Gizmo’s new Firefly battery bank, working out the details

Ben Ellison

Ben Ellison

Panbo editor, publisher & chief bottlewasher from 4/2005 until 8/2018, and now pleased to have Ben Stein as a very able publisher, webmaster, and editing colleague. Please don't regard him as an "expert"; he's getting quite old and thinks that "fadiddling fumble-putz" is a more accurate description.

122 Responses

  1. Anonymous says:

    What an ugly installation, sorry.

  2. serenity says:

    Finally another one using a correct way to wire batteries. I have been using method 4 for many many years. When I show it to there boaters, their eyes glaze over and they must think “well he came from Amsterdam…so he must be…..”
    Method 4 keeps all the battery cable in the large battery boxes ( each with 4 batteries) I have and only 2 cables out
    Then I use method 3 to combine all 3 boxes so 12 batteries correctly balanced.

  3. Bill Bishop says:

    Anon, beauty is in the eye of the beholder. As someone who does more of his fair share of installing and dealing with new technology this is a thing of beauty to me, especially given the fact it’s not completed yet. On the first pass you do the best you can to get new tech in, working, and safely wired. This act alone usually comes with plenty of Whiskey Tango Foxtrot moments, and the need for clever creative MacGyvering along the way. Then you go back and improve it. Nice work Ben, and a great write up on this new battery tech.

  4. Ben Ellison Ben Ellison says:

    Thanks Serenity and Bill! Maybe I wasn’t clear how temporary that install is — no cables have been cut yet — or maybe Anon is having a bad day. Constructive criticism welcome, though.

  5. Maybe Anon doesn’t like the color scheme 🙂
    PS Blue Sea and Marinco are sister companies, so are they really competing?

  6. Ben Ellison Ben Ellison says:

    RC Collins of Compass Marine added an important comment to a Forum thread about monitoring individual battery temperatures:
    And, Norse, you’re right, I forgot that Blue Sea and Marinco now have common corporate ownership. Let’s hope that plays out well.

  7. Jeff Zurkow says:

    I like MRBFs because they protect the wiring all the way to the battery (and make it easy to comply with ABYC’s 7-inch rule). With multiple MRBFs, you can run cables of different gauges direct to the battery, while protecting each cable with a suitably-sized fuse. For example: a 4AWG to the alternator, a 2AWG to the house panel, and a 00AWG to the parallel switch.
    Putting fuses between individual batteries in a bank is also possible, but seems like overkill to me. If this is what Alden has in mind, I’d be interested in the rationale for it.

  8. Marc Dacey says:

    I concur, and as a guy who is buying six L-16s in the next two weeks that’ll comprise a big house bank for a passagemaker, this is timely.
    I have no doubt your AGMs are fit for your purpose, and I love them for boats on moorings and start batteries due to their slow loss profiles and the fact you can lay them down. But in the long run, it’s a proper and thoughtful and INFORMED installation that will make any battery or bank last longer and work safer. I particularly like that Blue Sea Terminal Mount Fuse Block Kit, because I have more than just a bilge pump I would like permanently wired and I dislike the usual rat’s nest approach.

  9. Jim Hebert Jim Hebert says:

    The recommendations for the parallel wiring of multiple batteries were very interesting. People who have a really thorough understanding of a problem tend to uncover subtleties and details that are overlooked by others. Thanks for pointing to that information.

  10. Richard Gard says:

    Ben, I didn’t notice how much you reduced your total Ah capacity with the firefly batteries from your Ah capacity with the old batteries?
    Being able to routinely discharge to 80% instead of 40% is a beautiful idea.

