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…
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.