OK, this post is for all you sailing technology nerds out there. Lithium batteries are the newest and hottest thing that every cool sailor suddenly has to have. They are being promoted by some people who really don’t know what they are talking about, and they are enormously profitable for the sellers. But are they the right choice for you? Probably not, but read on to see…

For literally centuries, the only really useful kind of battery for storage of large amount of power was the lead-acid battery. They are cheap, they are well understood, and they work. There was no practical alternative. Technology has (finally!) marched on, and various Lithium battery types are on the market for cruising boats. In many applications Lithium batteries are clearly and simply the best, but that does not mean they are best everywhere.

They command a HUGE price premium, and promise all kinds of great and amazing benefits. Are they worth it the significant extra investment? In my opinion (and that is worth what you have paid for it!) for the typical cruising sailor the answer continues to be a resounding NO. Now that might change as the costs and capabilities change but as of the end of 2019, the number of sailboats for whom Lithium batteries make economic sense, or even technical sense, is very small. Let’s have a look at why.

I have taken the liberty of pulling a major seller’s list of reasons why you should buy a Lithium battery, and will comment on them point by point. In almost every case, the argument in favor of Lithium batteries is slanted by selecting the LEAST good lead acid battery as a comparator. When we look at the latest, and best, lead acid batteries, a very different story comes to the front. Most of the benefits that are claimed for the Lithium batteries are accurate, or as close to accurate as you can expect from the marketing department, but the vast majority of them are also irrelevant to the cruising sailor.

• SAFETY – Inherently safe Lithium Iron Phosphate (LiFePO4) chemistry. Safer than flooded or AGM batteries.

This is a difficult point to find a lot of backup information for. Certainly, lead acid batteries (of the sealed, valve-regulated type) have an excellent safety record when properly installed and maintained. Any large battery bank will store a huge amount of energy in a very compact space, and that is a potentially dangerous thing. When dealing with this much energy there are many types of failures that can cause problems. I’ll call this one a draw. If there was NO possibility of any Lithium battery EVER having ANY kind of issue, modern sealed valve-regulated lead acid batteries are safe enough that the difference is not worth much to me.

I wonder exactly what the writer of this statement meant by “inherently safe.” The phrase “intrinsically safe” has a very specific meaning well understood by engineers. Why did the author choose not to use it? Maybe because “inherently safe” doesn’t really mean anything…

• MULTIPLE BUILT-IN SAFETY FEATURES – Internal BMS provides full protection from overcharge, high or low voltage, over-temperature, excessive charge or discharge current, etc.

This is a classic salesman’s trick of handwaving a bug into a feature. Lithium batteries require a complex electronic BMS (Battery Management System) because they do not tolerate over or undercharging and temperature extremes. Lead acid batteries do no need these complex electronics to work reliably. I don’t see this as an advantage of the Lithium battery at all, it is just more to go wrong.

• DROP-IN REPLACEMENT – Available in most standard industry sizes. Some sizes available in 24v and 48v, eliminating need for the undesirable practice of wiring batteries in series to increase voltage.

This is TRUE, kind of. First, most boats use 12V systems and do not require series wiring. For those boats that do use 24 V systems, inexpensive battery balancers completely eleminate any issues with series connections. Claiming this a a magic benefit of Lithium is a misdirection, because balancing charging voltage to the individual cells is actually far MORE important for Lithium batteries, and is one of the reasons they require the complex electronics in the BMS.

• LIGHTWEIGHT – 50-60% less weight than lead-acid equivalents.

Here is the ONE place where the Lithium batteries win, flat-out, hands down, no argument. If you are sailing a boat where weight is a critical issue for performance, and you have the money, then go for it. Are you listening all you hotshot catamaran sailors? Also, boats where physical space is limited, can get more energy storage out of the same physical space if they use Lithium.

BUT…. On my boat, the weight of the batteries is used to trim the boat and balance the weight of the genset. Removing weight from the battery bank would NOT be a good thing, it would leave my boat with a permanent list to port. Of course I COULD add lead ingots to replace the lost weight and retrim the boat, but I think we all agree that would be rather silly.

• LONGEST LIFE – Up to 10X longer cycle life than lead-acid equivalents.