  11. Compass Marine says:

    The Blue Sea MRBF fuses are great products that can also be mounted to busbars but they are limited to 300A and would be a poor fit for any bank that could ever be used for starting an engine as large as Ben’s.
    MRBF to Busbar (background):
    On sailboats with small AUX engines 300A is almost always perfectly adequate to handle all loads, but as we push into larger engines, and the need to accommodate the potential for thrusters, large inverters or large engine starting requires larger fuses & wire.
    Best practice would be to size for any bank which can be paralleled in or switched to, in an emergency situation, or for any reason, and that bank should also carry over-current protection that can handle the starting loads without a nuisance trip.
    Rather than loading up battery terminals or battery switches with spaghetti, I much prefer to use off-battery busbars for negative distribution as well as positive/switched & positive/always-on distribution. Blue Sea also makes a bunch of hubs that can be used for this.
    What is lacking in the industry is a good high-amp fuse bus using ANL or Class T fuses where we can fuse for higher loads such as main battery bank protection, thrusters, inverters etc. and be able to bus them together.
    I am often forced to create my own ANL or Class T distribution bus with copper bar stock (McMaster-Carr) and red adhesive lined heat shrink. Marinco now has some plated copper busbars for their new Pro-Installer line that can work OK for busing large fuses too.
    As for fuses between batteries I have yet to see an internal short in a battery that could develop enough current between the good batteries and the bad one to trip a fuse that would not normally get tripped by the ships other high amp loads.
    I currently have two internally failed AGM batteries in my shop, one orbital and one flat plate, both of them now 10V batteries as opposed to 12V. No matter what I do I have not been able to trip a fuse between a brand new AGM and the bad one even when one is at 100% SOC and the other battery has already failed internally. When the over-current protection is sized to not nuisance trip for the max load the vessel will develop it would seem a rare situation where fuses between batteries would trip when otherwise properly sized.

  12. Anonymous says:

    Excellent points, Compass Marine. I confess I’ve been influenced by recent projects on auxiliary sailboats, where starting currents were modest and there was limited room to mount busbars. And thanks for the info on inter-battery fusing; I suspected it was unnecessary, but hadn’t done any research.

  13. Very timely info again here. I just purchased three G31 Firefly batteries to replace 5 LifeLine G27’s on my sailboat.
    I had also considered busbars to simplify some of the wiring at the battery side. I don’t have 100% of it in place, as space is a constraint as Anonymous mentioned, but that’s the route I would like to go as well.
    I have used the Blue Sea MRBF fuses for a while on my boats, and love them – they were installed first thing on each battery.
    I’m glad I don’t have to deal with loads larger than 200A such as Ben’s big engine starter – seems like there is a product gap there right now.

  14. Tim Onders says:

    The point of using fuses between batteries is primarily for safety in working around the batteries.
    Modern AGMs have extremely high short circuit current ratings. For example, my G31 TPPL batteries have a short circuit current rating of 5000 amps. For the three batteries in the battery box, that’s 15,000 amps if I ever do anything wrong while working on the system.
    In balanced usage, I’m never actually passing more than about 50 amps per battery (charging or discharging), so I can put 50A terminal fuses on each battery and make the whole system much more inherently safe. Because it’s fused at each battery, it also reduces the necessary maximum disconnect rating of the fuses (both the battery fuses and the main fuse).

  15. Dan Corcoran (b393capt) says:

    Ben what changes are you making to your charging systems to support the new Fireflys?
    Do you already have heavy duty alternators? Tempurature sensors? Higher capacity charger? Larger power cables to your charging systems? Are Battery switch and fuse holders (not just fuses) rated for the higher potentials of these new batteries? Anything you have on a future to do list?

  16. Andy Brown says:

    Just a quick comment. ABYC E-11 does not require a fuse between the battery and the starter. The general reasoning being that over the long term a fuse in the starting circuit could fail due to fatigue and then the boater may be unable to start their engine in an emergency.
    So far the industry really hasn’t found out a good fuse solution for the starter.
    Even though I would love for everyone to buy tons of MRBFs, I wouldn’t install them if I was already installing class T fuses.
    Andy Brown
    Blue Sea Systems

  17. John Hinckley says:

    Terrific discussion. Thanks for sharing it.
    Love to see a schematic when you’re finished.
    I’m about to replace 3 4D gels on my BHM 28, and am following this with great interest, for the battery decision, as well as how I fuse and cable them up.

