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Firefly batteries and the rest of the charging system

Apr 8, 2011
461
Hunter 36 Deale, MD
So I’m looking at replacing my house batteries on my 2009 H36 within a year and doing some research. I’ve read the AGM thread and done some reading in MarineHowTo.com as well. I’ve installed a Victron BMV-712 monitor, and am nearly done installing 160 watts of solar run thru a Victron 100/20 MPPT. Currently have PO installed 2 4D Deka (WM branded) 198 Ah AGMs and a ProMariner ProTech i 1230i Plus shore charger, and a stock 60 amp internally regulated alternator and (new) belts on the 3YM30. The reason I’m considering replacing is the batteries are original (I’m fairly certain), and capacity loss is substantial, though I’m still working on a test as MaineSail prescribed to get a tighter figure on capacity remaining. My usage is mostly day/weekend overnight sails, with occasional 7-10 day trips away from the slip. Otherwise the boat is plugged in and the charger engaged. My energy consumption is 130Ah/24 hrs away from the dock. I sail on the Chesapeake bay.

I’m leaning toward 3 paralleled Group 31 110Ah Firefly batteries. I’m primarily interested in thoughts on what I may need to do with regard to my alternator and/or shore charger, if anything. I would like to maximize the life on the Firefly batteries. Other suggestions welcome.
 
Feb 26, 2004
21,899
Catalina 34 224 Maple Bay, BC, Canada
Why bother? With your use, non-WM brand fla wet cell deep cycle batteries will do just fine.
 

Johann

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Jun 3, 2004
250
Leopard 39 Pensacola
Since you will have to run the engine about every other day to charge the batteries when you’re out on the hook, I’d look into upgrading the alternator to reduce engine run time and make more efficient use of the diesel. Maybe a CMI-80 with external regulation.

As far as the shore charger, Firefly recommends charging at .4C about once a week if you’re deep cycling. So that’s 132 amps for 3 firefly G31s.
 
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Johann

.
Jun 3, 2004
250
Leopard 39 Pensacola
After thinking about it, how about going for a 150 amp Balmar with a serpentine belt, and leaving the shore charger alone? You will typically be deep cycling on the hook, so why upgrade both the shore charger and alternator when I think it would be about the same cost to just go big on the alternator.
 
Jan 11, 2014
7,735
Sabre 362 113 Fair Haven, NY
I'm with @Stu Jackson on this. Why spend the extra money on Fireflys? True deep cycle FLA batteries will do just fine. Think about 6v golf cart batteries, 4 of them will give you ~440 amp hours. It will need a new charger, one that can support about 50 amps, like a Sterling 1250. A larger externally regulated alternator will be helpful too, but no need to go with a 150 amp Balmar for your usage. The CMI 90 amp should be sufficient for your plans. 4 new GC batteries, a new charger, and a new alternator and regulator will set you back about the same as 3 Fireflys alone.
 
Nov 14, 2013
198
Catalina 50 Seattle
I'm with Stu and dlochner on this. With your use case, FLA isn't just fine, it's appropriate. If you upgrade your batteries, you'll need to upgrade your charging to take full advantage and also to prevent damage.
 
Feb 6, 1998
11,436
Canadian Sailcraft 36T Casco Bay, ME
I’m leaning toward 3 paralleled Group 31 110Ah Firefly batteries. I’m primarily interested in thoughts on what I may need to do with regard to my alternator and/or shore charger, if anything. I would like to maximize the life on the Firefly batteries. Other suggestions welcome.
If you go Firefly you will want at the very least a new charger and at a bare minimum external regulation. This makes the whole cost a rather big pill to swallow, about 3.2K to 4K in total and that is if you DIY the entire thing....

One big mistake I see many Firefly owners doing is not discharging to 80% DOD.. The value in these batteries is that you can use 80% of the capacity and still exceed the cycle life of a typical deep-cycle AGM (the WM AGM's are not a true deep-cycle AGM) that is discharged to only 50%. Because of the PSOC tolerance it often means a smaller bank than you previously had by at least one battery. With only three Firefly batteries you'll actually have more usable capacity, about 264Ah usable, than you had with the Deka 4D's, 198Ah. Heck just two Firefly batteries will yield 176Ah usable compared to 198Ah for your WM/Deka's..

