Vehicle electrical loads

Nonwoven materials such as cellulosic fibers have never been successfully used in lithium batteries. This lack of interest is related to the hygroscopic nature of cellulosic papers and films, their tendency to degrade in contact with lithium metal, and their susceptibility to pinhole formation at thickness of less than 100 fjim. For future applications, such as electric vehicles and load leveling systems at electric power plants, cellulosic separators may find a place because of their stability at higher temperatures when compared to polyolefins. They may be laminated with polyolefin separators to provide high-temperature melt integrity. 

While the vehicle is at idle (engine at minimum speed), most electrical loads are maintained. The low output of the alternator under these conditions, as shown in Fig. 11.8, means that the battery may need to boost voltage through discharge, and the burden on the battery may be substantial depending on the accessory load and the time at idle. For example, a 1-kW deficit for 5 min will discharge a 50-Ah battery by 13.9%, for which a flooded lead add would only survive 1000 cycles (cf.. Fig. 11.4). 

Table 11.12. Proposed vehicle loads and their relative effect on electrical loads [7].

In parallel with this new class of load demands, the power required by more conventional loads increases, too. The power demand of a high-end vehicle increased from less than 500 W in the 1960s to more than 2kW in 2000, For the next decade, automotive engineers predict a sharp electrical load increase to about lOkW [40],... 

The development of vehicle electrical systems operating at a nominal 42 V and its standardization with ISO is underway. A working group of the European Vehicle Electric System Architecture Forum has prepared a draft standard for this new voltage level [58]. The 42-V level was chosen as a compromise between the demand for an increased voltage to achieve higher efficiency in electrical and electronic components and the exclusion of safety hazards, even under extreme operational conditions like load dump, etc. The term 42-V PowerNet , or simply PowerNet , is now established worldwide [9,11,59]. 

A number of companies have been developing this battery including Gould and Electrofuel, and large sized batteries are expected to be demonstrated by 1990. In addition to Argonne, the British Admiralty laboratories has been developing this battery for submarine applications while other potential applications include electric vehicles and load levelling. 

The direct current (DC) output of a fuel cell stack will rarely be suitable for direct connection to an electrical load, and so some kind of power conditioning is nearly always needed. This may be as simple as a voltage regulator, or a DCIDC converter. In CHP systems, a DC to AC inverter is needed, which is a significant part of the cost of the whole system. Electric motors, which drive the pumps, blowers, and compressors mentioned above, will nearly always be a vital part of a fuel cell system. Frequently also, the electrical power generated will be destined for an electric motor - for example, in motor vehicles. 

In fuel cell vehicle systems, for best performance and highest eificiency, the fuel cell stack, the DC/DC converter, battery, power inverter, and the motor should be treated like one system. If the amount of hydrogen flow to the stack is higher than that required by the electrical load, then energy is wasted in the exhaust. If the fuel flow is less than that required by the electrical load, then the impedance of the stack increases thus overheating the stack, and possibly leading to cell reversal. Hence, it is necessary to match the amount of hydrogen flow to the stack to meet the desired electrical load at the output. 

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