Boat batteries are the heart of it all. Whether used to power accessories or simply to start an engine, not too much happens aboard most powerboats without a battery. In today’s marketplace, boat owners are faced with an array of battery types to choose from. Traditional wet-cell and sealed lead-acid batteries compete with absorbed glass mat batteries and those using one of a few lithium chemistries to store and provide electricity.
This article compares AGM batteries to lithium batteries to help you know what to look for when buying batteries for your boat.
AGM Defined
Absorbed glass mat batteries are the most advanced type of lead-acid battery. AGM batteries hold a charge longer, can charge faster, and require less maintenance than traditional flooded-cell lead-acid batteries.
Lithium Defined
We are focused on batteries using lithium-iron-phosphate (LiFePO4) chemistry, not the smaller lithium-ion batteries found in phones and computers. LiFePO4 batteries can hold much more charge for their weight compared with AGM batteries, and they can utilize that charge down to almost zero percent. The depth of discharge (DoD) of an AGM battery is somewhere between 50 and 80 percent, and they are usually used for starting and deep-cycle applications.
Cost
AGM batteries cost less per amp, or per “battery size,” than lithium batteries. For example, an average current price for a 100-amp-hour LiFePO4 battery is about $700, whereas the same size AGM battery can be purchased for about $400. However, because a LiFePO4 can use more of its charge before needing to be recharged, that comparison is not completely fair. Still, AGM batteries are much less expensive and will start your engines and power your marine accessories, just not for as long or as many amps. High-draw accessories such as trolling motors do make the case for lithium batteries.
Read Next: How to Install a Wireless Marine Battery Monitor
Charging
AGM batteries can be charged by many typical marine battery chargers and marine engine alternators. While AGM batteries definitely benefit from a charging system optimized for their chemistry, even much of the older equipment already in the field is likely to charge AGM batteries. LiFePO4 batteries require a special charging system. Unless you have a new engine or a special alternator installed, most older marine engines won’t charge them. LiFePO4 batteries will charge in about half the time, though, if the proper charging system is in place.
Self-Discharge
The rate at which a battery loses charge just sitting in disuse is self-discharge. Old wet-cell batteries may have lost 10 percent of charge per month to self-discharge. AGM batteries self-discharge at about 1 to 3 percent per month. Use your boat once a month, at least, and finding a dead battery to self-discharge will be rare to nonexistent. Do note that LifePO4 batteries self-discharge at about 0.5 to 3 percent per month, an even slower rate than AGM batteries.
Lifespan
Battery life depends a lot on the application and owner care. Two different boaters might get two different lifespans from identical batteries. Lithium batteries, by their nature, should last longer, however, and deliver about 1,000 discharge cycles. New, advanced AGM batteries, such as Odyssey AGM2 batteries from EnerSys, are rated at 400 discharge cycles to 80 percent DoD and offer plenty of engine-cranking capacity. When shopping, balance LiFePo4’s longer lifespan and higher cost against the lower total cost of ownership of AGM batteries.
Weight
AGM batteries weigh more than LifePO4 batteries, but, unless you need a large bank of batteries to power a trolling motor, a powerful audio system, or some other high-draw appliance, the weight difference might be negligible. Many boaters get all their battery needs served by two or three batteries total on the boat. Start getting into four and six (and more!) battery banks, and then the weight difference does become meaningful.
Conclusion
So, will AGM batteries do the job for your style of boating? If you are not running high-amperage accessories such as trolling motors and powerful audio amplifiers for long periods of time, then the answer is yes, AGM batteries will serve you well and at a lower purchase cost versus lithium.
