2 Styles of 1/8" plugs are common - stereo (3 terminal) and mono (2 terminal). It is common for the ground to be connected to the housing for audio and does no harm as a negative for a battery system. If you get a stereo plug it will also substitute for a mono plug in a stereo jack. Plug it in and read the voltage off the plug terminals to get polarity for your charger.

John