Firefly Carbon Foam Batteries Are Great, But Read The Fine Print

Let me start off by saying that I’m a huge Firefly carbon foam battery fan boy. And, further, I think that for many usage profiles these batteries are the best alternative for live-aboard long-term voyagers.


That said, I have noticed in the comments here at AAC, and in reviews and commentary published on other sites, that there is a fundamental misconception about this technology.

Here’s a typical quote (removed from the site since) as an example:

Depths of discharge to 80%-100% of rated capacity. This can be done without any permanent sulfation or loss of capacity.

(Emphasis mine.)


As the screen shot from the Firefly data sheet (above) clearly shows, that statement is wrong:

Regularly discharging these batteries to say 80%, instead of the more commonly recommended 50%, will cut their minimum expected life—it’s a good idea to use minimums when evaluating manufacturer claims—from 3600 cycles to just 1000, a drop in capacity of 3.6 times.

Or, to put it another way, the above claim of “without any permanent sulfation or loss of capacity” is, to put it kindly, nonsense. And yet I’m hearing that repeated often enough for it to become accepted wisdom.

The point being that someone who buys these batteries and regularly—occasionally should not be a big problem—discharges them to 80% is going to be deeply disappointed with the service life.

Firefly Against Others

That said, on an absolute basis against other lead acid batteries, the Firefly claims are still impressive.

Compared at 50% discharge:

Above is a life-cycle graph from LifeLine’s manual showing an expected life of 1000 cycles.

And below is one from the Rolls Battery company here in beautiful Nova Scotia (we residents all have to say that five times a day to keep our health care card) that claims about 1250 cycles for their liquid-filled batteries.


But, wait, all of these graphs are, as far as I can see, unverified manufacturer claims. So are Firefly Batteries going to outlast LifeLine AGM and Rolls liquid-filled by over three times?

Beats me, but based on my own experience with LifeLine and Rolls (years ago for the latter), and taking into account that Firefly is a newer company that may, shall we say, be a little more aggressive with their claims than more established companies who have had longer to learn the lessons of how destructive it can be  to make overly-optimistic claims, I’m guessing that the reality lies a little lower—still impressive, though.

Limited Case Sizes

And while we are thinking about fine print. Another thing to take into account before buying Firefly is that they only come in three case sizes, which makes it impossible to build a fault-tolerant battery bank using their 2 or 4 volt cells without making it huge, or alternatively connecting way too many of their smaller-case 12 volt batteries in parallel. See Further Reading for more.

Still Require Full Charge and Equalization

Update December 5th. I just checked out the manual for FireFly batteries (see further reading) and it’s important to note that it recommends:

  1. A full charge to 100% once a week.
  2. Periodic “Restoration Charge”.

The former will require thoughtful use of a decent sized array of solar panels or shore power, and the later would be very difficult to do without shore power.

Still Great

Now, don’t get me wrong, the above numbers are still impressive and all the more so because Firefly Batteries can be left in a state of partial charge for long periods without being damaged as much as other types of lead acid batteries will be.

The Greater Takeaway

That said, there’s a larger issue here:

We all need to guard against our enthusiasm for new technology blinding us to the fine print and inconvenient facts like the case size and charging requirement issues above. And, further, it’s a good idea to apply a healthy dose of scepticism to manufacturer and/or dealer claims and always dig deeper into the specifications and manual before buying.

Further Reading

Like what you just read? Get lots more:

Learn About Membership

Notify of
Inline Feedbacks
View all comments
Marc Dacey

Form factor dictated our choice to stick with lead-acid batteries as much as any intrinsic chemistry or DOD cycling attribute. We could have taller batteries (and therefore a larger bank, desirable to us because of where we could put it under the mast on the centerline) and mount them securely together close to the charger/inverter. We are careful not to go below 70% SOC, which means we should get more cycles out of them than with those with deeper drawdown habits. But I’m under no illusions that others could easily do this on most production boats, and indeed, one of the biggest complaints I get from sailors looking to expand their banks are the difficulties with cable runs and fitting many battery form factors in advantageous places. A final comment is that lead-acid batteries are very widely available; our L-16 Crowns are used in solar installations and a variety of forklifts and can be shipped most places without comment. Their chemistry is exceptionally well-attested and need not rely on manufacturers’ claims, which lean toward the best-case scenario when not actually drifting into exaggeration.

