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Training Wheels

Gel Versus Firefly Carbon Foam Batteries

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Looking at getting new group 31 batteries. How do gel and Firefly compare? Seems like they have similar attributes, but which has the best bang for the buck? Are Firefly's that much better if they're not getting fully charged on a regular basis? Which can handle more deep discharges? 

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Compare the cycle count vs depth of discharge for the Firefly vs a Deka g31 gel.  At 80% depth of discharge you get twice as many cycles.  You get 1000 cycles at 80% depth of discharge on the Firefly vs 1000 cycles at 50% DoD on the Deka.  The Firefly has 3600 cycles at 50% DoD, which is the 20% DoD count on the Deka. 

Capacity is also higher, 120ah at c/20 rate for the Firefly vs 98ah at c/20 rate for the Deka. 

The Firefly batteries are really expensive (but still a lot cheaper than Lithium), but seem to be the best options.  I have 2 and they've met my needs. 

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Thank you, just the info I was looking for!

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Fireflys are the real deal, particularly their resistance to sulphation when not being brought back regularly to 100% SOC. Supply is limited so if you find some don't muck around.

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There are really only two points of interest in the battery world. At one end, you have flooded lead-acid. At the other end, you have LFP. Everything else is some compromise in the middle and most add cost faster than they add value on that curve.

In my experience, Gel, AGM and Firefly very rarely really make sense as they're tweener solutions and only work if you have a tweener goal such as 2-3 years of moderate cruising and then sell the boat. If you're a day sailor with a slip and shore power who does a little beer can racing, the smart money is on lead acid and knowing you're going to replace them regularly.

If you have a 5+ year world cruising outlook, LFP will pay for itself despite the large investment, mostly due to reducing wear and tear on your diesel that results from long, low-load charging. If you have a 2 year outlook and don't care about weight, you're right back to flooded lead acid.

Without understanding your goals and cost sensitivity, it's really not possible to recommend a solution.

 

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Can you explain why specifically in the context of Firefly?  What is your personal experience with them. 

LFP is great, but a proper installation for even a moderate size bank is a lot more money.  It isn't just the cost of the batteries, it's also the cost of new alternator, solar, and shore power chargers.  Those also need to be addressed with Firefly, but most are pretty easily reconfigured to work with them. 

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9 hours ago, jack_sparrow said:

Fireflys are the real deal, particularly their resistance to sulphation when not being brought back regularly to 100% SOC. Supply is limited so if you find some don't muck around.

Are there any public, reliable, disinterested sources that back up Firefly's claims?

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Maine Sail/Compass Marine did a big test for Practical Sailor that did.

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Practical Sailor did a compatible test with other battery chemistries, Nigel Calder also did a field test and there are the users themselves like Alex W. Their niche is users who want a sealed battery and can't justify the capital expense of LFP as they might only go cruising say for few weeks bulk charging only and then once back on shore power bringing them back to 100% SOC with little capacity loss. However quite frankly Moon is right that for the vast majority of applications it is hard to go past FLA if you are not hell bent on sealed.. 

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At least here in Oz, if you do any offshore racing (Cats 1-3) you have a choice of one - all batteries must be sealed.

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Alex,

You say that LFP is a lot more money than Firefly, and you're right. But Firefly is a lot more money than FLA, too.

It's also true that every charge source (alternator, shore power, wind, solar, etc) must be re-evaluated when you change battery technology. This is true of FLA, gel, AGM, Firefly and LFP. It's actually less important with LFP provided your charge sources won't overcharge as LFP doesn't need a strict maintenance charging regime.

If your charge sources can be reconfigured, they can be reconfigured for LFP.

You ask "Can you explain why specifically in the context of Firefly?" - why what? Why are they a tweener solution?

To an extent, that depends on your goals, which haven't been stated. I generally look at batteries from one of three perspectives:

  1. The low-budget daysailer - usually boils down to kw-h/dollar
  2. The no-budget racer - usually boils down to kw-h/kg 
  3. The offshore cruiser - usually boils down to kw-h/cost-of-ownership

For #1, cost is such a big lever that the lowest cost batteries almost always win. Sears or KMart is almost always the right answer. With a modest solar panel, they last a season or two. Here's a sealed, useable 75AH battery for US$99 based on trade-in. That's your bang-per-buck leader, and it's sealed.

For #2, when budget is not a concern, almost any analysis will end up at LFP.

For #3, if you really look at total cost of ownership, the enemy is engine hours and LFP's acceptance rates combined with big alternators can't be beat.