  18. Anonymous says:

    Glad to see more updates on the Firefly batteries. Certainly an interesting new option.
    However, specific to your installation, what’s the story on installing blade fuses, such as in the Blue Sea 5024 block, directly on the positive bus of a battery bank capable of delivering 1000A+ short circuit. Isn’t the ABYC spec that they need to be downstream of a properly AIC-rated fuse/breaker?

  19. Carl Nelson says:

    Are you using the Firefly BEMS?
    I don’t quite understand it. Does it work on both charge and discharge? Is it an alternative to “balanced cabling”. Is there an advantage to using both?

  20. Ben Ellison Ben Ellison says:

    Hi Carl, I don’t entirely understand the BEMS either, but note that “It is designed to equalize old and new batteries connected in a series string…”
    So the BEMS is not useful for an all parallel 12v bank like mine, but might be useful for a 24v boat bank that is partially serial and partially parallel. (Actually, according to the spec sheet it can be used with 2, 4, 6, and 12v batteries in series strings totaling 24 to 48 volts.)

  21. I checked the interrupt capacity specs of some Blue Sea blade fuses for the 5024 you mention — they were all 1000 Amps. Does that reassure you?

  22. Anonymous says:

    I’m just a tad worried about the fuel filter and lines above the batteries. I’d move that too.

  23. Ben Ellison Ben Ellison says:

    The hoses and filter are actually part of the hydraulic steering system, plus neither of the well accredited surveyors who’ve checked out Gizmo while I’ve been present had any problem with the house batteries in that same location.

  24. Robert says:

    I saw your note about using a solar charge controller that isn’t really multi stage nor adjustable. I installed a Midnite Solar KID solar controller on my boat about 12 months ago and couldn’t be more happy with it. I drive it with 2 flexible 135w panels in series (theoretical 270w total with peak voltage about 60v into the MPPT controller). It has adjustable bulk, absorption and float voltages and a ton of other features. Seems very well built and efficient.

  25. Ben Ellison Ben Ellison says:

    Thanks, Robert; the Midnite Solar controllers look interesting:
    But my Blue Sky 2521i was about half the cost, has held up well, and does have bulk, absorption and float charging stages. They are not adjustable but pretty close to Firefly ideal, as noted above. Here’s detail on my 2012 solar install:
    If I make a change, it will likely be the Victron BlueSolar MPPT 100/30. It’s multi stage and voltage adjustable, and I think it can also integrate with the Victron BMV and a Color Control GX for some pretty specatular off boat power monitoring and also conversion to NMEA 2000. I’m researching the possibilities.

  26. Compass Marine says:

    Your BS 2512i is programmable but you would need the Blue Sky IPN-Pro Remote or the Blue Sky UCM module in order to do that. Currently you have the basic IPN Remote. The IPN-Pro or the UCM will give you full programing of the absorption & float voltage settings as well as many other features such as absorption time.
    That said the Victron controllers with the optional VE Direct BlueTooth dongle allow you to track the controller on your phone, program it from your phone and they cost less (when buying new) than the equivalent controller plus IPN-Pro from Blue Sky..
    A Victron 100/30 plus the BT Dongle will run you about $270.00 for both. The IPN-Pro Remote alone will run you about $200.00 to add to your existing controller or the Blue Sky UCM would run you about $270.00.
    The Midnite KID is a great little controller but I feel they missed the mark on some fronts. The awkward size, mounting options, and no remote option being a few of them. While it is a great controller for boats it can often be a bulky & awkward installation.
    The new Morningstar ProStar 25 controller is also quite nice but again more costly than the Victron.

