Firefly batteries (and lithium)

David Vogel

Hi all,

I replaced 12x failing AGMs in Feb’20 with 10x Fireflys (wired as 5x series-pairs, no tie between the mid-point, and no battery balancers, but these upgrades are both planned for the coming quarter).

This provides a capacity of 5x116Ah=580Ah @ 24V. My normal ‘floor’ for discharge is 50%SoC, giving useable capacity 290Ah, versus the useable capacity of the AGMs (with a routine discharge floor of 80%SoC) of 126Ah @ 24V. The difference provided by the flexibility to accommodate longer periods between charge (e.g. a cloudy afternoon, or day, or extended time away from the boat), is noticeable. We also use less gas for cooking, preferring by a large margin now to use a single induction hob - this is an essential aspect in reducing the hassle-factor associated with prolonged remote area cruising.

For installation, the FFs were slightly taller than the AGM. So I did need to shave about a cm off the aft end of the battery securing bar under the battery compartment lid. I retained the two best of the old AGM as ‘spacers’, so the battery compartment remained full. These AGMS remain wired up, but without the electrical tie between each of that pair.

As an aside, I am newly considering using the spare pair of old-but-good AGMs to power an independent 24V bus for the HF, and thereby eliminating annoying sources of interference such as the inverter, fridges, and the wind-gen. But that is a topic for another day.

In service, I use the externally regulated 175Ah Leece-Neville high-capacity alternator to do a monthly charge-to-float of the FFs (initial Charge Acceptance Rate is up to 160Amps), and do try to do a charge-to-float once a week using the original 100A Dolphin charger (initial charge acceptance rate is normally 86-95 Amps). The external regulator seems to handle periods of extended motoring within spec, so I do not need to manage that aspect by manually switching the alternator in and out – something less to worry about whilst on-passage.

I do check each individual battery’s voltage and state-of-health whenever we have a more-than-fleeting access to shore power (which is somewhere between every 9 and 12 months), and manually balance each battery and then pair by charging each individual battery to extended float, discharge the pair, then charge the pair back-to-float again. If, after this treatment, any individual battery seems a little off, I give it special attention, but this has happened once only, and not seen again. BTW, each FF battery is stamped with it’s own serial number, so it’s easy to keep track of which one is where, should I wish to mix-and-match the batteries around the battery compartment. Also, I have never taken the whole battery bank down to 0%SoC.

So far, the batteries are performing well, and I recommend them as a convenient ‘almost drop in’ route to anyone considering lithium, but unwilling or unable to make the leap. For me, I did the upgrade in The Marquesas, where both expertise and access to spare parts, or any extra bits’n’pieces needed for extended electrical work, is very sparse – so it was a matter of necessity, doing an upgrade from AGM, with least possible introduced risk. Since then, the lithium-on-a-boat movement has progressed in leaps and bounds, and I have seen some excellent DIY and specialist-contractor-assisted installs of lithium upgrades. If faced now with failing AGMs, and with suitable support around me, I would probably consider to go diret to lithium, but there are other factors also to consider (such as charge and inverting needs & capacity).

The prices of the lithium batteries and other components has plummeted, and is now almost on par with (and in some cases now cheaper than) the 10x FFs – depending on how much you are willing to go direct to the battery manufacture in China, and DiY. In any event, assuming a 5-year service life for the FFs (and I might reasonable expect somewhat longer), when it comes time to replace the FFs, lithium technology will have matured further, and it will make the decision to upgrade to lithium a no-brainer. As others have mentioned, having sufficient charge capacity (solar) is a necessary adjunct to lithium on-board. I am presently in planning to replace our 4x100W panels with 3x450W, which will enable reduced genset run-times in the immediate timeframe, and establish a nice springboard for the later transition to lithium.

Hope this adds to the discussion, and provides food for thought.


SM#396, Perigee
Fiji, almost bound for NZ