NiteRider tail light

Is there any reason not to connect the NiteRider Universal tail light to a
generic 12 volt battery? They claim it's only compatible with their
"digital" six and 13.2 volt systems, which presumably use some sort of
voltage regulator -- does that imply that the tail light is a six volt light?

Apparently there was an older, non-digital, 12-volt model. If anyone knows
where I can find one of those, please let me know.

Thanks,

Mark