The most difficult kind of recommendation is when someone comes along with a constrained problem:

  1. Best battery system I can install for $1000
  2. I want xxxAH to fit in this existing battery box
  3. Best solution I can install this weekend

What problem are you trying to solve? What are your constraints? 

Fleetwood, when you need a sealed battery, your choices aren't one - they're all but one. Only flooded is excluded.

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You left out those of us who race and cruise our, and do have a budget...

By my math 2 Firefly batteries are cheaper than 3 Lifeline and weigh less and take less space, but give me a greater usable amp hour rating and work better for partial state of charge applications when cruising.  LFP is out of my budget, but these aren't.  FLA isn't legal for lots of racing, including races in my plans.

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I didn't leave you out. I listed the applications with which I'm most familiar. I specifically acknowledged over constrained problems and asked about your goals. You still haven't articulated any and that's where the problem arises.

Your race, you cruise and you have a budget. You're apparently size and weight sensitive, too. You want more for less, too, but that's an uninteresting data point because who doesn't?

What's your budget? What's 1kg weight savings worth to you? What's an hour of engine run time worth to you? What's your 24 hour AH budget for racing and for cruising? How do you prioritize cost, weight, capacity and size? 

Without sharing objective criteria for your design, you're the only one who can participate in your decision making.

 

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I have existing space for two batteries (group 27 is ideal, group 31 just fit). I have a $1k budget for batteries and would like to get 10 years our of them, but 5 would be okay.  When I'm cruising I need to go up to 4 or 5 days between marinas and without using the engine much and with a small solar setup (a 50w panel, it could be up to 100w).  My fridge uses 30ah/day average, up to 40ah/day. Other stuff is 10-20ah/day.  I need to use sealed batteries.

Running the engine in anchorage or while sailing isn't an option that I'll consider.

Any additional batteries would need to fit under the quarterberths and so they would lay on their sides.  I'd prefer not to have the extra weight or to add the blocking to support batteries there.

Budget is flexible, I can spend more on batteries, but it comes from my diving or sail or other budget.

This is far more specific than any specification that you mentioned.  You are just being argumentative.  I answered anyway.  I think many people sailing 20+ year old racer/cruisers who both race and cruise would have similar requirements.

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Solar in the PNW isn't really a good investment, you'd be better to spend the money on batteries or an alternator. You don't say anything about an alternator, so I'm guessing it's a 50 to 80A stock alternator with automotive regulator and also doesn't do much. So your only real charge source is shore power, which isn't a very reasonable budget allocation. Certainly in the PNW, there's a lot of motoring when cruising, but there's none in your "specification", which isn't actually very specific at all.

Your overall budget of 75% for refrigeration seems a little high, and your total requirement of 50AH x 5 days, isn't that high, but for $1000 you don't have many options. As is usually the case, focusing on reducing your demand might be be the best thing to do. 50AH could be as little as 20 minutes of engine run time per day with the right charging system and a 100AH LFP system makes a lot of sense.

At 80% DOD, you can't get what you need from a pair of Fireflys - they're good for about 90AH each and even with three, you're stretching it. You'll finish your five days on the hook with only 20Ah remaining, which isn't much margin. Four fireflies will run you nearly $2000 before tax and you still have no way to charge them without getting to a marina.

Basically, I believe your goals and your budget simply don't add up. I'd recommend taking a serious look at doing a shitty, temporary installation with some cheap gels and investing in realistic charging first and then good batteries later. Along that path, if you could figure out how to drop your consumption 10-20%, that will likely pay for itself when you're sizing a real battery system for a solid charging system.

Finally, if you're thinking you're getting 5-10 years, then your budget is $100-$200 per year, which is pretty slim. If instead you think of your budget as even $50 per month, that's a $3000 budget over 5 years or 6k over 10, which opens up a world or possibilities. And you can make that investment over time, even 3 years at $1000 per year will completely change the equation.

But I'm just argumentative ...

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Solar in the PNW works just fine, thank you very much. Our summer days are long and it's one of the driest places in the US during the month of August, so cloud cover isn't even an issue during peak cruising season. Count on about 70% of peak rated panel output if they're oriented horizontally and more if you can point them. 

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To me (a budget minded offshore cruiser) FLA mades more sense on cost of ownership because we had 550W of solar panels. Very very rarely did we have to run the engine for battery charging. So high acceptance wasn't important. 

