Many of these party lights have ten lights in a string.  Plug the set into
the wall and unscrew one light.  If the lights all go out then the string is
a 'series connection' and likely each party light is a 12v rated lamp.
Reconnect in parallel and you will have a 12v parallel connected system that
will work just fine off a battery.  OR you could use a 100w inverter to
power the little jewels and then you can use a conventional dimmer to
control em.

p