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

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

Published Dec 4, 2020 33 comments

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.

Claim

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.

Let me quote from oceanplanet.com 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.)

Reality

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.

Accuracy?

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

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Meet the Author

John Harries

John was born and brought up in Bermuda and started sailing as a child, racing locally and offshore before turning to cruising. He has sailed over 100,000 miles, most of it on his McCurdy & Rhodes 56, Morgan's Cloud, including eight ocean races to Bermuda, culminating in winning his class twice in the Newport Bermuda Race. He has skippered a series of voyages in the North Atlantic, the majority of which have been to the high latitudes. John has been helping others go voyaging by sharing his experience for twenty years, first in yachting magazines and, for the last 12 years, as co-editor/publisher of AAC.

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