For the past several years automotive electrical and electronic functions and features have driven the vehicle power budget up by 100 W to 140 W per year. These loads are coming from electronics in vehicle dynamics, telematics and entertainment systems. The automobile's primary electrical energy storage component, the starting-lighting-ignition (SLI) battery is experiencing higher demands for peak power and continuous loading because of these trends. It is well known that power cycling reduces battery life and shortens its warranty interval, even in benign temperature environments. Cycling and load switching contribute to electrical noise on the vehicle PowerNet, which has the potential to corrupt the operation of the 30 to 40 or more microprocessors and microcontrollers in today's vehicle, as well as communications links between the distributed controllers.
Enter the ultracapacitor, a member of the electrochemical capacitor (EC) energy storage component category, which stores its charge through dissociation of electrolyte salts in the form of an electrochemical double layer capacitor (EDLC or DLC for short). What is unique about the ultracapacitor is that it stores energy in the same form that it will be used — as electricity. Unlike the electrochemical, or Voltaic cell that relies on mass transfer, a Faradaic mechanism that leads to wear out, the ultracapacitor has no such wear out mechanisms and stores its charge in an electrostatic field.
However, the ultracapacitor possesses two fundamental differences from these conventional capacitors: For one, it has phenomenal surface area available in its highly porous carbon electrodes on which to store charge. Maxwell's BOOSTCAP® ultracapacitor carbon electrodes possess electrode surface areas of nearly 3000 m2/g (0.3 hectare). To put this into perspective, consider that 1 kg of such activated (i.e., highly porous) carbon has one square mile of area. Second, the charge separation in the ultracapacitor is one half a molecular diameter (~3 nm). Capacitance is surface area divided by charge separation distance, which, for this illustration is 1012 (3×103/3×10-9). Ten raised to the twelfth power is extraordinary and this is what puts the “ultra” into ultracapacitor. The ultracapacitor is a very large and efficient accumulator for electrons.
The automotive power challenge has been how to deal with electrical consumers having high peak to average demand. When peak power exceeds average power by a factor of three the alternator will no longer be capable of sustaining the load. The alternator is sized to meet vehicle average loads. For peak load demand, the alternator is no longer capable of sourcing the full load current and consequently the vehicle PowerNet voltage drops as the storage battery is called upon to make up the difference. This is the battery cycling that causes wear out.
The ultracapacitor in a vehicle PowerNet application is up to the challenge because any variation on the PowerNet voltage supply will be met with an immediate discharge of accumulated charge from the ultracapacitor to the electrical consumer or an immediate recharge.
With appropriately placed ultracapacitors as distributed modules, these PowerNet voltage transients will be absorbed. One of the ultracapacitor modules' benefits to vehicle electrical distribution systems is that it will inherently stiffen and smooth the voltage supply without incurring the added weight or warranty of additional batteries.
Ultracapacitors are real and ready. Costs are competitive with any battery system in terms of cycle life energy throughput. The ultracapacitor is an “install and forget” component. As hybrid electric vehicles become pervasive in the marketplace, the power challenges noted in this article are exacerbated and the consequences of PowerNet variability are more pronounced. In summary, the challenges facing automotive electrical systems will benefit from ultracapacitor distributed modules.
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ABOUT THE AUTHOR
John Miller is vice president advanced transportation applications at Maxwell Technologies and a former research engineer at the Ford Motor Company where he worked on electric and hybrid vehicle programs.