Power distribution control coordinating ultracapacitors and batteries for electric vehicles

Electrical energy storage is a central element to any electric-drivetrain technology - whether hybrid-electric, fuel-cell, or all-electric. A particularly cost-sensitive issue with energy storage is the high replacement cost of depleted battery banks. One possibility to ease the power burden on batteries and fuel cells is to use ultra-capacitors as load-leveling devices. The high power density of ultra-capacitors allows a significant reduction in the power fluctuations imposed on the remaining electrical system; however, the same ultra-capacitors have a very low energy density and therefore must be used sparingly and with coordination. A control strategy for coordinated power distribution is a central issue for ultracapacitor-supported systems. Toward this end, several control methods are implemented on an electric vehicle equipped with a battery/ultracapacitor system with the goal of improving battery life and overall vehicle efficiency. A particular goal is to obtain both a peaking load control and a frequency-weighted coordination between capacitor and battery in order to mitigate transients in the battery current demand. A key control design issue is that the control objectives vary with respect to vehicle velocity, driver's power demand, and state-of-charge of both the batteries and ultracapacitors.