 outlineimportant regulations for this product.Type Name DescriptionEconomic Cost maximum of $3000Sustainability Reliability minimum of 100,000 milesManufacturability Dimensions 20.5” l x 8”w x 4” hHealth and Safety Safety complies with UL specs 2231-1 and 2231-2Environmental Power Consumption monitoring system will consume a maximum of 5WTable 2: Practical Design Constraints2.2.1 CostTo build a successful product, the cost must be kept as low as possible. The most expensive part of thebattery system will be the batteries themselves. Some HEV battery systems contain battery packs that costapproximately $6800 to replace [1]. The batteries for this HEV battery system will cost around $96 foreach 12-volt pack. Since 23 battery packs will be needed, the total battery cost will be around $2500.