Another area folks don't account for is the usable capacity that can be stored quickly when cycling to 80% DOD. This means your charging system is working as efficiently as it can and you can restore about 60% of your capacity (80% DOD to 80% SOC, at charge efficiencies pushing 98% where with flooded or standard AGM your looking at only 30% of your energy being returned at a high efficiency. This also means that your alternator & regulator had better be ready to handle a long bulk charge duration.

That being said, your described use does not really warrant a dire need for Firefly or even AGM batteries for that matter. For a few hundred dollars some 12V Golf Car batteries (T1275 or J150 Trojan's), or 6V Golf Car batteries in series/parallel, will allow you to dial in your charging system so that when or if you decide to go Firefly, your system is ready for it..
 
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May 7, 2012
946
Hunter e33 Maple Bay, BC
That being said, your described use does not really warrant a dire need for Firefly or even AGM batteries for that matter. For a few hundred dollars some 12V Golf Car batteries (T1275 or J150 Trojan's), or 6V Golf Car batteries in series/parallel, will allow you to dial in your charging system so that when or if you decide to go Firefly, your system is ready for it..
Friday, I replaced 4 x 6V US Battery Golf Car batteries with Trojan T125s FLA. I had seriously considered, in fact was on a wait list for 4 x Firefly Oasis AGM G31 batteries. But in the end I could not justify the total additional expense when the FLAs had given me 7 full years of 100+ days per year away from our marina. Quality control issues, as stated by Nigel Calder in Sail Magazine (September 2018), was also a factor in my decision.
 
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Dec 4, 2018
46
Balboa 27 Denver
Glib response and I think you might consider ventilation concerns with wet cell replacement for arm.
 
Apr 8, 2011
461
Hunter 36 Deale, MD
Glib response and I think you might consider ventilation concerns with wet cell replacement for arm.
I genuinely appreciate the thinking and depth of responses from highly experienced forum members. Between their prolific - and incredibly informative - postings, I'm weighing my options for this battery replacement/system upgrade. Since I'm thinking the comments through and haven't made a decision I thought it only polite to acknowledge the comments and say thank you. So I'll have to disagree with you that my comment was in any way insincere. But hey, YMMV.

Fair point about ventilation with wet cells. They'd go into a fiberglass battery box whose top is screwed down, in a huge cockpit locker which is otherwise mostly empty. There don't appear to be any penetrations thru the box (e.g. its not screwed DOWN, which Maine Sail points out could risk leaking battery acid out of the compartment in the event of fluid loss in the box). Not entirely sure it vents outside significantly; if anyone has ideas on whether I should take overt steps to vent the battery compartment I'm certainly interested in thoughts.

Can you clarify what you mean by 'replacement for arm'?
 
Dec 4, 2018
46
Balboa 27 Denver
I genuinely appreciate the thinking and depth of responses from highly experienced forum members. Between their prolific - and incredibly informative - postings, I'm weighing my options for this battery replacement/system upgrade. Since I'm thinking the comments through and haven't made a decision I thought it only polite to acknowledge the comments and say thank you. So I'll have to disagree with you that my comment was in any way insincere. But hey, YMMV.

Fair point about ventilation with wet cells. They'd go into a fiberglass battery box whose top is screwed down, in a huge cockpit locker which is otherwise mostly empty. There don't appear to be any penetrations thru the box (e.g. its not screwed DOWN, which Maine Sail points out could risk leaking battery acid out of the compartment in the event of fluid loss in the box). Not entirely sure it vents outside significantly; if anyone has ideas on whether I should take overt steps to vent the battery compartment I'm certainly interested in thoughts.

Can you clarify what you mean by 'replacement for arm'?
Whoops. Mean AGM
 
Jan 11, 2014
7,735
Sabre 362 113 Fair Haven, NY
Fair point about ventilation with wet cells. They'd go into a fiberglass battery box whose top is screwed down, in a huge cockpit locker which is otherwise mostly empty. There don't appear to be any penetrations thru the box (e.g. its not screwed DOWN, which Maine Sail points out could risk leaking battery acid out of the compartment in the event of fluid loss in the box). Not entirely sure it vents outside significantly; if anyone has ideas on whether I should take overt steps to vent the battery compartment I'm certainly interested in thoughts.
Battery boxes should have some ventilation. One common worry is hydrogen off gassing from the charging process. When an electric current is passed through water, the water molecules break down in to their constituent parts, Oxygen and Hydrogen gas. While hydrogen is explosive there is very little hydrogen stored in a battery and hydrogen being the lightest element quickly dissipates given half a chance.

