Battle of the Batteries: Which is right for the job?

Oct 11, 2022 by Ralph Gutierrez

With the high-demand technology of today’s vessels, choosing the right battery is crucial.

Let’s look at performance-cost comparisons of flooded lead acid, AGM, and lithium batteries. The service requirements of flooded, or “wet,” lead acid batteries are well-known: Acid levels need to be checked periodically and distilled water must be added to keep the lead plates covered. These batteries are prone to spilling corrosive acid and venting explosive gasses. Newer flooded batteries may be labeled “sealed,” but that does not prevent dehydration and off-gassing. Flooded batteries are generally designed to be either “starting” (high cranking power) or “deep cycle” (long duration), but not both.

The AGM (absorbed glass mat) battery has become a popular choice for many applications. They beat wet batteries in both maintenance and safety, and generally outperform the older flooded technology. Interestingly, the AGM battery is, in fact, a lead acid battery that is constructed in a way that prevents spilling and off-gassing by employing regulating valves. The lead plates are wrapped in fiberglass mats and tightly placed in the battery case, then the case is sealed and acid is injected. This new construction method makes them spill-proof and explosion-proof, with no maintenance or off-gassing. Most AGM batteries can perform very well in both starting and deep cycle applications. They may be double the cost of flooded batteries but can last up to five times longer, making them a cost-effective solution.

“But, my buddy has lithium!” you may say. And he probably spent big money for them. Here’s why: Lithium has a much higher performance density, but is much lighter — 65% lighter. Think more power in a smaller and lighter package. They can be discharged and recharged much faster and many more times than either wet or AGM batteries. Lithium batteries are superior for deep-cycle applications, and when charged properly, they will last a very long time. What could go wrong? Well, there have been some growing pains — yep, some fires. New chemical compositions and a better understanding of how to charge them have mostly mitigated those risks, but check with your insurance company because some won’t insure your boat if using lithium.

In conclusion, you get what you pay for. Want a big house bank in your sailboat or for off-grid living? Maybe a flooded, deep-cycle battery bank is for you. Got four big outboards on your fishing machine and still like to hang at the sandbar? I’m thinking AGM. Got specific applications and some extra cash? Lithium, baby! 



The AGM vs. LiFePO4 smackdown!

The AGM battery is the popular choice for most vessels, but recent advances in performance and safety have turned the LiFePO4 (lithium iron phosphate) battery into a strong contender. Add in the skyrocketing cost of lead used in AGMs compared with the decreasing overall cost of lithium, and the choice becomes less clear. Here is a breakdown of the advantages of each to help you decide which is best for your needs.


Most lithium batteries are for deep-cycle application only and best for depth of discharge (DOD), meaning the deeper the discharge the fewer cycles the battery will have. Lithiums are not affected by DOD, while a 50-80% DOD is recommended for AGMs. Lithiums can run down to 0% and be recharged to 100%, while AGMs will lose capacity over time. Most deep cycle AGMs have a 550-cycle life rating, while lithiums will provide up to 2,000.

Verdict: Lithium has the edge.


Battery weight is often a problem for smaller boats; not so much on larger vessels. There is a significant weight difference between the same case size of the two batteries, with lithium being about 65% lighter on average. Also, lithium batteries are becoming more popular with trolling motors, as one battery will replace up to 48 volts.

Verdict: Lithium comes out on top.


Safety is by far the biggest concern when it comes to lithium batteries. AGM batteries are valve-regulated and non- hazardous, with no danger of exploding or causing a fire.

Lithiums have had problems with this in the past, but the technology has improved and is becoming safer. Most of the time the problem is due to not having the battery charger set correctly for the lithium profile.

Verdict: AGM has the upper hand.


AGMs remain less expensive due to the components used. However, in the long run, the longevity of lithium will offset the upfront cost.

Verdict: AGM upfront, but lithium long term.