a portable , lightweight and inexpensive emergency power pack to be hooked up to the lighting system of a vehicle trailer , such as a semi - trailer , camper , mobile home or the like , for the purpose of flashing all or some trailer lights on and off while the trailer is unhitched on the side of a road so as to warn passing motorists of its presence . the device includes a battery , an on - off switch , a flasher and an illuminating light . the case for supporting the battery and other components is made of a unitary , molded plastic piece , and includes a cylindrical housing on one side thereof that contains an adapter plug for connecting to a mating plug in the vehicle trailer . auxiliary warning lights may also be strung from a special connector on the unit , while an alternate embodiment may be utilized in the shop to test the various electrical systems on the trailers .