When you use your stereo, is that draining straight from the battery even when the boat is running. This may be a stupid question, but i have heard stories of boats not starting up even when it was off for a half hour and the stereo running.

If you stereo draws more than your alternator puts out it will kill the battery when the motor is running. This is a concern in outboards or older boats or boats with large multi-amp systems running over 3,000 watts.

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. <BR> <BR>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.

Remember though, if the boat is at idle, or slightly above when at cocktail speed, the alternator is not spinning fast enough to put out much. At slower speeds even a smaller system is actually draining the battery while running.