How to Maintain Batteries

Most boaters know of the importance of prepping your engine in advance of the winter season.  But taking care of your battery is important too.  In this article, I address some common questions regarding winter battery maintenance and offer some tips for how to protect your batteries to increase their longevity and to help ensure they will be ready to go next spring.

What are the Risks?

All batteries naturally discharge when not in use.  For example,  traditional lead-acid batteries in good condition will discharge at a rate of approximately 3%-5% per month. The rate is considerably less for AGMs and especially lithium batteries.  (It should be noted, however, that the rate of discharge for all battery types will be considerably higher in warm environments since heat is harder on batteries than cold.)

Over time, the accumulated discharge could be substantial which could harm your battery or shorten its life.  Generally speaking, it is a good idea to keep your battery near full capacity during the off season.

Is It Ok to Leave the Battery in My Boat Over Winter?

Whether or not it is Ok to leave your battery in the boat or store it elsewhere depends on how long your boat will remain unused and whether or not you have the proper battery charger.

If you only plan on being away for a short time – say 1 to 2 months, then you are likely fine to keep your battery in the boat.  However, if it is not connected to a charger, then you should disconnect the terminals so that there is no parasitic draw that could prematurely discharge the battery.

Also, if you have a battery maintainer or tender (also called a float charger), just connect your battery and you are good to go.  As the name implies, the maintainer will keep your battery at, or very close to, full charge throughout the winter season.

On the other hand, if you will not be using your boat for a considerable time (say, 3 months+) and you don’t have a battery maintainer, then it would be best to bring your battery indoors and keep it in a cool, dry location.  More on this a little later.

Is a Battery Maintainer and Trickle Charger the Same Thing?

A battery maintainer constantly monitors the state of your batteries and will alter the rate of charge based on the discharge level.  Once your battery is fully charged, the unit will cease charging and go into “float” mode.  However, many older trickle chargers simply deliver a slow, steady current and will keep going even after the battery is completely full which could cause damage.  The key is to determine whether the trickle charger you intend to use is “Smart” and will stop charging once the battery is at 100%.

Here is an example of a quality battery maintainer that gets excellent reviews on Amazon and is similar to the one installed on my fishing boat.  Made specifically for boats, this maintainer is waterproof and works with a variety of battery technologies including lead-acid (flooded, AGM) and lithium.

How to Properly Maintain Your Boat Battery Over Winter

As mentioned above, if you have a battery maintainer or Smart Trickle Charger, all you need to do is connect your battery.  Of course, this is also a good time to make sure your battery terminals are clean/corrosion free and that the fluid levels are appropriate.

If you don’t have a battery maintainer or don’t have power at your boat storage location, then you should do the following:

  • Disconnect your battery. This will prevent any parasitic draw from occurring and will make it easier to care for your battery over the winter.
  • Clean your battery terminals. This will make it more efficient to charge.  Baking soda and water and a wire brush is good for this.
  • Check your fluid levels.  If your battery is not sealed or “maintenance free”, pop the cap to check the fluid levels.  If low, add some distilled water but be careful to not overfill as this could bubble out when charging.
  • Bring the battery up to full charge. Attach your charger to the battery to restore it to full power.  Disconnect when finished (if using a standard charger or trickle charger that is not “smart”).
  • Store your battery indoors in a cool, dry location. Ideally, the location should not be susceptible to sub-freezing temperatures because it is possible for batteries to freeze if they are in a discharged state.  And a frozen battery is not a happy battery.
  • Top Off Your Battery Periodically. Every couple of months or so, put your battery on a charger to get back to full strength.  Just remember to disconnect when finished to avoid cooking the battery.

Can You Store Batteries on a Concrete Floor?

A common misconception is that you should not store batteries directly on a concrete floor because it could discharge them.  Because of this belief, it is often suggested that you should put a block of wood or some other insulating material underneath the battery to prevent discharge.

While this might have been true years ago when battery cases were made of inferior materials, it is not an issue with modern batteries.  So, feel free to store your battery on your basement or garage floor over the winter – provided that it is dry and not subject to sub-freezing temperatures.

Follow these tips and your battery and your budget will be happier next Spring.

Please note that I provided a link to a product that I personally have on my boat (NOCO Genius Battery Maintainer) for your convenience.  If you click on the link and buy the listed product or any other, I will receive a small commission that will help support BoaterInput.


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Can You Leave Marine Batteries in a Boat Over Winter?” Web Blog Post.  Battle Born Batteries.  30 March 2022.


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Jerry Mona - BoaterInput

About the author

Jerry Mona is an avid boater and angler and long-time boating industry insider. With over three decades of experience, he is often considered to be the leading research expert with boaters and has helped numerous manufacturers and trade associations to understand the needs, wants, attitudes and behaviors of boaters. He now shares many of his insights about boats and boaters for free on his website.

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