  27. Ben, the Blue Seas MRBF’s are a great solution for a lot of applications,with one significant drawback – they add almost two inches of total height to the existing post. The stud itself is 1.75″ long, and then the rubber insulator cap goes over that.
    When fitting these to existing boats equipped with battery boxes, it almost guarantees you won’t be able to get the lids back on the box!
    I’ve run into this before, and suggested to Scott McEniry over at Blue Seas that they consider designing the fuse holder with an offset, so the total height is still comparable to the original post – that was in 2014, so I guess my idea didn’t get far.
    I see the Firefly’s have a bolt terminal as opposed to a post, so you not have an issue in Gizmo…

  28. “… MAY not have an issue on Gizmo.”

  29. Compass Marine says:

    If you have lead post battery terminals the solution in the pic below works with MRBF’s. For Lifeline, Odyssey, Northstar Firefly etc. you can make a right angle 90 out of copper bar stock but I find a short jumper to an ANL or Class T or short jumper to a busbar mounted MRBF works too.
    Horizontal MRBF:

  30. Thanks guys – but by the looks of that picture, you’re still not going to be able to keep it all inside the box. The only difference is now it hangs out the side, instead of poking through the lid!
    I’m all for creative solutions, and I appreciate your tip – but I think the best solution is just to re-design the profile of the holder in a “Z” type shape, and perhaps reduce the length of the stud as well (which seems overly long). I love about 99% of Blue Seas products, but I think there’s room for improvement here…

  31. Ben Ellison Ben Ellison says:

    Thanks from me too, but I’m not planning to use MRBF fuses on each battery. I think the accidental short risk when working without a cover is manageable, and the exceptionally flat top design of the Fireflys allows for a snug protective cap.
    But here’s a question: these pre-made Ancor 18-inch 2 AWG gauge cables look like they’ll do the job linking each battery to the Powerbar terminal blocks…|328|2290011|2289961&id=193184
    …but what do you all think?

  32. Stephane says:

    Ben, do you use a charger for shore power to 12V charging, and if so which type? (Is it the Balmer or Victron your refer to in your post?) I recently also bought 2 Fireflies that will use a programmable Victron 75/10 solar charge controller (and Bluetooth dongle to allow monitoring from a smartphone) for charging at sea/anchor, but have found it hard to find any chargers for shore power that will float at 13.2V – most seem to float at 13.4 or 13.5V instead, and that is not recommended per the Firefly specs (will reduce battery lifetime). It seems harder to find a suitable programmable shore charger than a good programmable solar charge controller.
    Thanks for sharing your experience redoing your battery bank – lots of very useful discussion here and in the earlier posts!

  33. Ben Ellison Ben Ellison says:

    Hi Stephane,
    I installed a Victron MultiPlus 12/2000 in 2012, details here:
    At the bottom of that entry is a “VE Configure II” screenshot showing how you can totally customize the charging voltages and much more.
    So, yes, I can set the Multi to float at 13.2v, but I’ll need to borrow a special Victron USB dongle (Victron data protocols are really confusing). The Bluetooth dongle seems great, especially for professionals who work on many boats.

  34. Compass Marine says:

    The Sterling ProCharge Ultra charger has a custom profile setting that can be used to set 14.4V & 13.2V. They largest model is 60A.

  35. Ben, the Ancor pre-made cables you referenced are great from a quality and value standpoint, if they meet the AWG you need for your loads and happen to be the correct length. I prefer to make up my own cables, because I can cut them exactly the correct length, which I believe was part of the discussion in terms of equalizing voltage drop among the batteries. Also, on short lengths, you can crimp the lugs on the correct “clock” orientation for the terminals they are attached to, which avoids twisting the cable to make things line up. But this requires some rather expensive tools which you may not want to acquire for infrequent jobs…

  36. Compass Marine says:

    Email me your lengths and lug sizes and I can make your battery cables to the exact dimensions you need. Measure twice cut once.. (wink)

  37. Completely agree with RC and Grant. I make my own cables for the same reasons cited above – exact length and loss, and orientation of the lugs. I’m also slightly OCD so having cables that aren’t the exact length drives me insane.

