I would say -5 is safe. I also agree the -15 is horrible to have on the disc, I have had several calls about people smoking there subs. The first response the give when I explain clipping is " I set my gains with the smd dd1". A bad ass product does not fix poor judgement.
The charger will kick it self off while you playing as It is seeing a load and thinks there a problem with the battery. I,know Nathan will chime in with a more technical reason, but you get the idea.