How to Tell If Boat Battery Is Bad? Bad Symptoms To Look For

A battery is one of the essential parts of a boat. Without a battery, a boat will not start, and it will not power your electricals on the boat. Even after taking proper care of them, they will fail in the end, intimating you to replace them. So, what are the bad boat battery symptoms?

Signs of a bad boat battery are either electronics onboard won’t work, the boat won’t start, voltage shows below 12.4V after recharge, fails in a load test, physical wear & tear. A bad battery can still put 12.6V but can’t hold it for a long time under load, so a clear survey is a must while replacing one.

If you want to know precisely whether the battery is working or not without spending time confirming the warning signs, you need to load test it. The battery may show appropriate voltage with no load and also won’t be able to maintain the same voltage under load.

The simplest way to tell a battery’s condition is to take it to a battery retailer and ask them to load test it. It’s also the least expensive and most reliable technique to find out the battery’s condition quickly than others. Without further talking, let’s see the bad boat battery symptoms in a detailed way.

Related post – How often to replace a boat battery? Check this article to know when to replace a battery on the boat without unnecessarily replacing them.

What Are The Symptoms Of A Bad Boat Battery?

Electronics On The Boat Won’t Work (If Any)

Some of the easy ways to tell your battery on the boat is not working or not is with the help of the electronics onboard (if you have any). All the electronics connected to the power source, such as depth finder, chart plotter, bilge pump, GPS, fishfinder, etc., will not work with a dead battery.

Since all the electronics on the boat are connected to the battery or working with the help of a marine battery, if any of them are not powering up at all, it clearly tells you that it has something to do with the power source (which is the battery) unless if the device is damaged.

However, if you don’t have any electronics on the boat (if the electronics are not in a working condition or won’t work at all), there are many other easy ways to check the battery condition, which we will discuss now.

Related post – Boat won’t start and just clicks? Check this article to know all the reasons why the boat won’t start, and clicks along with a detailed solution.

The Boat Won’t Start With A Bad Battery

Now we are talking about a starter (cranking) battery, not a deep cycle battery that is primarily used to power the electronics on the boat. As we all know, boats use a different battery for starting the motor alone. If that battery fails, it won’t start the engine.

The starter battery is connected to the solenoid, which is connected to the starter, and that is connected to the ignition switch. The moment you turn on the key switch, the battery needs to send power to the starter motor, which is connected to the starter ring gear.

That starter ring gear is attached to an internal combustion engine that is part of the assembly that transfers the torque from the starter motor to the engine’s crankshaft to start the engine. So, if the battery is not working, it won’t send the power to the starter motor, the engine won’t start.

However, there could be many other reasons why your boat engine won’t start, such as bad starter motor, solenoid, bad connections, problems with the boat engine, etc. So just don’t automatically replace a boat battery if the boat engine is not starting; check other symptoms that won’t start the boat as well.

Related Post – How To Tell If Boat Starter Is Bad? Check this article to know more about the bad signs that a bad starter will show and how to deal with them in a detailed way.

Physical Loss (Wear & Tear, Cracks, Bulges, Leaking, etc.)

More than anything else, bad visuals can tell you more, and you can decide on that factor most times. You can decide that by just looking at the battery if the battery has bulged, has cracks on it, leakage, or even bad terminals will cause the battery not to work properly.

Broken or loose terminals are dangerous and can cause a short circuit. There would be some indication of burning or melting when a battery short circuits, which means all the electronics on the boat will not work. That produces a lot of heat and sometimes even causes the battery to explode.

There is a difference between “battery terminals look clean” and “battery terminals are clean.” So remove the clamps and clean the clamps and the posts, then tighten and retest. And you better check the ground connector wire as well. These connections can get loosen easily.

If any of the wires fell right off while disconnecting any of the wires, it needs a replacement. So the visual inspection of the battery is also crucial, and the bulge implies overcharging, and cracks mean need a replacement if it failed a load test.

Drop-In Voltage After A Recharge (<12.4V)

Things mentioned above are the symptoms of a bad boat battery which you will see if the battery stops working. Now we will confirm those symptoms by voltage checking the battery. You can get a good multimeter for less than $20, and you can test it all by yourself. The procedure is as follows.

