Re: Firefly batteries

Paul Dowd and Sharon Brown

Hi Bill,


When you say this:


All the batteries on the 0 to 12 volt side are tied together. Likewise, all of them on the 12 to 24 volt side


Do you mean that the low side batteries of each 24v pair are all connected together with all their negative terminals, and similarly the high side?


Another possible hitch is that I’ve been told by a UK supplier that FF recommends no more than four pairs, but I believe you mentioned that a 54 would normally take 12. Would you know anything about this? It is the G31s I’m talking about.


Hi Arno,


You say that 12 batteries would not fit. I note you have hull 121 and wonder if Amel may have resized the compartment in the interim from #98? According to my measurements on record it will be 2mm short on the width but assume some inaccuracy in this measurement.





Paul Dowd & Sharon Brown

S/Y Ya Fohi, Amel 54 #98
tel: +44 (0)7710 466619

skype: pauldowd


From: <> On Behalf Of Bill Kinney
Sent: 08 October 2021 14:54
Subject: Re: [AmelYachtOwners] Firefly batteries



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

Ya Fohi - Amel 54 #98

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