Battery Upgrade

I have previously described some of the issues that a cruising boat has with batteries.  As our batteries have started to show their age, we replaced them with what we hope to be a long term solution.

Firefly Battery Update

We have had our Firefly batteries in full-time cruising service for a couple of months now, and think we can give a preliminary report on the results we see vs. what we expected.  After 20 years and nearly continuous cruising, our Onan genset–with over 7000 hours–is aging.  Given that replacement of the genset is a very expensive proposition, reducing its run time will postpone the day when that trigger needs to be pulled.
Where we started:
We have an older Amel Super Maramu (hull #160) with a 24 volt house bank assembled from eight Group31 12 volt batteries.  Our charging sources are:
  • a 630 W solar array with a Victron MPPT controller;
  • the 220V genset powers a very sophisticated 70 Amp Victron Inverter/Charger, and a rather dumb 50 amp Techpro charger;
  • And finally, a 50 Amp internally regulated alternator on the main engine.
Our previous battery bank used Lifeline AGM batteries.  They worked well for 6 years, but had lost about 30% of their original capacity.  They were not in a rapid decline, but were slowly fading.
With that system, our routine was to run the generator for about 90 minutes each morning. The stopping point was when the accepted charge current dropped below 18 amps in the absorption phase of the charge cycle, which corresponded to about 83% charged.  The solar panels then took over, and the batteries were typically at 95%–or better–state of charge by the end of a solar charging day. This process kept the batteries well charged, with minimal potential for undercharge leading to sulfation.
The downside was that the solar panels spent most of the day in absorption phase of charging, meaning their full potential output was not available to be stored in the batteries and we were running the genset more than we really wanted to. Typically we saw about 50% of the potential energy from the panels was discarded in the absorption or float stages of the charge cycle.
Our usage of electricity is dominated by our two refrigerators and one freezer, and our watermaker.
What we wanted.
Our reason for going with the Firefly batteries was centered on the claim that they do not need to be brought to near 100% charge to avoid sulfating.  They can be operated routinely and constantly in a partial state of charge with out long-term damage.  If true, this meant that the normal daily charge could be kept below the point where the solar panels were current-limited by their controller.  All, or nearly all, of the capacity of the panels would be able to be stored in the batteries, thereby reducing the need for generator run time.
What we see—so far
This has worked exactly as we expected. Our generator time has been cut by more than 1/2.  We now run the generator routinely every other day for no more 60 to 90 minutes. If the weather is especially sunny we can easily go two days. If we did not run the watermaker, we’d be running the generator only for stretches of cloudy days. If this change in generator run cycle postpones buying a new generator for one year, the extra cost of the batteries will have been worth it even if they only live five years.
The actual stored output from the solar panels is now higher because they are no longer throttled by the ability of the batteries to accept amps. The batteries routinely cycle between 60% and 85% state of charge over two days.
The charge acceptance rate for the Fireflies is very much higher than the old AGM batteries.  Even as the battery state of charge rises above 80% we see the batteries accepting amps as fast as we can make them, it’s only as they get above 85 to 90% that the amps begin to throttle down.
Our conclusion is that the batteries are delivering the benefits we had hoped for in the short-term.  The still unknown factor is: will they have the full expected life span when operated under a nearly constant state of partial charge—as the manufacturer claims? Only time will tell…
Other Thoughts
There are properties of the Firefly batteries that we are not really taking advantage of.  For example, compared to most other lead-acid battery types, they can be deeply discharged without damage.  It is nice to know that if we had need to drain them deeply, it wouldn’t be a problem.
It is certainly POSSIBLE to run other battery types with this kind of charging regime and see similar results, with one serious caveat:  Constantly keeping most other types of batteries below 80% charge would condemn them to a fairly early death.  If I believe the Firefly specifications, we could see as much as ten years life from this set using them as we do.

4 Responses to Battery Upgrade

  1. Fantastic write up as usual. Thank you Bill!


  2. Bill Kinney says:

    “Not Damaged” is an relative term. Firefly batteries, like all batteries, have a reduced number of total cycles at deeper discharges. In the real world, we don’t actually care about number of cycles, but rather the number of kW-hrs a battery can store and return over its life.

    There is a balance point in any battery based on the energy stored and returned over its lifespan. A battery that will store and give back over its life 1000 kW-hrs of power when discharged 25%, and 1000 kW-hrs when discharged 50%, and 1000kW-hrs when discharged 75% has what I would call a “linear lifespan”.

    Most modern batteries are more or less linear down to about 50% discharge. A battery that returns 1000 kW-hrs at 25% disharge and at 50% discharge, might have a life power return of only 500 kW-hrs if discharged 75%. Atypical specification would be 1000 cycles at 50% discharge.

    My understanding, is that the “breakpoint” where the lifecycle becomes shorter in terms of life-time power stored for Firefly batteries is much deeper than others.

    The Firefly specification sheet calls for 50% discharge to give ~3600 cycles and at 80% discharge ~1000 cycles. Not many other battery types would claim 1000 cycles at 80% discharge. Since we are keeping our routine discharge depth to significantly less than 50%, I should expect >3600 cycles. Of course expectation might not match reality. Other unavoidable things might age the battery than simple discharge cycles. Temperature for one. All ratings are at 25C. Firefly suggests that battery lifespan will be halved for every 10C increase in temperature.


    • James Alton says:

      Bill, Thanks for the concise and informative response. I now better understand the reasons that the Firefly batteries may be a good choice for boating. The improved absorption rate combined with not having to top the batteries up each time really sounds especially good to me. I note that the manufacturer seems to back these batteries quite well with their warranty. Congratulations on cutting your genset time in half on average, I am not sure of what the cost of running your genset works out to per hour but I have seen numbers of around $5 per hour for fuel, maintenance and depreciation. I guess that the genset cost savings should be factored into the cost of the Firefly. I will continue to research these batteries. Do you know if any special conditioning is needed to keep the cells balanced? Do they gas at all under normal use? Best, James


  3. James Alton says:

    Bill, A great update on the Firefly batteries, they certainly sound promising. You mention that they will not be damaged if they are deeply discharged. So are you saying that the expected number of total cycles does not decline due to deep discharges? I know that with most batteries, cycle life is very much tied to DOD. I hope that you get the 10 years that you are hoping for. James


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