Whenever you see a salesman use a term like “up to 10X” you know it is a really bad comparison, based on the worst case possible. Using a high quality lead acid battery like the Firefly there is a forecast cycle life of 3600, while the lithium battery claims 7300 under similar usage profile. So the “Up to 10X” actually becomes 2X–maybe.

My Firefly batteries have a 6 year warranty period while the ReLIon marine Lithium batteries are covered for only five. Why do they not put their money where their mouth is?

There is another issue with battery lifespan that is not often discusssed by the proponents of Lithium batteries. Batteries age and degrade even if they are not cycled. A battery that might last 10 years if cycled twice a day (7300 cycles) might only last 15 years if cycled once a day (5475 cycles). That can have a HUGE impact on per cycle cost. Ask to see the data on this before you assume that your expensive new batteries will last “forever.”

On my boat normal usage has me cycling batteries every other day. So even the 3600 cycle life forecast by the manufacturer of Firefly batteries gets me 20 years of life–in theory. Why would I pay more to get a long life than this?

Another thought on lifespan… A battery bank is a fragile thing. To get the maximum lifespan out of it you need to take VERY good care of it. Many sailors do not have the technical background or the interest in the details to really carefully manage their battery banks for maximum lifespan. The other thins, is that sometimes even to the best and most careful of us, “stuff” happens. Maybe you are just better than I am, but I’ll be very surprised if I can maintain a perfect charging scheme every day for 10 or more years.

• MORE USABLE CAPACITY – 25-50% more capacity than lead-acid equivalents.

This is a silly number. I do not care how many “boxes” it takes to store my power, I care about how much it costs, capacity per dollar. Other people might care about capacity per pound. Who really cares how many boxes it takes? And again, the “50%” number is a worst possible case comparison. It is absolutely true that for the same usable energy storage, you need fewer Lithium batteries than lead-acid ones, but with batteries like the Firefly that difference is 25%, NOT 50%.

• CONSTANT POWER – Full power available throughout discharge. Voltage does not drop like lead-acid. Voltage drops by only 0.3 volts between fully charged and 80% Depth Of Discharge (DOD).

This is sort of true, and completely irrelevant. My battery bank drops from 12.8V at 100% change to about 12.2V at normal discharge levels. Who cares if it only dropped to 12.5V? This is also a gross misuse of the word “power.” Either the person writing this doesn’t understand the word, or they are trying to pull the wool over your eyes. A small drop in the voltage during the normal usage cycle does NOT mean there is less power available!

• FULL CAPACITY AVAILABLE UNDER HEAVY LOADS – Unlike lead-acid, 100% capacity is available whether a 1 amp or 100 amp load.

Another case of yes, it is true, and completely irrelevant to a cruising sailor. There certainly are applications where this matters, but a typical sailboat almost always draws power at a slow and steady rate. High amp draws are infrequent and of very short duration and have no real world effect on the battery bank’s capacity.

There is an additional very important issue here. Lithium batteries are limited in how fast they can output power, significantly so. One of the benefits that people use to sell lithium batteries, is the total installed capacity can be lower, because you can drain them deeper. But be very careful of this primrose path if you sometimes call on your battery to lift some heavy loads! An example:

On our Amel we have a 464 A-hr lead-acid battery bank. By most calculations we could install a 250 A-hr lithium bank and have the same usable capacity. BUT… they wouldn’t work! We call on our batteries to run our bow thruster. The thruster uses about 484 Amps, which our lead-acid batteries deliver without a flinch. If you tried to put out this much power from the typical 250 A-hr lithium battery bank, the battery’s control system would shut them down—which could be a very nasty surprise as you pulled into that tight dock. To have sufficient instantaneous capacity to run our bow thruster, we need essentially a 500 A-hr battery bank, totally eliminating the cost savings the sales brochure claims we can see by installing a smaller bank!

• TEMPERATURE TOLERANT – 2.5X more efficient operation at low temperatures than lead-acid. Safely operational up to 150°F (65°C).