Were I building a boat from scratch, or cruising coastal, I would not exclude this tech, as it has promise, even in beautiful Nova Scotia.


Ocean Planet are being *very* careful with their language there.
“This can be done without any permanent sulfation or loss of capacity” may be entirely correct, if – after repeated deep discharges – it is still possible to charge them to 100%, discharge to 20%, and get the same amount of energy out of that cycle as when they were new. But they carefully left out any reference to total life span in that statement! It is quite possible (indeed likely, based on what little I know of the chemistry) that frequent deep cycling doesn’t compromise capacity, but does reduce the number of years / cycles to total failure.

I would still size a Firefly-based bank for 80% to 50% nominal cycling under typical house loads, but I’d be much more willing to relax the “edge case” requirements. Consider the case where on-passage loads look a fair bit higher than anchored loads – thanks, autopilot & radar. I’d normally use the on-passage day as the 80%-50% basis figure, because on a bank with a 1500 cycle life at 80%-50% that drops to 500 cycles at 80%-30%, getting surprised with an early battery replacement in Vanuatu would really suck. But if it’s 3600 vs. 1800 cycles, well, maybe we don’t mind dipping into that extra margin for 60 or 80 days a year, and can accept a lower total nameplate capacity – similar cost, less weight & bulk?

Max Shaw

That is such a fun example Matt as I actually had to do an unscheduled battery bank change in Vanuatu. Our batteries started to fail as we were on passage from the Marshall Island to Vanuatu. Thank goodness for extra battery cables to jump the bad batteries. Irrelevant to this topic as they were bog standard 6V deep cycle and replaced with bog standard 6V deep cycle as that is what we could get shipped into to Port Villa. Loading the batteries into a panga on the beach was the fun part. We had hoped to be more creative with the house bank change but alas it will have to await the next house bank replacement (hopefully not for a few years). In the meantime I can watch the Firefly, lithium, etc discussions from the sidelines.

Thanks for all the great work on this site !


SV Fluenta

William Koppe

Hi John,

I speced my Firefly bank based on a usable 80% DOD compared to a Lifeline bank at 50% DOD. As it turns out, that gives me 1150 cycles vs 1000 cycles.
Interesting to note that Lifeline cycles drop from 1000 at 50% to 500 at 80% or half.
Firefly drop from 3900 cycles at 50% to 1150 at 80% or 2/3rds.
Not sure I would like to see what a constant 80% DOD would do to the Lifelines.

Andy Lee


I was looking into buying Firefly directly from the manufacturer in India (evidently, this is where they are made now) for my live-aboard catamaran notwithstanding the shipping cost would be close the cost of battery. However, once I learn from the manufacturer that owner should “equalize” the battery every week to extend the life of the battery, i decide to go with CALB lithium instead.

I never truly understood their rationale as no other lead acid battery brand requires frequent battery equalization. I always understood equalization is something you do once in the blue moon. I am no expert on this, I would love to hear you and other think about the equalization of Firefly topic. Even if I made mistake, I like to know why.

Andy Lee


I was corresponding with Firefly technical team. See the exchange below. I am happy to forward you the email chain but I thought I just cut and paste the relevant pages for now as I don’t want to post Firefly technical team contact without their consent.

I heard about Firefly from Jeff Cote at Pacific System, and aware of Nigel C’s review. I thought Firefly has the benefit of lithium (#cycle , energy density), the durability of proven technology and a reasonable price – best of all worlds:).

