It’s important to check your marine battery before taking off and after it’s been sitting for a couple of weeks or months. One in ten calls for help happens because of a dead battery or other electrical problem.
The voltage your marine battery should read when fully charged depends on the size of your motorboat’s battery. A 12-volt marine battery should read 14.4 volts when fully charged, while a 24-volt marine battery should read 28.7 volts when fully charged.
With that said, there are other important facts about marine batteries you should also keep in mind. This includes the difference between the voltage of a full charge battery and float voltage.
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How Do You Tell if a Marine Battery is Fully Charged?
Most people read the voltmeter gauge located at the helm or boat’s steering wheel, to tell if a marine battery is fully charged. However, this is not always the most accurate reading.
Expert boaters use what’s called a digital multimeter tool that attaches directly to the marine battery. To use this handy tool, simply power it on and set the meter to DC volts. From there you’ll place the probes onto the battery terminals and note the reading.
Your most accurate reading of a marine battery is by checking it under load with the digital multimeter tool. Have someone crank the starter while you monitor the multimeter on the battery terminals.
If the reading on your multimeter drops below 9.6 volts while under load, then it’s a telltale sign that your battery is taking its last few breaths.
What Voltage Should a Marine Battery Read When Fully Charged?
Most motorboats run on a 12-volt marine battery. So for a 12-volt marine battery, it should read 14.4 volts when fully charged.
Now with that said, it’s important to remember that a 12-volt battery will only be over 14 volts for a few seconds while it’s being charged. After your marine battery is fully charged it will naturally decrease a couple of volts which is considered the float voltage.
Below is a chart that shows you the number of volts on a 12-volt battery and what they indicate.
| 12-Volt Battery Range | What the Volt Range Indicates |
| 12.65 – 12.77 volts | The battery is fully charged |
| 12.45 – 12.54 volts | 75 percent battery charge |
| 12.24 – 12.29 volts | 50 percent battery charge |
| 11.99 – 12.06 volts | 25 percent battery charge |
| 11.75 – 11.89 volts | The battery is almost dead |
What should a 24-volt marine battery read when fully charged?
If your battery is twice the size, meaning you have a 24-volt marine battery, then it should read 28.7 volts when fully charged. Similar to what we said about the 12-volt battery, a charge over 28 volts will only be there while the battery is charging.
Once the battery is no longer charging, it’s natural for it to dip down a couple of volts. Similar increments in volts as the 12-volt battery can be applied here.
On average, a 24-volt marine battery should read approximately 24.96 on a multimeter tool when it is fully charged. On the flip side, a 24-volt battery voltage is empty at approximately 21.96 on a digital multimeter.
What Should a Float Voltage be on a Marine Battery?
According to MasterVolt, your float voltage is determined by the type of marine battery that you have. There are three lead-acid batteries and one lithium marine battery. The three lead-acid battery types are wet cell, gel cell, and absorbed glass mat (AGM).
Float Voltage for Gel Cell Marine Battery
The float voltage for a gel cell battery is 13.8 volts for a 12-Volt battery. For a 24-Volt battery, you are looking at a float voltage of 27.6 volts.
Float Voltage for AGM Marine Battery
Both the gel cell and AGM marine battery have the same float voltage. So, the float voltage for an AGM battery is 13.8 volts for a 12-Volt battery and 27.6 volts for a 24-Volt battery.
Float Voltage for Wet Cell Marine Battery
The float voltage for a wet cell marine battery is 13.25 volts for a 12-Volt battery. For a 24-Volt battery, your float voltage will be 26.5 volts.
Float Voltage for Lithium Marine Battery
For a 12-Volt lithium marine battery, the float voltage will be 13.5 volts. On a 24-volt lithium battery, the float voltage is 27 volts.
| Type of Marine Battery | 12-Volt Battery Float Voltage |
24-Volt Battery Float Voltage |
| Gel Cell Battery | 13.8 volts | 27.6 volts |
| AGM Battery | 13.8 volts | 27.6 volts |
| Wet Cell Battery | 13.25 volts | 26.5 volts |
| Lithium Battery | 13.5 volts | 27 volts |
What Voltage is too Low for a 12-Volt Marine Battery?
A voltage level below 11.6 volts should be your cutoff for a 12-volt marine battery. This holds true for all three lead-acid battery types. Operating a battery at 11.6 or lower can damage your battery for good.
The absolute lowest level that a 12-Volt marine battery can go under load is 10.8 volts. However, just because you can run that low, doesn’t mean you should. Stick with the 11.6 volts to be safe!
Like we mentioned before when your multimeter drops below 9.6 volts while under load, then your battery is practically dead.
Conclusion
We hope we’ve answered your questions regarding what voltage a marine battery should read when fully charged. It’s valuable to remember that a full marine battery charge is at its highest level when being charged and then it dips down a bit in its floating charge.
Whether you own a 12-volt or 24-volt marine battery, always make sure to test the charge of your battery before you leave the docks. Stay safe out there folks!
Amazon best-selling author & coauthor, Misty Compton is an outdoor enthusiast, raised on water sports, fitness, and writing. CEO of two companies, Daggerwing Publishing House LLC and Organized Otter Administrative Support LLC, she writes and edits content providing great reads, and assists companies remotely by helping them get organized.