  38. Ben Ellison Ben Ellison says:

    Thanks, RC, Grant & Steve, but actually 18 inches is a perfect length for all eight Method 3 (star pattern) battery connects in this lucky case. With the PowerBars on center of the bank and at similar distance to their respective poles, equal length cables make sense (and are easy to figure out — 18 is the longest run to the four target poles from either PowerBar ;-).
    At $23 a piece for what look like quality factory-made Ancor cables with right size lugs, it seems hard to justify custom cables (tho thanks for the offer, RC!).
    However, I may use red cables on both sides because Ancor’s black pre-made cables have 5/16 lugs. That will probably cause consternation from the “peanut gallery” here, but, heck, the Covey Island Boatworks folks who originally wrestled so much 4/0 cabling into Gizmo — it wasn’t needed, but it works great — fairly often used wrong color cable but simply marked it correctly with tape or shrink tubing.
    But I’m still in “listening to advice” mode and not too hurried (it’s going to be a late launch for other reasons).
    So far no one here has objected to 2 AWG cables for those individual battery connects, but it’s not too late 😉 I’ve been advised that it’s plenty big enough and see in the Ancor tables that 15 feet of 2 AWG — ten times my lengths — can carry 100 amps with a maximum 3% voltage drop (i.e. heat creation).
    But then again I don’t fully understand what “Instantaneous P: CA/CCA ….. 800/600” means on the Firefly Oasis G31 spec sheet. Anyone?
    Also while looking for amperage specs I noticed again in the Firefly manual that not only is 13.2v float strongly recommened but “The Oasis does not require a float charge.” They are different!

  39. Ben,
    2 AWG cables should be fine for battery interconnects.
    CA = Cranking Amps
    CCA = Cold Cranking Amps
    From (and many other sources):
    “Cranking amps are the numbers of amperes a lead-acid battery at 32 degrees F (0 degrees C) can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12 volt battery).
    Since it is more difficult for a battery to deliver power when it is cold, and since the engine requires more power to turn over when it is cold, the Cold Cranking rating is defined as: The number of amperes a lead-acid battery at 0 degrees F (-17.8 degrees C) can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt battery).”
    I never did find the G31 Spec sheet you referred to online, but as you’re dealing with a house bank as opposed to a start bank, you really don’t care about cranking amps anyhow. Your research to this point on Amp-hour capacity, depth of discharge, etc. is far more relevant to what your usage will be. Hope that helps –

  40. Ben Ellison Ben Ellison says:

    Sorry, Grant, I didn’t express myself well. The Firefly spec handout (and the manual) are at the bottom of this page:
    The spec table is a little odd and I wasn’t sure it meant 800 CA and 600 CCA, but I guess that’s right. And I guess those values are OK with the 2 AWG cables, which was my thought. Thanks!

  41. Anonymous says:

    As a head’s up to readers, I contacted Firefly to purchase batteries, and found out that only Hybrid Power Supply — — is authorized to distribute Firefly in Canada. Some websites claim otherwise, but they are off-base for doing so. Not sure if there is any legal implications, but people need to respect business boundaries and it appears some people aren’t doing that.

  42. Thanks for the link, Ben – I see the table now, and it IS confusing! And I’m as baffled as you regarding the meaning of “Instantaneous P”. That’s a new one on me.
    By the way – a couple of other options for your negative cables would be to buy the black pre-made cables and carefully drill out the 5/16″ terminals to 3/8″ – or just re-terminate them with the proper lugs (yes, you would lose an inch of length in that case).
    But really – I think Gizmo deserves some proper custom-made cables if you’re going to all this trouble anyhow – and besides, the DC negative really ought to be yellow, not black!

