Predicting capacitor run time for a battery/capacitor hybrid source

Perhaps the most promising near-term application of double-layer capacitors is as a load leveler for a battery. In this application the capacitor is connected in parallel with a battery to form a battery/capacitor hybrid source. The capacitor provides the peak current demanded by the load, thus reducing battery wear and allowing a reduction in battery size and weight due to the lower current-handling requirement. Described in this paper is a procedure to predict the run time for the capacitor when the hybrid source is supplying a pulse power load. The battery is modeled as an ideal voltage source in series with an equivalent series resistance, and the capacitor is represented by an ideal capacitor in series with an equivalent series resistance. The procedure is verified experimentally using a bank of 10 F Panasonic Gold capacitors and a power supply to emulate a battery. This hybrid source is supplying a constant output power of 50 W through a commercial DC-DC converter. Calculated run times are within 10% of the run times measured in the experimental setup.