However, from Firefly technical team’s answer, I got the feeling, Firefly is not as durable as I initially thought and lifecycle benefit requires owner’s discipline to equalize the battery so I move to a different direction.

I always wonder if I made the right decision and would love to hear what others who are more knowledgeable than me in marine electricity to weigh in.


On Fri, May 29, 2020, I wrote

“ Firefly team,

Thanks and let me digest some more.

Thanks for your speedy reply. The answers surprised me though.


On Fri, May 29, 2020 at 3:11 PM wrote:
Dear All ,

My reply to Mr.Andy’s queries…

My (Andy) Question

Q2). I was surprised to learn that to extend battery’s life we should equalize once per week. As far as I know, almost all lead acid battery manufacturers don’t even require this equalization frequency. Since one of Firefly’s selling point is its resistance to sulfation THEN why one needs to equalize so often? What am I missing?

ANSWER (Firefly)

We have no control over the usage pattern of the customer / Application. In Solar applications the input of 103 % on a sunny day is not enough to maintain sustained output in subsequent cycles till end of service life.

The DOD % can go to 100 % more often in both Solar and Non solar Applications. With less available time for charging and no input on sunless days particularly in Solar applications with 80 to 100 % DOD , the cell voltages in the battery bank will be non uniform with some cells lagging in voltages during charging and eventually degrade. This is the reason for equalization charge at pre determined intervals may be 10 ~ 15 days once though once in 7 days is preferable.

During winter , this may be required more frequently in a solar appliction. This is recommended to eqalise the voltages of all the cells and to sustain the output during cyclic usage and keep the batteries in a healthy condition. This has nothing to do with sulphation.

Q3) (Andy) . From Lead-Acid’s Sweet Zone, How to get more energy out of your Solar Batteries & Panels by Presenter Mukesh Bhandari COO,Author Kurtis Kelley, Firefly International Energy Peoria
Illinois, dated: Feb 2014, it suggests that to gain solar charging efficiency, one should NOT charging the Firefly to 100% or during the Floating stage since a lot of energy is lost because of gassing.

Am I correct? More importantly, we should discharge the battery to 20% of SoC and recharge it back to 80% of SoC. This is your Set Point of State of Discharge at 20%. If we were to keep the battery between 20-80% of State of Charge, wouldn’t it reduce the total lifecycle of battery comparing to keeping 50-100% of SoC.

AnSWER (Firefly)

It is true. Operating @ 20% ~ 80 % SoC is for better charge acceptance from
solar panels. Operating @ 50 % ~ 100 % SoC results in early drying out of VRLA
battery as gassing voltage of battery happens at 80 % SOC.

80 ~ 100 % input phase results in more electrolysis and positive plate corrosion and batteries’
losing electrolyte volume with comparatively reduced service life.

Andy Lee


thanks, your explanation makes more sense if I were to replace technician’s reply “equalization” with “fully charge” then charging the battery to 100% SOC once a week is not that onerous to owner.

Dick Stevenson

Hi all,
Is being fully charged once a week (or pretty regularly) a characteristic of all AGM batteries? I have never lived with AGMs but I have heard that admonition for years and never known how important it was. It is one of the reasons, probably the primary one, that I never went over to AGMs as I lived at anchor for long periods and my batteries rarely got over 80% SOC.
Thanks, Dick Stevenson, s/v Alchemy

Michael Lambert

Edited. Disregard. I had to ask the question in order to figure it out in my own……..

Gino Del Guercio

My wife and I are full time liveaboards on a 44 foot sloop with solar but no generator. We bought four Fireflies four years ago. We generally discharge them to 80 percent each night but occasionally go to 50 percent and have dropped them to 20 percent on occasion. We have not noticed any drop in performance. This year I bought the new Balmar SG200 battery monitor, which has a “battery health” indicator. This might just be Voodoo but it says my batteries are still at 100 percent health at over 1,200 cycles. I was very happy to see this and it matches my experience. If they ever wear out I would definitely buy them again.

