Hebner reckons that batteries are very reliable technology, but as any parent the day after Christmas knows, batteries wear out. Even rechargeable batteries eventually loose their capacity. These store and release energy on a continuous basis. The versions that the UT engineers are looking at are designed to fit in the existing space provided for a chemical battery. They also have a greater storage capacity than their chemical cousins and could easily last the lifetime of the space station itself.
Flywheel batteries are a cleaner and more economical source of battery power than chemical models.
The flywheel is a simple technology, as old as the potter's wheel, though the models under scrutiny for the ISS are slightly more advanced. They are made of a composite material, mounted on a titanium shaft and levitated on magnetic bearings. The spinning wheel stores the energy coming in from solar cells by revolving faster and faster. Energy being withdrawn at any time has the opposite effect, slowing the madly-turning spindle. The flywheel that would be used in the battery on the space station is about 1.5 feet in diameter and nearly three feet long.