Depends on how its wired, some systems maybe linked to the ignition while others maybe constant draw. A battery disconnect is suppose to open the circuit on the positive side which would stop all draw from the battery its connected to. best to look at what else is connected at the terminals.
Example would be if you have a new stereo system which requires a constant 12v battery supply to keep memory, another 12v supply that can be switched or wired, and constant ground. the memory wire will be a direct connect to the battery (or at least fuse terminal with battery for constant voltage), the switchable 12v could go though acc. switch / or ignition switch and in some cases also connected directly to a fusible terminal.