4 x golf carts = 450 a.hr @ $600

(useful a.hrs = say 200 a.hr)

Say you only get 3 years from your batteries (we got 3.5+) so annual battery cost = $200

VS

Firefly 31  3 x 100 A.hr = 3 x $468 = $1400

(useful a.hrs 240 A.hr. don't think you can really go to 100% Discharge so I used 80% figure

"capable of 3X the number of deep discharge cycles than that of other lead acid batteries"

So maybe it lasts for 9 years? Annual cost = $155

If it lasts for 7 years you are in FLA ballpark.

I am frankly surprised at how cheap the Firefly batteries are for higher useful capacity. Caveat: only 24 months warranty, 48 months pro-rata warranty in marine use. So maybe you don't really get 7 years out of them!

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Moonduster: My system that I described does work.  I have two Firefly batteries. I have an anemic 35 amp Hitachi alternator. I have a 50w solar panel. I can go 4 days without plugging in, I just did it on the cruise that I'm on.

Our biggest power drain is the fridge, we're cruising with a baby and have instruments on for 3-5 hours average (because we sail short distances) and go to sleep when it is dark so there really isn't anything else left except for charging cell phones.  All of our lighting is LED.  Our instruments take 1.5 amps when the autopilot is in use (most of the time). 

The deepest discharge during our trip was 120ah and we averaged 20ah per day on solar (as reported by a Victron battery monitor and MPPT charger), so a 5th would be possible.  Our peak solar days are 300wh (25ah).  On a mostly rainy and grey Thursday we got 70wh.

I don't think FLA or AGM would handle the deep discharges or be happy with the incomplete full charges.  LFP would certainly work but cost a lot more.

I understand how to amortize costs but there is also the issue of upfront capital. We bought this boat a year ago and our digging through the standard backlog of work. $3000 in batteries is cheap per year, but expensive now (especially since we still need a new main and a bottom job).  I already had a single Firefly (kept from my old boat), so buying a second was the cheapest option we had.

 

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I was also looking at the firefly's but chose the outback 200NC (nano carbon) instead.  The form factor was the key as I had height to give.  The benefits of the DoD and capacity are great.  I have no issues to date.  They have been installed for only 4 months .

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I second Alex's comments above -- I used two Fireflys Oasis batteries and saw them work very nicely in summer PNW conditions in combination with solar.  This was while racing in R2AK 2016 on a F-31 trimaran, so no heavy loads (no fridge), but 24h/day usage of chartplotter & VHF, plus nighttime LED nav lights, and some occasional autopilot, over 6 days (note: no engine, so no non-solar recharging at all).

I never saw our state of charge drop below 60-65% full at night based on the min voltage of 12.47 by the last night (with some light loads still on), and it would go right back up to nice bulk charge voltages of >13.6V daytime (though not perfect - 14.4V is recommended).  I was very happy with our setup -- some other boats were out of power by the end, but we were still nicely charged. 

We also gained good weight savings given the higher usable capacity, allowing us to cut down from 3x to 2x batteries, very welcome on a weight-sensitive multihull:

- 3x existing WestMarine Gel batteries: 200 lbs for 140Ah usable capacity

- 2x Firefly: 150 lbs for 185 Ah usable capacity @$990

The Firefly cost seemed very reasonable compared to Gel and even AGM or FLA, given that 50lbs weight savings vs. our existing Gel batteries (which tend to be heavy) and better overall capacity and lifetime (and we also preferred sealed batteries in case of capsize). None of the FLA/AGM alternatives were really that compelling in comparison:

- 3x WestMarine FLA: 167 lbs for 140Ah usable capacity @$650

- 3x Lifeline AGM: 170 lbs for 140Ah usable capacity @$935

Our final setup:  

- 2x Firefly ($990 with tax)

- 100W solar panel from Amazon for $150 (putting in about ~420Ah/day on avg)

- Victron MPPT BlueSolar 75/10 solar controller for $90

- Victron Bluetooth Dongle for $50: really nifty to monitor solar panel & battery via smartphone.  See the nice screenshot below - we started sailing around Day T minus 7-8: you can see the daily yield is ~420Ah after that, and the min battery voltage per day starts dropping gradually to 12.47V by day T minus 2, not shown; note "consumption" is shown as zero as there were no loads directly on the solar controller.

IMG_4808.thumb.PNG.0aa00f745a2e9a5e5902c93724064884.PNG

 

 

 

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Another happy Firefly user here.  I have a 38' club racer/weekender that I sail off a mooring.  No solar or wind and I rarely see shore power.  Batteries live at 40%-80% most of the season, with rare (2-3 times/year) trips to 100%, either from plugging in at a marina or long motor runs.  On my third season with them and so far, so good.  My unscientific estimate is that they haven't lost any capacity.

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