A good marine battery is essential to make the most of your time on the water.
AGM Batteries are still the most popular choice because they are sealed and virtually maintenance-free.
In this article, you’ll learn about the pros and cons of AGM batteries and tips on how to choose the right one for your boat.
Let’s get started.
8 Things to Consider When Buying AGM Batteries
1. Advantages of Using AGM Batteries
Lead-acid AGM batteries are built with a well-known and safe technology. They’ve been constantly improved since the 1980s, and they can last many seasons if looked after properly.
They’re also relatively inexpensive if the alternative is using lithium batteries.
Another great thing about them is that you can charge them with a regular car battery charger.
2. Downsides of AGM Batteries
If you have ever carried a car battery, you know how heavy lead-acid batteries are. AGM batteries are just as heavy or even heavier if you opt for a larger capacity.
AGM batteries can’t be discharged below 50% of their capacity without a significant drop in their lifespan.
Good AGM batteries will give you 200-300 charge/discharge cycles, which you can consider good enough, depending on how often you go out fishing. However, lithium batteries can do even over 3000 cycles, which means you don’t have to worry about buying new batteries for a long time.
Another important limitation is that AGM batteries have to be stored fully charged. Otherwise, they will quickly start deteriorating and losing capacity.
3. Know the System Voltage
The voltage of your trolling motor will determine how many 12-volt batteries you need.
A 12V trolling motor requires just one 12V battery; for 24V and 36V systems, you will need to connect 2 or 3 batteries in series. More batteries of the same size mean more capacity and more range available.
4. How to Select the Right Size?
How much run time you will get out of your battery mainly depends on two factors: battery capacity and the current draw of your trolling motor.
Battery capacity is measured in Ampere-hours or Ah and current draw in amperes (amps). In theory, if your battery has 100Ah capacity and your motor draws 20 amps of current, then you can calculate its run time by dividing 100/20 = 5 hours.
However, as we said before, if you want to keep your AGM in good condition, you should only use 50% of its rated capacity. In the case of a 100Ah battery, you really only have 50Ah available, so your run time at the same speed will be 2.5 hours.
5. Check Available Space and Battery Dimensions
It’s tempting to use large AGM batteries that will give you more run time.
However, do check how much space you have available, and remember that you will need more than one marine battery if you have a 24v or 36v trolling motor.
If you have enough space but your boat is light, think about how the extra weight in the desired location could affect your boat’s balance and handling.
If you want to keep your battery in a specially designed battery box, you will need to check if the fuse is the right size for your motor.
6. What Type of Terminals Should a Battery Have?
Battery terminals are frequently overlooked when choosing a battery. There are several different types, with all their pros and cons. Have a closer look at the terminals, and – if possible – read user reviews.
If your battery has threaded terminals, you should read the battery specifications and find out what torque you should use to tighten them.
You will need to get an appropriate torque wrench to do that.
7. What About Flooded Lead-Acid Batteries?
Both flooded and AGM batteries are lead-acid batteries. The main advantage of an AGM battery is that it’s encapsulated and spill-proof.
You can also charge an AGM battery much faster (up to 5 times), and it can better withstand freezing temperatures.
Another reason why I recommend AGM over flooded batteries is that they are better suited for a boating environment, where motion and vibration are commonplace.
8. Get the Correct Battery Charger
You can charge a 12v AGM battery with a car battery charger, but make sure it has the correct current output for the size of the battery you want to charge.
It is normally recommended to charge your deep-cycle AGM battery with a current limited to 20% of rated capacity. For a 100Ah battery, this equals 20A of current.
Now, if you took a 20A battery charger and connected it to a small 35Ah AGM battery, this could lead to the battery heating up and gassing, and damage to the battery.
Frequently Asked Questions
What size battery do I need for a trolling motor with 30 lb thrust?
It will most certainly be a 12v battery. When it comes to its Ah rating, It really depends on how heavy your boat is. On an aluminum jon boat, your 30-lb motor will draw much more current than on a kayak.
Can I install my AGM battery on its side?
Most AGM battery makers allow installing a sealed AGM battery on its side. However, you should check it with your battery manufacturer. It is always recommended to install the battery upright and NEVER upside down.
How do I wire an AGM battery for a 12v trolling motor?
If you have a 12 volt trolling motor with just a single battery, the wiring of your system is straightforward:
- Make sure the speed of the motor is set to “0”
- Connect positive (+) of the motor to positive (+) of the battery
- Connect negative (-) of the battery to negative (-) of the motor
How do I connect AGM batteries in series for 24 volts?
If you have a 24 volt trolling motor, you will need to connect two 12 Volt batteries in series:
- Make sure the speed of the motor is set to “0”
- Connect positive (+) of the motor to positive (+) of Battery 1
- Connect negative (-) of Battery 1 to positive (+) of Battery 2
- Connect negative (-) of Battery 2 to negative (-) of the motor
How long can you store an AGM battery?
AGM batteries can be stored for up to seven years. However, it doesn’t mean you can leave it for that long unattended and hope it will perform like new. If you want it to last that long, you will have to keep it fully charged.
Still, there will always be some chemical aging and damage affecting the plates, which means you will have significantly less available capacity than before when it was new.
