But when I turned of the engine, I got this warning "Battery discharge warning. Please use the system with engine running" hence I asked this question.
That warning comes up every time "something" is pulling power when the car is off. My dash-cam gives me that warning every time I turn it on and it has been installed for quite some time now.
Eventually, yes but you would have to leave it on for a very long time, without driving the car... I'd toss a small switch in the circuit but only because leaving lights on bugs me...