Gone Outdoors

What Causes My Boat Battery to Go Dead Overnight?

by Will Charpentier

When the boat's battery is fully charged on the day you go boating, but as dead as a doornail the next morning, there are four potential trouble spots. These four trouble spots include an external short, an improper motor shutdown, the battery selector switch, and, the most expensive, but easiest to fix, a battery that's so far past its prime that it won't hold a charge. One requires a repair, two are cured by a small change in behavior and the last requires a replacement.

External Short

On boats with a generator or power inverter, there's a green ground wire that links the DC ground block to the AC system. This wire is intended to prevent a high-voltage system from grounding to the water that surrounds the boat. If this wire is missing, disconnected or too small to carry the charge, the boat's entire electrical system will ground to the water, in what marine engineers call a "dead short," depleting the battery quickly.

Pulling the Kill Switch

If you routinely shut an outboard motor off by pulling the lanyard that links you to the motor's kill switch, instead of turning the key to the "Off" position, this is the same as leaving your car's ignition in the "On" position all the time: The battery will drain well before "overnight." Although the motor stops, the boat's battery is still in the "On" position, and, as in the case of the car, the battery will die long before the next sunrise.

The Battery Selector Switch

If your boat is equipped with multiple batteries -- one to start the motor another to run the on-board entertainment -- it's also equipped with a battery selector switch that allows you to switch between the two batteries. The battery that's not in use is charging as long as the motor runs. The selector switch also has a "0" position so that neither battery runs, and both can charge at the same time. If you fail to set the selector switch to the "0" or "Off" position after heavy use, when the battery isn't needed and should be charging, you'll find the battery depleted in the morning.

Bad Battery

Sadly, old batteries eventually lose their ability to accept and hold a charge; when this happens, it's time to turn them in for recycling and purchase a new battery. Check the "expiration date" on the battery -- if the battery is at or near that date and won't hold a charge overnight, odds are you'll need to replace the battery before your next outing.


  • "Evinrude Repair Manual -- 2.5 to 250 HP Models, 2002-2007"; Seloc Marine; 2007

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