Paul,

Our electrical system is basically all Victron and we have installed their Cerbo GX to monitor and control all the bits and pieces.  The key key piece for the batteries is the BMV Battery Monitor.  In addition to all the usual functions of bank voltage and amp usage tracking, it can monitor the difference between the "top" of the bank and the "bottom".

All the batteries on the 0 to 12 volt side are tied together. Likewise, all of them on the 12 to 24 volt side.  In the ideal world, the voltage inside each group would be exactly one half of the overall bank voltage, or (nominally) 12 volts. If a single battery was to develop an internal short that half of the battery bank would drop in voltage, and the other half would rise, increasing the difference between them.

Normally our batteries run with a 0.00V difference. Toward the end of a hard charging cycle we can see up to a 0.04V difference.  We have an alarm set to flag if the difference gets above 0.06V.  So far, it has never gone off.

I have attached the voltage graph from our last full discharge and rapid recharge.  The discharge rate was pretty steady at 50 Amps for about 10 hours.  The charge rate was all the batteries could take, or 200 Amps whichever came first. It was pretty warm that day, so the absorption voltage was a bit lower than you might expect.  The top and bottom difference increased as the batteries approached fully empty, during the steepest part of the bulk phase, and in the middle of the absorption phase, all of which is normal behavior.



Bill Kinney
SM160, Harmonie
Annapolis, MD, USA

Paul,

We have a full set of Firefly batteries, and love them.  We periodically do a complete discharge to dead flat, and a rapid recharge as Firefly recommends as a "restoration charge."  Every time we we have done this we carefully monitor the amount of power discharged.  Each time the amount of power they have delivered has been above the original specification sheet value, and it is still the same as the day we installed them. The last I looked, it seemed that Firefly bumped up the rated specification of the batteries from 116A-hr to 120A-hr which is more in line with what we measure. We take good care of them, but while out on the hook, we certainly do not bring them fully charged every day.  They don't care.

For the way we use our boat, they give us all the benefit that a set of Lithiums would, at a significant discount.  They never have had any trouble with powering the bow thruster, or high loads out of the inverter.  At a steady 50 amp draw the voltage stays remarkably steady all the way down to about 15% SOC.

Some issues to watch for:

Firefly recommends that the batteries be charged at between 0.2 and 0.5C My recollection of a 54 is that you have 12 G31 batteries.  That would give you a bank of 720 A-hrs.  Charging that large a back at a rate of 0.2*720 = 144 Amps might be a challenge with a standard charging setup.  Our older SM has a 480 A-hr battery bank.  We have a DC generator that outputs 150 amps at 24 volts, so it fits well within the minimums recommended.  If you can't reach the recommended minimums, they request you talk to their technical staff for an alternative procedure.  I don't know what this is, but  it might be worth investigating up front if it might be an issue for you.

Early in their production run Firefly had difficulty with the occasional battery developing a shorted cell.  Left undetected, this can bring down the entire bank  in short order because of charge imbalances.  It is not a problem unique to these batteries, any type of lead acid battery can have this problem, but it seemed Firefly had more than their fair share.  We had this happen to us, and Firefly was 100% helpful and completely took care of us under warrantee. Our current monitoring system doesn't allow this to happen without us knowing about it very quickly.

We have 615W of solar. Our normal daily power usage is about 3kW-hrs, and solar can make about half this. During our normal charging cycle while at anchor in good weather we end up in the morning at about 75% SOC.  Over a sunny day, the solar panels bring the batteries up to about 90%.  The next morning, the batteries drop to about 60-65% SOC.  The generator starts in the early morning, runs for 80 to 90 minutes, and brings the bank up to 93% where it shuts down, and the solar panels have the bank at 100% by 3PM.  And the same two day cycle repeats.

We find we get excellent utilization for the power available from our solar panels, and the generator runs most of its time at high load, where it is most efficient.  For us, minimizing expensive generator run time is important.

We have looked very hard, and sharpened our pencils a lot, and for us we just don't see a benefit to Li batteries that justify the higher costs.  But every boat works differently, and everybody has different needs.

Bill Kinney
SM160, Harmonie
Annapolis, MD, USA