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Select The Right Ultracapacitor Solution
Date Posted: November 05, 2009 12:00 AM
As can be shown when two power sources are connected in parallel, the current draw from each will be inversely proportional to the internal resistance of the devices. In this case, theory predicts that at the instant the ignition key is engaged, the ultracapacitor bank will provide most of the current and will supply approximately 80% initially. The battery will gradually pick up load as the capacitor discharges. However, the initial current supplied by the ultracapacitor bank will start the ICE and reduce the current drain on the battery.
The parallel configuration will present a lower internal resistance as seen by the load. Again using the numbers in this example, the internal resistance of the power source is about 3 m . When the starter motor load is connected to the power source, it will draw about 415 A, providing about 4300 W, which is more than enough to start the ICE.3 Because the internal resistance of the ultracapacitor bank is so much smaller than that of the battery, the maximum I2R internal loss is on the order of 600 W, and the smaller current drawn from the battery reduces the initial I2R loss of battery to about 150 W.
In general, the use of an ultracapacitor in combination with a battery is an excellent way to increase the overall power density of the power source and decrease the strain on the battery. As illustrated in this example, a smaller battery could be used since the available power of the hybrid power source is so much more than required. In any case, where energy storage with highpeak power is required, it is very likely an ultracapacitor device will be useful.
References
1. For volatile memory backup and clock backup power in computers and other electronic devices with small energy consumption, an ultracapacitor can be used without a battery.
2. The energy required is 6750 J = ½ C(V2 upper – V2 lower); C = 2 × 6750/ (12.02 upper – 9.02 lower) = 214 F
3. The currents from the battery and ultracapacitor bank can be calculated more precisely, and the method may found at www.ioxus.com in an application note.