Lee Corwin

John have you rethought this subject now that drop and play BMS/LiFePO4 units are more available at almost a reasonable cost? Seems the window of time for foam carbon being a good economic choice has gotten smaller now that costs are comparable. See them as still being a very viable choice for retrofit but not for new builds where you would have no need to swap out any other component.

Evan Effa

I agree with you Lee.

There doesn’t appear to be much real user feedback available yet but the Trojan Trillium Drop-in LiPO4 batteries may well be a better performing and more cost effective option than the Carbon Foam FireFlies…

Evan Effa

Interesting details here.

The Firefly documentation in 2017, when I bought my bank of (6) 12V Firefly batteries said nothing of any equalization routine.

I have not been doing anything except to bring the batteries back up to 100% SOC once back to port.

When out cruising, we do not allow the bank to dip below 50% SOC ( mostly because I was paranoid about the voltage dropping below 12.1V).

We try to get the SOC up to at least 90+% when recharging.

Having said that, an unexpected power loss in my boat shed in 2018 accidentally dropped the bank to 100% DOD for what might have been 2 days. I immediately recharged the bank and installed a cell based battery monitor.

Without ever having done an ‘equalization routine’, I see the bank not seemingly performing as well as it did in that the voltage drops to ~12.5 V by ~ 100Ah discharge.

It will stay above 12.1V with ~ 300Ah discharge but the voltage is less for any given SOC than it was when new.

Thanks for referencing the revised 2020 Firefly Manual, it looks like with the changes in the instructions, I should take each battery, isolate it from the bank & discharge to ~ 11.0 V and do an an aggressive 0.5C recharge?

This looks to be a hassle. (Playing with 4-0 Battery cables isn’t my favorite activity). I wouldn’t want to do it more than once a year but maybe it would improve the performance of the house bank.


Evan Effa

Other changes noted for 2020:

Installation to allow for air space between each battery etc. (!)

I have battery temperatures monitored by both my alternator regulator and the main charger and do not ever see excessive temperatures; but, I have no idea how the individual batteries vary in temperature.

Given some of these revised maintenance and installation instructions, when I go to replace this bank, I will be much more inclined to go for LiPO4.


Evan Effa

Revised above comment : (Edit privilege timed out)

Thanks for referencing the revised 2020 Firefly Manual, it looks like with the changes in the instructions, I should take each battery, isolate it from the bank & discharge to ~ 11.0 V and do an an aggressive 0.5C recharge? (It’s quite possible that despite my working hard to equalize the length of each lead to each battery that within a 6 battery bank wired in parallel there could be disparities between the batteries causing uneven discharge and recharge currents to each individual battery.)

Doing a “Refreshing Charge” to each individual battery looks to be a real hassle. (Playing with 4-0 Battery cables isn’t my favorite activity). I wouldn’t want to do it more than once a year but maybe it would improve the performance of the house bank?

Evan Effa

Hmmm, How big is big enough?

It’s not that easy to get up to 0.5C (~ 300A) charging rates without disconnecting some of the house bank batteries.

I have a Magnum Charger Inverter that puts out ~ 120A + a Xantrex 40A charger which reliably produce a total charging current of 160A when the batteries are in early stages of Bulk Charging. Adding the engine’s 120A alternator adds only a little more to that in that I’ve only seen maybe 180A max. (I probably need to adjust the Balmar external regulator to improve the alternator contribution.)

Maybe discharging to something below 12.0V would provide a better conditioning charge?

Some of these details represent the “Fine Print” you referred to in your title for this article. How to keep the Carbon Foam batteries in top form?

Marc Dacey

Of course, I am referring to the standard old-school plate-style batteries, yes, with the standard charging regimens and the expected duty cycles. My grasp of this suggests that it is the internal architecture, not the electrolyte per se, that separates the various designs.

Simon Fraser

Surely “discharge to 80% of rated capacity” is just that… discharge of 20% of the capacity.

