Suggestions for replacing Firefly batteries

[Gianluca]

Hello, I have 3 Group31 Firefly that have performed well for the last 6 years as house batteries on my boat. I want to replace them but Firefly is not longer available.
Any recommendation how to proceed to get the best AGM replacement batteries. Best for me is high Ah and long life….
Thanks,

[smac999]

I would look at the battle born group 31 lithium.

what do you currently have for shore and engine charging? and what is the biggest load on them?

[Gianluca]

I have a PROsine 2.0 Inverter/Charger for shore and a 120 alternator for engine charging. Also 2 solar panels 110w/each. Lithium is not possible since I would have to change too many parts..

[smac999]

for agm, life line is among the top

north star is another good one

[Adelie]

What's your bank size?
What's your budget?
I assume that nominal house voltage is 12v.
Do you have any high demand uses like electric propulsion or electric galley?

[Gianluca]

The bank size is 3 group31 batteries. I would like to maximize the total Ah. I would like to know the options before adding the cost to the decision. Thanks

[Adelie]

By bank size I meant Ahr.

If I recall correctly Fireflies are 116Ahr each.
3 is a 348Ahr bank. Assuming 80% usable, that 278Ahr.

BattleBorns are a straight 100Ahr, 12v @ $849 each. Assume 90% usable. Sealed case so no repairs possible.
3 BBs are 270Ahr usable, $2,547 + tax and shipping.

SOK marines are 206Ahr, 12v @ $1,030 each. Assume 90%. Screw top plastic case so repairs possible.
@ SOKs are 371Ahr usable, $2,060 + tax and shipping.

Trojan T-1275s are 156Ahr, 12v, $238 each (extra $15 if you don't have a core), 50% usable. Real deep cycle so 1200 or so cycles is a reasonable guess at cycle life.
4 T-1275s are 312Ahr usable, $952 + tax and shipping.

Will Prowse made pleasant noises about the SOK which is why I included them here.

https://www.youtube.com/watch?v=zJmIS5tmzPU&t=83s

If you wanted to DIY LFP batteries they could be a lot cheaper.

Personally, I would not be happy using 90% of a LiFePo, I'd be happier cycling between 10%&95% SoC, so 85% usable.

Expected lifetime for LiFePo is going to be dependent on normal operating temps. Are you going to be operating a lot in near freezing conditions or near 95F?

[ItDepends]

Originally Posted by Gianluca

Hello, I have 3 Group31 Firefly that have performed well for the last 6 years as house batteries on my boat. I want to replace them but Firefly is not longer available.
Any recommendation how to proceed to get the best AGM replacement batteries. Best for me is high Ah and long life….
Thanks,

Lifeline is a premium AGM supplier that makes a drop in replacement for the Firefly G31. (I switched the other way a few years ago.) Consider Victron as well. They have a full line of batteries that they do a good job with, and some are size compatible with the G31 form factor as well as compatible with all of you existing charging gear.

My own opinion is that if the AGM's have served you well, and do what you need, there is no reason to spend the big bucks for a lithium conversion. If you are happy with a 6 year lifespan, why would you spend 3X the cost to get 18 theoretical years?

[Adelie]

I forgot to ask, are you set on AGMs?
One of the 2 Achilles heels of AGM is that if you really are using 50% of capacity you should be recharging at at least 0.2C (70A), 0.4C (140A) would be better. That means a high output alternator.

The other is Partial State of Charge (PSoC) cycling, not recharging to 100% almost every discharge cycle. Firefly's are less suceptible to PSoC cycling than regular AGMs. Converting to a regular AGM bank would likely see less life for exactly the same use.

If you really want a maintenance free battery get a Gel. If not, get regular FLAs.

[goboatingnow]

Originally Posted by Adelie

I forgot to ask, are you set on AGMs?
One of the 2 Achilles heels of AGM is that if you really are using 50% of capacity you should be recharging at at least 0.2C (70A), 0.4C (140A) would be better. That means a high output alternator.

The other is Partial State of Charge (PSoC) cycling, not recharging to 100% almost every discharge cycle. Firefly's are less suceptible to PSoC cycling than regular AGMs. Converting to a regular AGM bank would likely see less life for exactly the same use.

If you really want a maintenance free battery get a Gel. If not, get regular FLAs.

Agree entirely , AGM have advantageous of little use or use ability on a boat and disadvantageous ( PSOC) that are all too prevalent.

Don’t bother use good conventional SLA.

[Rumrace]

Are Battleborn and a few other internet deals 3.2 Chinese batteries stuffed in plastic cases with domestic names. It’s a great Chinese method to deflect the defect on the importer dodge UL in the process. Pay more buy domestic

[goboatingnow]

Originally Posted by Rumrace

Are Battleborn and a few other internet deals 3.2 Chinese batteries stuffed in plastic cases with domestic names. It’s a great Chinese method to deflect the defect on the importer dodge UL in the process. Pay more buy domestic

Doesn’t mean their bad or good just examine them.

I’d certainly never accept Li batteries when I can’t access the cells and BMS

[sailorboy1]

Originally Posted by Gianluca

Hello, I have 3 Group31 Firefly that have performed well for the last 6 years as house batteries on my boat. I want to replace them but Firefly is not longer available.
Any recommendation how to proceed to get the best AGM replacement batteries. Best for me is high Ah and long life….
Thanks,

Assuming that if like me you only spent the $$$ for the Firefly batteries to not worry about the PSOC issues of lead acid batteries, then there aren't any AGM batteries to suggest. I feel your best answer is now the packaged LFP system that the Electric Car Parts Co sells. They have a 400ah setup that is "reasonable" compared to what the recent of FF batteries were.

Don't know why you feel you need to replace your FF, but hope you tried to "recover" them.

[ranger58sb]

Originally Posted by Gianluca

Hello, I have 3 Group31 Firefly that have performed well for the last 6 years as house batteries on my boat. I want to replace them but Firefly is not longer available.
Any recommendation how to proceed to get the best AGM replacement batteries. Best for me is high Ah and long life….
Thanks,

I have had good service from both Odyssey Extreme and Lifeline AGMs.

Former were G31s (PC2150s, but they have a new name now...). Each bank of 3x lasted in the range or 11-12 seasons, but that was also with light use and with near-daily recharge except for a couple of 1-2 weeks at anchor every year. When anchored, we recharged with the genset 2x/day, coinciding with meals (electric galley) and heating water.

The Lifelines were GC2s (GPL-4CT) in the previous boat. I think only 4-5 seasons on those before we changed boats... but they were still going strong and the new owner (for a couple years) hasn't reported any issues. We also just replaced a pair of flooded 8Ds with Lifeline 8D AGMs in our current boat, but haven't yet enough time with those to report longevity.

-Chris

[phantomracer]

Going to be in the same boat at some point. my 6 year old fireflies are performing quite well, like new. However, when they do give up the ghost, will be looking for alternatives.

Going to be a choice between LFP and revamping the charging system, for a multi thousand dollar project, or pop a couple Costco GC-2 6v FLA batteries in for $100 each, beat them up, get 2-3 seasons on them. Conservatively, 10-12 sets of the golf cart batteries (or more) will be the break even point for converting to LFP