dead battery

[DOODLEBENJI]

recently tried one of these units that plug into the 12v socket on a running car and one to the other car with a flat battery which is supposed to give enough charge after about ten minutes to start the car with the flat battery,this unit failed but could have caused my other car to have problems has any one tried these units ?

[T_Pot]

It was my understanding the unit holds charge and plugs into the lighter socket of the flat car, there is not another car needed ????

[DOODLEBENJI]

THATS THE ONE TYPE THE ONE I HAVE NEEDS 2 CARS