like mikeski mentioned, whenever the motor is running and the alternaotr is running the stereo will ALWAYS draw FIRST from the alternaotr, and only go to the battery for reserves if the alternator cannot supply enough juice. Remember the battery is also a load on the alternator just like the amp. If you run an outboard the charging systems in it are very weak and only meant to run lights and a little bit of equipment like fish finder etc, so you have to use more effecient setups with less power and better box design if using a sub, or run more batteries and keep them charged while parked at the dock with a trickel charger.
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