热度 17
2015-10-23 21:21
1934 次阅读|
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Today's internal combustion (IC) and EV/HEV cars are already pulling kilowatts from the battery and power subsystem, and the drain is clearly going to increase. The power train control, infotainment, ADAS (advanced driver assistance systems), and other functions each need "juice." Ten-kilowatt load factors are a very real possibility. The problem is that the standard 12.6 V (nominal) supply rail brings an inherent limitation. Since P = VI, simple math shows that each kW of demand will need about 80 A, and that's a serious amount of current. As the current drawn from the source increase, resistive voltage losses increase linearly as defined by V = IR, but dissipation increases with the square of the current: P = I2R. The solution to the loss challenge has been known since the early days of power (think Edison, Tesla, and Steinmetz) use higher voltages to reduce the current needed when delivering a given amount of power. It's the same reason that many homes and offices have 240 VAC mains for some equipment (electric dryers, EV chargers) in the U.S. instead of 120 VAC alone, and why industrial factory mains are often 480 VAC. To minimize the supply inefficiency dilemma, there's talk of migrating cars to a 24-V or even a 48-V rail. A recent article at IHS Engineering360, " 48 Volts: Time for a Jolt to Vehicle Performance? ,”explored the promise, potential, and even peril of higher voltages in cars. This is not the first time that there has been a push to using higher voltages in cars. About a decade ago, there was some effort to go to 24 V or 48 V, for the same reason that 48 V is now being promoted -- except the problem wasn't as severe or imminent as it is now. That 24/48-V effort failed for many reasons, but the underlying rationale on dealing with the physics constraints versus the load demands are unchanged. If it happens at all, the transition to 48 V will not be easy. The reality is that there is a significant ripple effect from such an apparently modest, easily defined change to a long-established standard. Increasing the nominal voltage affects cabling, connectors, DC/DC converters, small-motor drivers, regulatory concerns, fusing, failure modes, test and maintenance, dealer support, and more. It's a teachable moment to keep in mind when you hear people who don't understand system and product design casually suggest, and with confidence, "what's the big deal? Just go to bigger batteries." Regardless of the voltage rail selected, there is another limit on the electrical subsystem, only this time at the source rather than the current-delivery function. The car battery is not only a source of voltage, it is an energy-storage unit, and a standard lead-acid battery has density of about 40 W-hr/kg and 100 W-hr/liter. Even if you increase the voltage supplied by the battery by using more cells, there will still be issues related to how much energy and power (the rate at which energy is being used) a battery can actually deliver from a given volume, and if the alternator can provide energy and power to keep charging that battery. While non-lead acid battery chemistries (such as lithium-based ones) have much higher densities, the venerable lead-acid has some very favorable attributes (cycles, self-discharge, temperature performance) which make it a better fit in many ways for the basic task of starting the car and keeping its rails powered. Even EV/HEVs, with their high-voltage/power battery assemblies, are not immune to problem of powering all these other loads. You don’t want to be running that much-higher voltage bus around the car just to power accessories, nor do you want to drain the traction motor's batteries power them, among many other practical issues. What's you view on the desirability and practicality of going to 48-V battery packs and subsystems in both IC and non-IC cars? Will it happen this time, due to the inevitable pressures of more loads? Or are the associated problems going to make it a great idea in principle, but an impractical one in practice?