Street use? Yes, you need a battery to be legal, at least in California -- you would have to check our own State's Vehicle Code to see if you have a similar requirement:
CVC 24253. (a) All motor vehicles manufactured and first registered
after January 1, 1970, shall be equipped so all taillamps are capable
of remaining lighted for a period of at least one-quarter hour with
the engine inoperative. This requirement shall be complied with by an
energy storing system which is recharged by energy produced by the
vehicle.
(b) All motorcycles manufactured and first registered after
January 1, 1971, shall be equipped so all taillamps, when turned on,
will remain lighted automatically for a period of at least
one-quarter hour if the engine stops.