The other concern is sulfur compounds that off gas. These are not explosive but they are corrosive. When they escape and condense on moist metal they form acids and they can be corrosive. That's why you don't put batteries directly underneath expensive chargers and electronic devices. Again allowing these to dissipate slowly is helpful.

So, the battery box needs some ventilation and the area the battery box is in needs ventilation. The vents don't need to be big and they are not nearly as troublesome as heavier than air explosive gases such as propane and gasoline.
 
Feb 6, 1998
11,436
Canadian Sailcraft 36T Casco Bay, ME
Guys,

Please understand that the ABYC does not differentiate battery ventilation between standard open flooded batteries and valve regulated lead acid batteries such as GEL, AGM, TPPL AGM or Carbon Foam AGM, they all require ventilation. The only thing that differs between AGM and flooded, in regards to standards compliance, is the need for acid containment with batteries that could leak electrolyte.
 
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Apr 8, 2011
461
Hunter 36 Deale, MD
Update: In January 2020 a local supplier ran a sale on Firefly batteries, so after much consideration of the advice here and in other places on SBO (and Cruisers Forum) I decided to replace my old AGMs with two Firefly G31s - cognizant of the other system upgrades necessary and the cost involved (thanks to the discussion in this forum). I purchased a Sterling charger from MarineHowTo based on their recommendation and had it professionally installed (the 60 amp charging required some wiring upgrades I'm not qualified to handle). I've since programmed that charger and my Victron MPPT solar controller to match the manufacturer's recommendation for charginging/maintaining the Firefly batteries. The last piece is the alternator, which I'm also working with MarineHowTo to source an externally regulated solution that will fit my needs. Also, losing over 100 lbs from my port cockpit locker has fixed the slight permanent list to port that the boat has had since I purchased it 3 seasons ago.

But I do have a question. My Victron battery monitor allows me to set a low voltage alarm, and presently I still have it set to 12.2 volts, which is what I used for my AGMs. I'm wondering what an appropriate low voltage alarm should be for the Firefly batteries, given they are designed to be used to a greater depth of discharge than the AGMs? I ran across the attached test report on Cruisers Forum, which is from the manufacturer and details a voltage vs SOC% for the G31 battery at two discharge rates. My thinking is to set the low voltage alarm at 11.8 volts, which would represent a 70% depth of discharge. However, that is probably inaccurate since the battery would be under load. Should I set it lower? I do not have a starting battery, so I also want to make sure that my house Firefly batteries can still start my 3YM30 at the lower voltage available at the deeper discharge. Not sure where that starts to become a problem.

Perhaps I'm overthinking this. Curious as to your thoughts.
 

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Jan 11, 2014
7,735
Sabre 362 113 Fair Haven, NY
I don't have an answer, but another question. How large will the voltage drop be when the engine starts? The initial draw from the starter will cause a momentary voltage drop, will that trigger the alarm? Will it just beep and turn off when the surge passes, or will it stay on and need to be reset?

The GC batteries on my boat need to be replaced (some fool left the refrigeration on and the battery charger off too many times :facepalm:) and have been considering changing to Fireflys. So, I'm interested in your project.
 
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Feb 6, 1998
11,436
Canadian Sailcraft 36T Casco Bay, ME
Update: In January 2020 a local supplier ran a sale on Firefly batteries, so after much consideration of the advice here and in other places on SBO (and Cruisers Forum) I decided to replace my old AGMs with two Firefly G31s - cognizant of the other system upgrades necessary and the cost involved (thanks to the discussion in this forum). I purchased a Sterling charger from MarineHowTo based on their recommendation and had it professionally installed (the 60 amp charging required some wiring upgrades I'm not qualified to handle). I've since programmed that charger and my Victron MPPT solar controller to match the manufacturer's recommendation for charginging/maintaining the Firefly batteries. The last piece is the alternator, which I'm also working with MarineHowTo to source an externally regulated solution that will fit my needs. Also, losing over 100 lbs from my port cockpit locker has fixed the slight permanent list to port that the boat has had since I purchased it 3 seasons ago.

