Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. The concepts discussed in this section are known to the inventors but are not necessarily known to others or previously conceived or pursued by others.
It is generally known to provide typical lead-acid (Pb-acid) batteries for starting, lighting, and ignition (SLI) applications in a vehicle. Such Pb-acid batteries usually have a capacity of about 70 Ah and a voltage of about 12V. The weight of such Pb-acid batteries is typically about 21 kg and the energy density is often about 40 Wh/kg. One performance requirement for Pb-acid batteries for SLI applications is referred to as the “cold cranking current” requirement. Vehicle engine starting requires delivery of a relatively high current within a few seconds, which is especially difficult in cold weather conditions. However, such known Pb-acid batteries, in order to meet the cold cranking current requirement, are sized such that they tend to occupy a relatively large amount of space, and add weight to the vehicle platform.
Another drawback with conventional battery systems is the issue of poor charge acceptance. That is, in certain instances, the battery may not be capable of handling the high charge current, which may have an undesirable impact on the vehicle's energy regeneration capability. Accordingly, it would be desirable to provide one or more advanced energy source systems that are capable of efficiently meeting the cold cranking current requirements for engine starting while being packaged in a smaller and lighter device. Further, it would also be desirable to provide one or more advanced energy source systems that are adaptable for use with components associated with start-stop technology or components of the vehicle (e.g. to permit stopping of the vehicle engine during standstill periods and restart upon demand by the driver), or with components associated with mild-hybrid technology or components of the vehicle (e.g. to provide motor-driven boost or assist in accelerating a vehicle to a cruising speed), and electrical vehicle applications, and in a voltage range of approximately 10-400V, and more particularly within a range of approximately 10-100V.
One known improvement to advanced energy source systems for electric vehicles has been the development electrochemical capacitors to supplement standard Pb-acid batteries. Capacitors can release power more rapidly than a battery, and hence, are able to deliver bursts of power when needed.
Electrochemical capacitors are distinguishable from traditional electrolytic capacitors which store energy by charge separation across a thin insulating oxide film that is often formed by a controlled electrolytic oxidation process at an appropriate metal plate. Electrochemical capacitors are a special kind of capacitor based on charging and discharging at the interfaces of high specific-area materials such as porous carbon materials, or porous oxides of some metals. The porous electrodes create a large effective “plate area.” In the presence of an electrolyte, energy is stored in a diffuse double layer, created naturally at a solid-electrolyte interface, when voltage is applied. The thickness of the double layer is only about 1 nm, thus forming an extremely small effective “plate separation.” As a result, electrochemical capacitors have high volumetric capacitance density (10-100 times greater than conventional capacitors). They can store electric charge and corresponding energy at high densities in a highly reversible way. They offer advantageously fast charging or discharging rates as compared to most batteries of comparable volume, but their energy density is usually less, by a factor of 3 to 4, than that of batteries. Nevertheless, these properties are a valuable complement to standard Pb-acid batteries.
There is, thus, a need for improved electrochemical capacitors for use in advanced energy source systems suitable for use in electric vehicles.