  1. Charge your battery fully (100%) and let it sit overnight unplugged, and then only perform the test. Otherwise, it will give bad (inconclusive) results due to the charge fluctuations (low charge).
  2. Connect the multimeter’s negative wire (black wire) to the battery’s negative terminal and the positive wire (red wire) of the multimeter to the battery’s positive terminal.
  3. If those values go below 12.4 volts, you need to replace the battery. Do the test after disconnecting the battery.

So, it’s good to carry a small multimeter in the boat. Check this Cernova digital multimeter (linked to Amazon) if you want a multimeter, and it does your job better than most of the premium ones.

Check this short and helpful video on how to test your Boat Battery Voltage with a multimeter to get an illustrated view of the above explanation or procedure.

Fails In A Load Test

A voltage test is a good and quick reference but not the proper way to check batteries. A bad battery can still put out 12.6 volts but not for very long at all. It needs to be load tested to get precise answers. Load testing the battery will clearly give all the insights, and from that, you can decide.

For load testing a battery, you need a battery load tester. If you are doing it occasionally, once every 3 or 6 months, you don’t need to buy a load tester because almost any store that sells marine or RV or other batteries will usually load tests for free. So, you can take your battery and test it there itself.

But ensure to charge your battery fully before doing any tests on that battery. If you want to test the battery often, you can buy one load tester for less than $50. Check this ANCLE load battery tester (linked to Amazon), which can support large amperage (CCA) values, especially for load testing a battery.

That being said, we will now see how to load test a battery by yourself with a battery load tester.

  1. Charge your battery fully (100%) and let it sit overnight unplugged, and then only perform the test. Otherwise, it will give bad (inconclusive) results due to the charge fluctuations (low charge).
  2. Connect the negative wire (black wire) of the load tester to the battery’s negative terminal and the positive wire (red wire) of the load tester to the battery’s positive terminal.
  3. Select one-half of the CCA value on the load tester (you can see the CCA value on the battery, if you can’t find it, you can contact them).
  4. The test is performed for 15 seconds (the load is applied for 15 seconds). Check the voltage values and CCA values throughout the test for 15 seconds.
  5. During the test, if the CCA values on the load tester go below 80% of one-half of the battery’s CCA value, and if the voltage values go below 12.4V, it’s time to replace your boat battery.

Below is a table showing the voltage values of a 12V battery (all types). They determine the voltage levels at different DOD levels (source).

State of ChargeFlooded Lead Acid battery voltage(in volts)Gel battery voltage(in volts)AGM battery voltage(in volts)

After completing the whole test, wait for 1 or 2 minutes and perform the test again (multiple times) to get the best results out of it. Then finally, decide upon those values. Some load tester will directly tell whether it is good or bad. Like ANCLE load battery tester (linked to Amazon), it displays on its display whether the battery is good to use or not.

If you want to (load) test your battery quite often (once every month), it is highly recommended to buy a load tester, and you can get one for less than $50.

Check this helpful video on testing the boat batteries with a load tester to get an illustrated view of the above explanation or procedure.

Some Of The Easy Ways To Increase Your Boat’s Battery Life

There are some simple ways to improve your boat’s battery life. By doing them, your battery could last 7-10 years in total. We will now see how to increase your boat’s battery life.

Don’t Deeply Discharge A Boat Battery

Discharging a boat battery deeply often (75% or 50% or less) will decrease the battery’s total number of cycles dramatically. When a boat battery has been subjected to deep discharge, the amount of electricity that has been discharged is actually 1.5 to 2.0 times as great as the battery’s rated capacity.

One more thing to consider is a battery that has been deeply discharged takes a longer time to charge it fully (100%) than a normally discharged battery (not deeply). The occasional dip to 75% or 50% DOD (Depth of Discharge) is fine, but you need to try to recharge it up with 100% ASAP.

Generally, 50% is the max recommended DOD by most all AGM & flooded battery manufacturers. So, make sure you won’t get below that quite often. The less deep discharge you regularly go, the more number cycles you will get, which implies more life (years).