Once again, the statement is true, and is also not helpful on a sailboat. My batteries see temperatures MAYBE down to 55F and as high as 95F. Why would I pay more for performance outside this range? There is also a catch that people in cold climates should be aware of. Below 32F, very special charging regimes are required to avoid serious damage to a Li battery. In fact charging needs to be done VERY slowly at temperatures this low. So “temperature tolerant” is not even really a completely true statement. Just like with lead-acid batteries, running a set of Lithium in a high temperature environment (like a poorly vented engine room) will result in greatly shortened lifespan.

• FAST & SAFE CHARGING – Highly efficient charging. Will absorb maximum charging current available from charger, alternator, etc. until battery is fully charged. Can fully charge in 1-3 hours. Built-in overcharge protection.

Yes, and no. If solar power supplies a significant fraction of your boat’s electrical needs, a high charge acceptance rate is most likely pretty irrelevant to you. If you are charging by alternator is is VERY unlikely that your alternator will be able to supply full rated output for even 1 hour without seriously overheating. Your alternator controller will ramp back the output to keep the alternator from a literal meltdown, so the charge acceptance rate of the battery will be irrelevant. Also, Firefly batteries have charge acceptance rates that are probably higher than you can generate, so, again, the difference is not relevant to the typical sailboat.

If you can generate and feed 0.5C or 1.0C of charge current to your battery bank, then maybe you are a good candidate to spend the extra for Lithium batteries. My genset is close to fully loaded generating 0.3C (170A@24V) into my bank, and my Firefly batteries can take that all the way up to 85% SOC. A higher charge acceptance rate is not helpful to me.

• PARTIAL STATE OF CHARGE (PSOC) TOLERANT – No damage from partial state of charge operation or storage. Will recover from complete accidental discharge without damage.

This comparison is certainly true of older lead-acid battery types, but the Firefly carbon-foam battery is not limited or damaged by POS operation, so no help here.

• LONG SHELF LIFE – Low self-discharge rate, so battery stays charged for years if unused.

If you are not using your batteries for long periods of time, then spending the money for the superior lifecycle performance of Lithium batteries is pretty silly…

• MAINTENANCE-FREE – Plug, play, charge, and use.
• NON-HAZARDOUS – No watering. No gasses emitted. Inherently safer than lead-acid batteries.
• INSTALLATION FLEXIBILITY – Can be installed upright or on its side

All of these are features that are supplied by any good AGM or GEL style valve-regulated sealed lead acid battery, and don’t really count as advantages unique to a Lithium battery.

By my calculations, Firefly batteries have a total life cycle cost that is SLIGHTLY higher than a similar bank of Lithium batteries. The Firefly batteries would last about 10 years, (with daily cycling) and the Lithium bank 20 years. Break even would come after AT LEAST year 17. Even longer if the batteries are not cycled every single day. There is no way that anyone can realistically argue that an investment with a 17 year payback makes ANY financial sense. And I can see NO technical reason to use a Lithium battery for the vast majority of cruising sailboats.

If you are considering Lithium batteries for a cruising sailboat, consider very carefully exactly what it is you are spending all that extra money for. Don’t do it just becasue someone else tells you they are the “best.” Remember, that for a lot of people, the most expensive option must always be the best one… They cost a LOT more than even the very best lead-acid batteries, and you need to be SURE that that premium is worth it for the way you use your batteries. They might be a significant improvement over an oldfashioned flooded lead acid battery, but make the comparision to a modern, carbon foam, sealed valve regulated, battery that cost half as much as the Lithium option and you’ll likely make a good choice.

All of these comments can be put out of date pretty quickly, so pay careful attention to current specifications and prices. I strongly suspect by the time I need to replace my batteries, many of these issues will have shifted quite a bit, and my research will need to all be re-done.

Usable battery capacity desired	200	A-hrs @24V
Firefly minimum SOC	55%
Firefly Capacity/G31 unit	60	A-hrs
Firefly Units Needed	8
Firefly Cost per G31 Unit	$512.00
Firefly Total Cost	$4096.00
Firefly Cycle Life at DoD	3600
Firefly Battery cost per cycle:	$1.14
ReLiion Minimum SOC	30%
ReLiion Capacity/G31 Unit	50	A-hrs
ReLiion Units Needed	6
ReLiion Cost per G31 Unit	$1270.00
ReLiion Total Cost	$7620.00
ReLiion Cycle Life at DoD	7300
ReLiion Battery cost per cycle:	$1.04