Marine Starting Battery

When you’re out on the water, the last thing you want is to be stranded due to a failing marine battery. Your boat’s battery is a crucial component, powering everything from the engine to essential electronics. Recognizing the signs of a failing battery and knowing how to address the issue can save you from potential frustration and disappointing your guests. In this article, I’ll guide you through the telltale signs that your marine battery might be on its last legs, and provide practical solutions for getting your boat back in action if your battery does fail while you are on the water.

Please note that I have included links to some products referenced in this article for your convenience.  If you click on a link and buy something, BoaterInput will receive a small commission to help defray some of the cost to maintain this site. 

Signs of a Failing Battery

There are several different indicators of a failing battery.  Here are some of the key things to watch out for.

Visual Clues

Corrosion – Corrosion build-up on your battery terminals will reduce performance and may eventually cause your battery to fail. Clean by applying a baking soda + water paste with a toothbrush.  Apply petroleum jelly or terminal spray to strengthen the connection and inhibit corrosion.

Bulging/swelling – the cell plates inside your battery might expand due to overcharging or shorting of the terminals. If you see this, it is definitely time to replace your battery.

Leaking – a battery may leak for several reasons including overcharging, over filling, exposure to extreme heat or cold, leaving the battery discharged for a long period of time, excessive vibration or simply physical damage to the battery case. If the leaking is fairly minor around the terminals or caps, try cleaning the battery and monitor it to see if the problem persists.  If the leaking is fairly extensive or continues even after cleaning, it is likely time to replace the battery.

Performance Indicators

Slow Starting – perhaps the most obvious sign of a failing starting battery is that the engine cranks slowly or fails altogether when trying to start your motor.

Doesn’t Hold Charge – If your deep cycle battery draws down quicker than before, you might consider replacing it before long.

Intermittent Electronics – If installed electronics such as your fish finder or Multifunction Display (MFD), stereo or VHF radio flicker or turn off, this could mean either a failing battery OR a faulty connection between the battery and the device.  To determine which one it is, use a multimeter tool to check the voltage at the battery itself and at the connection to the electronic device.


Open-Circuit Voltage check – A multimeter is a simple and inexpensive way to check the voltage of your battery. Ideally, you should check the battery when it is fully charged and disconnected from terminals (or turn your battery switch to OFF if you have one installed on your boat).  Set the multimeter to DC volts and place the black lead on the negative battery post and the red lead on the positive post.   A reading of 12.6 V is a healthy, fully charged battery.  12.2 V indicates a 50% charge and a reading below 11.7 V is a fully discharged (i.e., “flat”) battery.  If you are unable to bring your battery back to a healthy range (12.5-12.6 V) after charging, then it may be time to replace your battery.

Load test – While the open-circuit test will show the batterie’s voltage potential, a load test measures how the battery performs under load such as when starting your engine or running a trolling motor. Most auto parts stores will do a load test for free.  However, if you don’t have an auto parts store nearby or simply prefer to do it yourself, you can purchase your own battery load tester like the one shown below (which will apply a 100 amp load).  However, one important word of caution.  A load tester like this is designed to create resistance and will heat up much like an electric stove top.  Therefore, you should remove your battery from the boat before performing the load test to avoid potentially igniting any gas fumes in your bilge.

How to Prolong Battery Life

There are a few steps you can take to help your batteries have a good, long life.

Keep Your Battery Charged (but don’t over charge). Leaving a battery in a discharged state is hard on batteries and will shorten its life.  However, overcharging a battery is harmful too.  This is where it is important to know the difference between a trickle charger and a Smart Charger or Battery Maintainer.  The former will apply a small, continuous charge but will not automatically turn off.  The latter tracks the state of charge and will “govern” the charge rate and even move to “float” when the battery is fully charged.

Below is an “on board” and stand alone smart charger that are very popular and receive excellent reviews on Amazon.  Both work with most battery types, including Deep Cycle and Lithium (LiFePO4).

Don’t Draw it Down Too Far.  If you draw your battery down too far, it might not be able to recover or you may reduce the number of charge cycles available.  I experienced this a little while ago when I had engine troubles and had to travel a fairly long distance with my trolling motor. One of my batteries would not recover after charging and so I had to replace both.

Recharge Quickly.  If you draw your battery down and leave it that way for an extended period of time, crystallization forms on the battery plates which is called sulfation. When this occurs, it may permanently reduce the performance and life of your battery.

