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Make Sure Your Boat’s Battery Are Fully Charged Before Storing It For The Winter.

Did you know discharged batteries can sulphate or even freeze to death if not left in a fully charged state before extended lay up. Many a good battery has been destroyed by being left in a discharged state for extended periods. It’s easy to check to see what state they are in, all you need is a simple multi-meter, or if your boat is equipped with one, the installed voltmeter will do.

There is a direct correlation between battery state of charge and a battery’s specific gravity, which is a measure of the electrolyte strength and charge level. The table below shows the correlation between what we call “open-circuit voltage” and percent charge. You never want to let your batteries fall below about 75% state of charge during an extended lay up.

  • 12.6 volts+ = 100%
  • 12.3 volts =    75%
  • 12.2 volts =    50%
  • 12.0 volts =    25%
  • 11.7 volts=      0%

If your boat doesn’t have an installed voltmeter, or its sort of vague (tens on a volt make a big difference here), connect a voltmeter set to DC volts scale as shown in the diagram below, from my book, The Powerboater’s Guide To Electricity.


Open Circuit Voltage

Once you are sure your batteries are fully charged, it’s a good idea to disconnect them from the boat by disconnecting the negative terminal connection. Make sure you don’t leave them hooked up to a charger all winter! This can be dangerous, and will damage your batteries. Once charged, the self discharge rate of the batteries will keep them up to snuff until June for sure.

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