Re: Firefly Battery Update.

Porter McRoberts

Wow. Great Intel Bill. Thank you 

Makes one rethink lithium in light of the insurance issues.   Or at least consider the firefly a greatly improved bridge to wait for insurers to figure out the low risk reality of lithium. 

Porter McRoberts 
S/V IBIS A54-152
WhatsApp:+1 754 265 2206

On Feb 3, 2022, at 2:27 PM, Bill Kinney <cruisingconsulting@...> wrote:

We have had some pretty extensive experience with our Firefly batteries at this point.  We had a couple lumps and bumps in the road early on, but things now seem to have settled in to good smooth operation. One of the unusual things about using a Firefly battery is the procedure Firefly calls a “Restoration charge.” After an extended period of operating at partial state of charge, to restore full capacity the recommendation...

...perform the restoration charge: charge the G31 to 14.4V at 0.2C to 0.5C and continue to charge until the current drops to 0.5 A on the G31  and apply a float charge of 13.5V( for 24 hours. Fully discharge the battery to 10.5V, and then repeat the same charge cycle.

This is also the perfect time to actually measure the capacity of our batteries, and monitor their health since we are discharging them from 100% full to as close to 0% as you can get. For our battery bank we do this by turning on a 1000 Watt space heater, which pulls about 50 Amps out of our 480 Amp-hour battery bank draining them fully in a little under 10 hours.


For our 480 Amp-hour battery bank Firefly’s minimum recommended charger capacity is 96 Amps, and a maximum of 240 Amps, with an optimum target of 192 Amps.  Our DC generator puts out 150 Amps, and our shore power inverter/charger does 70 Amps.  


In Early October 2020 we installed our current set of Firefly batteries.  After installation, initial charging and testing we did a full ”restoration charge” bringing the batteries from 100% full, down to dead flat, and recharging at 200 Amps


Firefly’s current specification sheet for the G31 version of their batteries lists the C10 capacity as 448 Amp-hours.  Our first test in Oct 2020 came in at 454 Amp-hours. It seemed we were off to an excellent start.  Our batteries managed to supply about 1% above rated capacity.


In April 21 after 6 months of use, we again ran a full discharge test, and the result was 459 Amp-hours. Even better!  Obviously this was not a trend that could continue for long…


In Sep 21 after 10 months another test:  453 Amp-Hours.


Jan 22 at 15 months: 448 Amp Hours, right on the original rated nameplate capacity.


I do not have any feeling for how precise these numbers are.  Are they +/- 1% or 10%?  No idea.  The measurement is a bit dicey, since I certainly do not have laboratory grade control of the discharge rate.  So I can’t yet say if there is a real trend here or not.  But the (very!) good news for us is after 15 months of constant use our batteries test out right at the original manufacturer’s capacity specification, or a tiny bit better!


Considering that an inexpensive set of flooded lead acid batteries would be approaching the end of their useful life at this age, we count this as a total win.  Will these batteries last long enough to justify the additional cost?  Maybe.  Maybe not.  But they are certainly on the right track.


For the way we use our boat, the performance of these seems to be an excellent match. 

Bill Kinney
SM160, Harmonie
Hollywood, FL, USA

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