Experimental Characterization of Hybrid Power Systems Under Pulse Current Loads

The electrochemical performance characteristics of a prismatic lithium-ion battery, ultracapacitors, and hybrid combinations at ambient temperature are considered. The effects of pulse current, duty cycle and hybrid combinations are studied. Current pulsing was shown to improved the total energy discharged from the lithium-ion battery and battery / capacitor hybrid combinations over that of a continuous current discharge of the battery alone. The hybrid combination of the 50F capacitor and lithium-ion battery exhibited a 20.3% increase in capacity as compared to a 2A continuous discharge of the battery alone. Furthermore, current pulse discharging improved the maximum current available from the hybrid systems at 10 amps for the 50F / battery hybrid versus 2.4 amps from the battery alone. The hybrid systems also exhibited less voltage swing during the pulse when compared to the battery alone. However, the energy and power considered on a mass basis was lower for the hybrids than for the individual devices. 1.0 Introduction For applications requiring relatively constant loads, either a lithium-ion battery or an electrochemical capacitor are chosen for low and high power applications, respectively. Lithium-ion batteries have the advantage of storing 2-3 times more energy per unit weight (~100 Wh/kg) than conventional technologies (lead-acid, nickel/cadmium). Capacitors have the advantage of providing extremely high power per unit weight (~ 500-700 W/kg). However, when an application demands considerable 3 variations in load (e.g., cellular phone, portable computers, electric vehicles) a hybrid device is needed that combines the high energy of the battery with the high power of an electrochemical capacitor. Such hybrids have been discussed previously [1,2], but no quantitative study has taken place to characterize their energy and power densities under pulsed load conditions. Here we show a series of Ragone plots (i.e., specific power versus specific energy) for a lithium-ion battery connected in parallel with a 5F or 50 F capacitor. The duty cycle (DCYC) of the pulse load is varied, and the electrochemical performance of the battery, capacitor and hybrid devices are compared and contrasted. 2.0 Theory Ragone plots relating pulsed power to a total energy on a specific mass (or volume density) basis is used for comparing the performance characteristics of various electrochemical energy storage devices. The total specific energy is defined as 0 1 () () ST E v t i t dt m      (1) where ST E is the total specific energy, () vt is the …