Avoid Extreme Temperatures. Excessive heat is especially hard on batteries and may shorten their usable life. But extreme cold can be harmful too – especially if the battery is stored in a discharged state which will make it more susceptible to freezing and permanently damage your battery.

Minimize Vibration. The plates inside a battery are fairly brittle and could crack or break with excessive vibrations.  AGM and Lithium batteries are less susceptible to vibration damage than standard lead-acid batteries but, at the very least, make sure your batteries are secured properly inside your boat.

Perform Periodic Maintenance. Besides keeping your battery properly charged, most lead-acid batteries (if not sealed) should be topped off with distilled water periodically to replace what had evaporated during the recharging process.  Also, corrosion on your battery posts will impact performance.  If you see corrosion, you can clean it with a wire brush or by using a baking soda/water paste to neutralize the battery acid.  You can also use this slick little tool to quickly clean your posts.

Once you clean your battery terminals, apply petroleum jelly or this special protective grease to inhibit corrosion.

For an extra level of protection, add these inexpensive felt washers to your battery posts to prevent corrosion from forming.  Just place the red washer on the positive post and the green on the negative.

Properly Store Battery During Off-Season. If you will not be using your boat for an extended period of time, you will want to take steps to protect your batteries.  This is especially important since batteries will naturally discharge over time.  At the very least, you should disconnect your batteries (if not stored in a wet slip) to avoid any parasitic draws, store them in a cool, dry place, and keep them fully charged (preferably with the use of a Smart Charger).  Check out this article on How to Properly Maintain Your Boat Batteries when not using your boat over winter or for an extended period of time.

What if Your Battery Fails on the Water?

Let’s say you were rocking the tunes at the beach or sandbar and now your engine won’t start, what do you do?  Before flagging down another boater for the dreaded tow back to the ramp, you might have a couple of options if you are prepared.

Give Yourself a Boost. This powerful little device can deliver up to 1000 amps to help restart a motor up to 6.0 Liters in size.  Plus it is water-resistant and spark-proof making it safe for marine usage.  Just connect the positive (red) and negative (black) leads, turn on the NOCO boost, and then start your engine when the boost LED is illuminated.  As an added bonus, you can even use it as a flashlight or to recharge your mobile devices.  This is a great and fairly inexpensive “insurance policy” to keep aboard your boat.

Use Your Switch to Combine Batteries. If your boat is equipped with a separate “starting” and “house” battery, you can easily combine these to start your boat in a pinch if you have the proper switch installed.  Just turn the switch dial to “All” or “1+2” or “Both” to draw from both batteries to start your motor.

Give Yourself a Jump Start. If you don’t have a switch but your boat is equipped with multiple batteries, you might be able to get a jump from one battery to another using a pair of jumper cables.  This is what we did when my neighbor’s boat failed after a day of fishing.  In this case, Mark jumped from ONE of his deep cycle (trolling motor) batteries to the cranking battery to start the motor.  However, one strong word of caution is that if the jump source batteries are banked (as is typically the case with trolling motors), be sure to disconnect and isolate ONE of the batteries to use as your jump source.  This is because most trolling motors run on either 24v (2 batteries) or 36v (3 batteries).  If you connect this to your cranking battery, you could cause severe damage to your motor which operates on 12V.

Get a Tow. If all else fails, you will have little choice but to flag down another boater or, if you happen to be on the ocean or in the gulf, contact Sea Tow to drag you back in.  If you are getting towed in by another boater, you should attach the line to your front eyelet (used to winch your boat onto the trailer) – not your cleats.  This is a stronger connection point and will pull your boat slightly upward rather than dragging the bow down into the waves.  For the tow boat, the line should be attached to a center tow ring (not an arch or tower) if available.  If the tow boat has neither and you need to travel a considerable distance, then the ideal approach is to use or create a bridle that attaches to both stern “eyes” to help balance and center the load.  To create a bridle, put a loop at the end of the main tow line from the disabled boat.  Then, pass a shorter line (about 2X of the beam of the tow boat) through the loop and attach to both stern eyes.  Below is an image of a pre-made towing harness (used for watersports) to give you an idea of what a bridle looks like.

By following these tips, you should be able to identify the early warning signs of a failing battery to help you avoid getting stranded on the water.   However, there are times when your first sign of a problem is when your engine will not start.  That is why investing in a portable battery booster, which you can use in your boat or vehicles, is something that every boater on mid-sized to larger bodies of water should carry.


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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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