Robert Hellier

Hi John,

Last summer my better half and I acquired a 36′ sailboat. In our delivery voyage back to Lake Ontario, after two weeks of anchoring, we realized that the 4 G31 AGM batteries in our 12V house bank were on their last legs. Initially we thought there was something wrong with the galley fridge as it would shut down and only the cooling fan would cycle on and off. But after some diagnosis, I realized the controller was acting up due to very low battery system voltages. To resolve, we had to run the engine to bring the bank up to a reasonable voltage and turn the fridge off at night, while keeping all other electrical loads to a minimum. Now that the boat is on the hard we are working on their replacement.

Initially I was planning to just switch out with 4 new AGMs, but your articles on the issues with AGM premature sulfation and the lack of ability to recondition them (except for the Lifelines of course) has given us reason to pause. We also considered LiFePO4s since the battery bank location is difficult to access at the back of the engine compartment and their lighter weight would’ve made changes much easier. Again, your warnings about the myth of “drop in Li batteries” has ruled that possibility out as well. Thanks for your timely insights!

All this has led us to two remaining options:

  • Four flooded G31s (probably East Penns) at around $CAD900
  • Three Firefly CF G31s at around $CAD1900

Given the somewhat deeper discharges that could be made with the Fireflies we thought a 25% reduction in capacity was possibly manageable, though we will retain the 4th position in the battery compartment in case we later decide to add another Firefly.

The more than double pricetag of the smaller Firefly battery bank is a bit of a gulper but it is nonetheless attractive due to the elimination of the need to check and water the battery bank combined with the greater DOD flexibility.

To save my back I’m also going to rig up a tackle above the battery compartment. There’s no way I could bench press a single 70 lb battery into that compartment while lying on my side in the quarter berth, let alone 3 or4 of them in succession!

Once I complete the installation, I’d like to accurately track the battery bank’s performance. This leads me to two questions.

  • How do you define and determine that a battery has gone through “a cycle”? It is not always self-evident – to me at least – that a battery bank has cycled. For example during a stay at anchor or at a mooring, the battery bank draw-down might run up and down by small degrees in a haphazard pattern, as loads and generation (esp wind and solar) vary. Is there a rule of thumb/ threshold that you use to determine when “a cycle” has occurred?
  • How do you manage to keep track of the cycles over a battery’s lifetime? Do you employ some nifty piece of electronics that gives a readout of the progression of cycles? Are you personally checking the DOD constantly to determine when a cycle has occurred and noting that into a battery log? Or are you determining your actual usage through a mix of general assumptions and occasional observations?

BTW how’s it going with your new Instant Pot?


William Murdoch

I have been using 3 group 31 deep cycle marine batteries from Sams Club in my Pacific Seacraft 34 house bank since 2004 replacing them every three years. They are East Penn batteries and currently sell for $US117.88 + 9.5% sales tax. .

We anchor out perhaps 150 nights a year usually on a 5 or 6 month trip to the Bahamas from North Carolina.

I replace them every three years moving them to other boats that I own (and to a truck that I once owned) where they live out the rest of their lives. I test them for their ability to deliver a high amp load for 10 seconds with a 100A carbon pile battery tester. I test their amp-hour capacity with a West Mountain Computerized Battery Analyzer which discharges the battery over a 20 hour period and produces an voltage vs amp-hr graph. With it I can watch the batteries age. My CBA IV has now been replaced with the new CBA V which can also do the high current testing.

Per & Meri Lovfald

Firefly Energy website is down. Says the domain has expired. I hope this isn’t a harbinger of bad news. I was ready to purchase a bank of three MCF-G31 batteries for our boat.

Per Lovfald
SV Sojourner

Brian Russell

Firefly Energy website is working today, but it appears their distributor network has had a shakeup. We are approaching the end of third year with the Firefly’s and have found them quite robust. Certainly much easier to install than lithium!