Fully Charge And Disconnect Them During A Long Term Storage

Fully charging a boat battery (100%) before storing it away for any long-term storage or winter is crucial. Any battery will self-discharge over time, and you can’t stop it from happening that. If it is not fully charged, it will hurt the battery’s life eventually, and it could die quickly.

And another thing to consider here is disconnecting the battery during the winter or long-term storage is crucial. If a battery is not disconnected, all the electrical devices onboard will slowly discharge the battery over time, even if they are turned off. So, disconnecting the boat battery during long-term storage is essential.

So, pull them out for winter storage, and keep them in the garage on a wood platform or a rubber platform for most of the winter. Or just disconnect them and leave them on a boat but charge them fully before leaving and using them.

Don’t Leave A Battery On A Charger Continuously

Charging a battery is essential, but overcharging it will kill the battery over time. This is one of the main reasons why the flooded battery will bulge outside. Overcharging a battery causes excessive gassing — the electrolyte gets hot, and both hydrogen and oxygen gas are generated.

On older vented batteries, the electrolyte could cook away, leaving the plates exposed and ruining them. On sealed batteries, the buildup of gases could cause the battery to burst. Even if you leave the charger connected continuously, at a small 2 amps, the battery eventually will die.

So, don’t put your battery on the trickle charge or any charger continuously. Many chargers are not as “smart” as they claim. Charge to full, then disconnect is often safer. Float or maintenance chargers such as a Battery Tender keep the battery charged without the risk of damage.

Don’t Keep A Battery In Hot Places

The temperature will affect the boat battery most because high temperatures will not only be a loss of battery’s capacity but also leaks or ruptures the battery as well. Store your batteries at room temperature or below. The recommended storage temperature for most batteries is 59 °F (15 °C) (source).

As temperatures increase, especially over 100 °F (38 °C), so does your batteries’ internal discharge. If a new fully charged battery is left sitting 24 hours a day at 110 °F for a month, it would most likely not start an engine. Cold weather isn’t nearly as hard on batteries as hot weather.

The chemistry just does not make the same amount of energy in bad conditions. A deeply discharged battery can freeze solid in sub-zero weather. So, store your batteries well during extreme winter and hot summers to avoid any problems.

Fully Charge A Battery As Often As Possible

While using your boat every day, the battery discharges. Fully charging the battery will improve the number of cycles. However, the number of cycles won’t increase beyond the manufactures max cycle count. Using a battery at lower DOD (50% and less) will damage the battery and puts more load than a fully charged battery.

It will eventually degrade the battery’s life if it is used while deeply discharged. Weekly or biweekly, getting your battery up to 100% as a minimum guideline will dramatically improve cycle life. So, charge it as often as possible, but ensure to charge it fully weekly or biweekly.

It’s OK to use a battery between 50% or 60% DOD and 80% or 90% DOD while cruising. But then try to get a good full 100% overnight charge at least once or twice a week.

Check Electrolyte Regularly On Flooded Batteries

Whenever you are working with a battery, you’ll want to take some safety precautions. Not only can the battery give you quite a shock, but it can also become dangerous if not handled properly. Ensure you have gloves and safety goggles and disconnect the battery before you start messing around with it.

First, clean the battery with a brush and a mixture of baking soda and water near the terminals and top. Then remove the plastic tops covering the cell ports with the help of a screwdriver or any. Once the covers have been removed, carefully clean away any dirt that may have built up underneath.

Now check the electrolyte levels inside each cell and add only the distilled water, not any other water form if electrolytes are not up to the level. A common rule of thumb is to add enough water to cover the electrodes or plates. Then cover the cell ports and charge it fully and then test whether it is working properly or not.

Bottom Line

The electronics on the boat won’t work, the boat won’t start properly, voltage drop below 12.4V even after recharge, fails in a load test, and physical wear & tear are the symptoms of a bad boat battery.

If you are unsure about it’s working, an easy way is to do a load test. The battery may show appropriate voltage with no load and also won’t be able to maintain the same voltage under load. If you don’t know how to load test it, you can take it to a battery store, and most of them will do it for free.


My name is Mahidhar, and I am passionate about boating. Every day I learn some new things about boats and share them here on the site.

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