The demand for better and more efficient power sources in the automotive industry has been a driving force behind research in battery technology, capacitor technology, and electronic power supply design. In order to utilize energy as efficiently as possible, automotive engineers began by reducing the gross weight of an automobile by replacing metal trim molding with lighter composite materials and by using lighter metals when possible, such as replacing copper core radiators with aluminum core radiators. These early efforts proved valuable but were just the beginning of better energy efficiency in automotive design. As computer control mechanisms and sensors became more sophisticated, fuel injected systems were developed that increased engine performance and fuel efficiency. The early efforts to make more fuel efficient automobiles focused on how to squeeze more useful energy out of the internal combustion engine.
Alternate power sources were largely ignored due to their expense. One of the more obvious alternative power sources is the battery, but the size and weight of batteries required for an all-electric car were not available, and this remains basically true at the present time. The next best approach was a hybrid power plant, a power plant that uses a small internal combustion engine (ICE) in concert with a battery pack. It is fair to say that hybrid power plants have come a long way in the last five years, however they still leave a great deal to be desired.
The internal combustion engine has remained dominant in the automotive industry because it has very high energy density (gasoline) and very high power density controlled by the rate of fuel ignition. This combination of energy and power density does not exist for batteries or fuel cells. To make the point even more clear, gasoline has an energy density on the order of 45 MJ/kg1 while batteries are, in most instances, more than an order of magnitude less having energy densities on the order of a fraction to a few MJ/kg2. Thus, it is not surprising that the latest entries into the energy efficient power plant still have an ICE component. In order to make batteries and fuel cells more attractive, their overall performance must be increased.
The use of ultracapacitors, while not a total solution to the problem, is nonetheless a partial solution. It is widely known that pairing a capacitor with a battery will improve the power density of hybrid supply, which has the added advantage of allowing the battery to operate without seeing large current spikes that would be present in the absence of the capacitor. The ability to prevent the battery from experiencing these large current spikes under load allows the battery to have a longer effective life. A typical starter battery, for example, will degrade very quickly if it is required to supply high current for any length of time. So-called deep cycle batteries are designed specifically to supply higher currents, but even such batteries with their thicker lead plates are not immune from damage due to repeated deep cycling. A parallel configuration of a battery with an ultracapacitor can dramatically reduce the deep cycling of the battery under heavy load conditions and thus extend the life of the hybrid power supply as well as providing a more efficient supply.
There is, however, more to the story. In most instances it is necessary to construct a “smart” supply; generally speaking it is necessary to do more than just connect a battery in parallel with an ultracapacitor and hope for the best3. The typical ultracapacitor has a voltage rating of only 2.5 to 2.7 volts and for higher voltage applications the capacitors must be configured in series strings for higher voltage stand offs. For example, an automotive application consistent with a nominal 12-volt system would require six ultracapacitors in series for a 15-volt stand off, which is necessary since voltages at that level are used for charging the battery, and it also provides design margin. As voltage requirements rise, a series configuration may not be the most economical approach. In some instances it makes sense to use a DC-to-DC converter, taking advantage of the boost characteristics of a switch mode power converter. In addition to the use of the many topologies available for power conversion using switch mode circuitry, a microprocessor controller may be necessary. For example, in a hybrid power source it is often desirable to disengage the ultracapacitor bank from the main power buss, or it may be desirable to monitor voltage levels on the buss and be able to disengage the capacitor bank in the event of a surge voltage on the buss to prevent damage to the capacitor bank. Obviously, the specific application will dictate the details of what is required.
Making ultracapacitors and designing integrated systems satisfies a customer’s power needs. The techniques available today include the construction of a capacitor matrix to achieve voltage stand off requirements (series string), overall required capacitance (parallel strings), the inclusion of switch mode circuitry for DC-DC, DC-AC, or other combinations, and the control circuitry, most of which is microcomputer based. Through evaluating ultracapacitor solutions, sales and engineering teams can benefit from cost analysis and come up with the best solution for a particular application. The use of switch mode devices and smart controllers extends the usefulness of ultracapacitors far beyond what most engineers are aware of today.
Chad Hall is the COO of Ioxus, Inc. Previously, he spent 14 years with Ioxus’ parent company, Custom Electronics, Inc. (CEI). His extensive mechanical engineering and business experience helped establish Ioxus from funding to factory to launch.
1 http://hypertextbook.com/facts/2003/ArthurGolnik.shtml - Physics fact book on line
2 http://www.energyadvocate.com/fw64.htm - Table - Energy Advocate
3 There are instances when this can be done, however they are low voltage and low power cases, which do not relate, in most instances, to automotive applications.