Choosing the Right Marine Battery

If you are interested in creating a new bank of batteries, it's really important to understand your options – so in this Navigator we explore the different types of batteries available for boats.

There are four basic types of marine batteries to choose from – Flooded (wet-cell), AGM (absorbed glass mat), Carbon Foam AGM and Lithium Iron Phosphate (LiFePO4). Depending on their construction, any of these batteries can function as Starter or Deep Cycle batteries. To make it possible to compare apples to apples, all of these are rated by energy output – expressed in amp hours – along with how many charge-cycles the battery should give over its lifetime. Each type has its own pros and cons, so let's look at each one.

Flooded Batteries

Also referred to as “wet cell”, flooded batteries use a reservoir of liquid sulfuric acid between their lead plates that produces hydrogen and oxygen when the battery is being charged. Because of the hydrogen, the batteries must be vented to allow the gas to safely escape. Without this, they would overheat and boil over – so you must ensure these batteries have adequate space for ventilation. Flooded batteries also need distilled water added regularly – so make sure you have a way to access them easily.

Pros:	Cons:
Usually cheaper than all other battery types	Must remain upright or the acid can spill out
Can last longer if well maintained	Need good ventilation
98% recyclable	More maintenance with the need to occasionally top up with distilled water
Easy to find and purchase all over the world	Cannot be installed near sensitive electronics
Handle overcharging better than most other batteries	Self-discharge rate is higher (6-7% per month) than other batteries

AGM

Absorbent Glass Mat (AGM) batteries are sealed, so they have don't have the same ventilation requirements of flooded batteries. While technically a type of wet-cell, in absorbed glass mat batteries the positive and negative plates are separated by an absorbent fiberglass mat that holds the acid electrolyte like a sponge. This minimizes the potential for electrolyte spillage. During the charge process these batteries have pressure valves that allow the oxygen from the positive plates to move to the negative plate where they recombine with hydrogen – producing the water that the battery needs – so no need to top up! While these are a sealed battery, they do have a pressure relief valve that can release excess pressure if needed – and so require adequate ventilation. AGM battery technology is improving, so it pays to do some research to make sure you are getting the best bang for your buck!

Pros:	Cons:
Maintenance free	Require a “smart” charger
Greater charge currents – so can use as a starter if needed	Sensitive to overcharging (keep under 14.4 volts)
Greater charge acceptance (up to 40% of capacity) so quicker recharge	Shorter life cycle in deep cycle applications than Gel or Flooded
Only a 3% discharge rate	Improper charging greatly reduces battery life
Resistant to vibration	Cost less than LiFePo4, but more expensive than Flooded
Spill proof, leak proof and submersible

Carbon Foam AGM

Unlike standard AGM batteries, Firefly AGM batteries replace most of the lead in the traditional grid structure of conventional batteries with carbon foam. Firefly's unique carbon foam structure provides many benefits over traditional lead and AGM batteries, including higher Ah and energy efficiency, longer cycle life, faster recharges, resistance to sulfation, continuous power through discharge (up to 90%), deep discharge capability (80% - 100%), and the ability to operate in partial states of discharge for days without loss of capacity.

Pros:	Cons:
Maintenance free	Expensive
Higher depth of discharge (80% - 100% vs. 50%)	May need to upgrade onboard charger
Can function as start or house battery	Currently only offered in 2 sizes
Higher life cycles (3 – 4 times normal AGM or Flooded)	Improper charging will reduce battery life
Partial state of charge for long periods doesn't damage battery
Faster recharge than AGM or Flooded
Wider operating temperature range
Excellent warranty

LiFePO4 (Lithium Iron Phosphate)

LiFePO4 batteries are making big inroads into the marine market and while expensive, have quite a few benefits over standard battery types such as flooded and AGM. Not to be confused with Lithium Ion, LiFePO4 batteries use different materials and are far more stable (i.e., safe) – making them highly suited to boating applications. LiFePO4 batteries are sealed and can withstand a much higher temperature range without any performance loss. They are less than half the weight, offer 3 times the life cycles, can take a far lower depth of discharge (up to 100% vs. 50%), and can accept much higher charging amperage than traditional AGM or flooded batteries – which means they charge in a fraction of the time to get back to 100%.

So what's the downside? These batteries are more expensive (but when you compare pricing, remember that you can use 100% of the stated Ah capacity of a LiFePO4 battery vs. only 50% of a lead or traditional AGM) and have particularly sensitive charging requirements that will likely require upgrading your current charging system. Those charging requirements make it imperative that you have a “BMS” (battery management system) for your LiFePO4 batteries (some batteries come with an integrated BMS and others require you purchase a standalone BMS). A BMS manages the individual cells in the batteries and keeps them “balanced” to prevent damage. That said – if you look at how many charge cycles you'll get (up to 10,000!), the difference in weight and size, and how much you'll save in reduced charge time – they become an extremely viable option onboard if you spend a lot of time away from the dock.

Pros:	Cons:
Up to 70% lighter	Expensive
Higher depth of discharge (up to 100% vs. 50%)	Need integrated BMS
Higher life cycles (up to 3X) = longer life	May need to upgrade onboard Charger
Up to 70% smaller footprint	Need to change charge profile below freezing (0°C or 32°F)
Faster recharge	Not suited as a start or bow/stern thruster battery
Consistent voltage during high loads	Not for the DIYer
Lower self-discharge than other battery types
Operates within wider temperature ranges (–20°C to 60°C)
Isn't damaged by extended time at high discharge

A Few More Important Things to Remember…

When buying a new set of batteries, it's imperative that you not only purchase the same type of batteries (Flooded, Gel, AGM or LiFePO4), but that they are made by the same company and manufactured around the same date. It's never a good idea to combine batteries that have different “birthdays”. If you do – the older battery will often negatively affect the performance of the newer batteries.

Having the same type/manufacturer of batteries is also critical as all batteries use different charge profiles (determined by their manufacturer) – so they cannot be connected to other types/manufacturers of batteries and be still be charged correctly. It's important to consider all of your needs onboard before making your choice.

One more important thing to remember – you must make sure your charger is set up with the correct charge profile for the battery type you have selected – or you may be buying new batteries a lot sooner than you expected! Today's “smart” battery chargers can usually handle any type of battery if you choose the correct settings. In addition, newer chargers monitor battery charge state to provide the optimum charge process – providing for the fastest recovery and ideal conditioning – which can lead to a longer life span for your batteries. If you haven't bought a new charger in a while and you are planning to invest in new batteries – now is the time to upgrade! Fisheries Supply carries a multitude of options, which you can explore here.

Finally, if you aren't 100% sure about how to set up your electrical system, we highly recommend that you hire an experienced installer.

We hope this Navigator has been helpful. If you have any additional questions or need clarification, check out the other battery topic links below, or feel free to contact our battery experts at (800) 426-6930.

Marine Battery Basics

How to Size a Marine Battery Bank

Charging a Marine Battery

Marine Inverter Basics