But I do have a question. My Victron battery monitor allows me to set a low voltage alarm, and presently I still have it set to 12.2 volts, which is what I used for my AGMs. I'm wondering what an appropriate low voltage alarm should be for the Firefly batteries, given they are designed to be used to a greater depth of discharge than the AGMs? I ran across the attached test report on Cruisers Forum, which is from the manufacturer and details a voltage vs SOC% for the G31 battery at two discharge rates. My thinking is to set the low voltage alarm at 11.8 volts, which would represent a 70% depth of discharge. However, that is probably inaccurate since the battery would be under load. Should I set it lower? I do not have a starting battery, so I also want to make sure that my house Firefly batteries can still start my 3YM30 at the lower voltage available at the deeper discharge. Not sure where that starts to become a problem.

Perhaps I'm overthinking this. Curious as to your thoughts.
Those graphs are "under load" at a 30A or 11A load. A 20 hour load would be 5.8A based on the current 116Ah Firefly batteries.. In the Practical Sailor PSoC testing, the tested Firefly (110.2 Ah), delivered 82.52 Ah's at a 20 hour discharge rate to a cut off voltage of 11.7V.

Put another way, stopping the discharge at 11.7V, under 5.5A load (20 hour rate), yields a 75% DoD.. None of these are resting voltage numbers, they are all under load.
 
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Apr 8, 2011
461
Hunter 36 Deale, MD
Those graphs are "under load" at a 30A or 11A load. A 20 hour load would be 5.8A based on the current 116Ah Firefly batteries.. In the Practical Sailor PSoC testing, the tested Firefly (110.2 Ah), delivered 82.52 Ah's at a 20 hour discharge rate to a cut off voltage of 11.7V.

Put another way, stopping the discharge at 11.7V, under 5.5A load (20 hour rate), yields a 75% DoD.. None of these are resting voltage numbers, they are all under load.
That makes sense, and should generally coincide with the Ah deficit on my Victron display. Practically speaking is that too low a voltage to successfully start the 3YM30 with , or is there still a considerable/reasonable margin of cranking still available at that voltage under load with ~58Ah remaining in the two paralleled batteries?
 
Feb 6, 1998
11,436
Canadian Sailcraft 36T Casco Bay, ME
That makes sense, and should generally coincide with the Ah deficit on my Victron display. Practically speaking is that too low a voltage to successfully start the 3YM30 with , or is there still a considerable/reasonable margin of cranking still available at that voltage under load with ~58Ah remaining in the two paralleled batteries?
Not too low with two Firefly batteries. These batteries were invented in the Caterpillar Special Projects Lab for starting heavy equipment that sat for long periods even in sub freezing weather. They were designed to be able to start massive diesels...
 
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Feb 21, 2005
1
- - Miami
Attached is an email exchange I had with a very helpful fellow at Ocean Planet Energy. They sell support Firefly batteries.

What’s the lowest voltage these batteries can withstand?
10.5v is fully discharged, 11.7v is @ 60-70% discharged. You can discharge them down to 80%, @ 11.5v as long as you bring them back with a charge rate of at least .4C. (.4C is 40% of rated capacity - 3X in series gives you 348ah of capacity, so periodically it is good to recharge with at least 120-140 amps. )

2. Charging my old batteries either with my solar panels or shore power resulted in readings of 13.6 volts. With the ships alternator (a dumb 80 amp gadget) 14.6. Can the Firefly handle that?
Sounds like the float is too high at 13.6v. Proper care of the Firefly requires resetting your shore power charger to the following parameters: Bulk 14.4v, Absorption 14.2v, Float - 13.4v. Your shore power charger and your solar chargers need to be reprogrammed to these parameters to properly maintain the batteries and to fullfill the warranty. I would also recommend upgrading your alternator to a higher output with a programmable controller. For your bank of 348 ah, I would suggest the MGDC 180a unit with a remote rectifier. This would giveI have attached some test charts , articles, the manual and the warranty document. I hope this is all helpful, and please don't hesitate to write with further questions. These are great batteries.

Tom Whitehead-Technical Sales
Ocean Planet Energy
cell (207) 838-9715
Office (207) 370-9112
www.oceanplanetenergy.com
 

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