9V Battery to DC Barrel Plug 2.1mm Internal center negative. Cable for connecting 9V batteries to pedals via their DC input jack. This cable is great for using batteries with pedals that do not have a battery compartment or for quickly switching from a power supply to a battery. This cable is 3 1/4” inches in length. The barrel plug has a 5.5mm outer diameter and 2.1mm inner diameter. Battery is optional.