  43. Ben Ellison Ben Ellison says:

    Yeah, yeah, but I’m a skinflint Yankee and how could Ancor made cables not be “proper”?
    Thanks to the Hybrid Power link just above your comment, I found another Firefly spec sheet that doesn’t list CA or CCA at all, just 500A max discharge at 77F:

  44. I’m sure they’re proper – except for the wrong size lugs! If you’re OK with black – maybe check out these as well:
    A custom-made 1.5′ 2AWG cable with heavy-duty lugs and heatshrink came out to $ 11.84 – pretty reasonable, I thought…

  45. note from the peanut gallery– yellow negative cables would go well with the color scheme and color code, and it’s easy to do with shrink-wrap.
    AWG 2 corresponds to at least 100 Amps in that table, per battery, so 400 Amps combined. How big is your max load?
    But note that the length given is positive and negative combined, so it’s not really 10 times your length. Still good though.

  46. Doing yellow cables with shrink tubing is fine, but getting yellow (or red or anything but black) means you aren’t getting the best possible insulation material. If you are using Welding cable for flexibility, black Hypalon is the ONLY choice of jacket you want.(Just ask RC!)

  47. Actually I meant yellow over red, but that is good to know. I’ve learned a lot from this thread.
    There’s an interesting article in the current issue of Wooden Boat about using Firefly batteries to replace the engine in a sailboat:
    WoodenBoat 250, May/June 2016
    Electric Auxiliary for a Sailboat — With solar recharge, off-the-grid cruising is in range by Jonathan Taggart

  48. Leo Starrenburg says:

    A lot of boaters over here have a start-, service- and bow thruster battery in their boat. A state of the art FET charge current distributor keeps them apart while charging and discharging. Am I wrong to say that an intelligent x-phase charger is useless in this set-up because the charger can’t ‘see’ (measure the voltage of) the batteries he is charging, so a constant voltage charger would be the only option when using a FET distributor and multiple batteries ?

  49. Leo, I think it would work fine. The FET charge distrubutor will be “on” when charging, so the charger will “see” the batteries downstream of it. The FET distributors work the same as the older relay versions, but faster and quieter without the drain and potential for mechanical failure.
    They only isolate the batteries when discharging.

  50. Arch says:

    I will be adding solar and rewiring my battery banks this summer so I’ve been following along, learning and gathering ideas. I thought I had my plan figured out using Blue Sea ACR’s to control charging of start and thruster batteries. Then came FET charge distributor??? Can someone point me to information about FET systems. Thanks.

  51. Anonymous says:

    Arch, try this link:
    I got the FET 3 output 100 A and it works great.
    w fr greetings, Leo

  52. Compass Marine says:

    Keep in mind that when using traditional diode or FET based isolators most charge controllers, smart battery chargers and wind controllers today are looking for voltage on B+ wire in order to work/turn on. This is a safety feature designed to prevent charging into a dead or internally shorted battery. Because isolators block the controller or charger from sebsing voltage on the input stud, before turning on, they make for a rather poor fit when using alternative energy or smart chargers to feed multiple banks.
    Some isolators feature a “key-on” excite terminal but this would require the isolator to always be powered, if you used other charge sources, such as solar for charging multiple banks.
    Combining relays or voltage sensitive relays (VSR’s)such as the Blue Sea ACR or the Yandina Combiner etc. do not suffer these issues. These devices are extremely reliable, so reliable that Yandina actually puts a lifetime unconditional guarantee on them. I have never seen a Blue Sea or Yandina Combiner fail.
    Another alternative for charge management would be to use a battery to battery charger such as an Echo Charger, Duo Charger or the Sterling Battery to Battery chargers.

  53. Anonymous says:

    I used a VSR in the past, it looked at the voltage of the starter battery and when that was high enoguh it changed over to the house battery. Now that I have three batteries I use a FET isolator. This does just what the name says: it isolates the batteries; no more, no less. When I connect a constant voltage charger to the input, all 3 outputs will deliver current to all the batteries, the amount of which varies with their respective levels of discharge. And yes, my FET isolator has an ‘energize’ input to connect to the + of a battery in order to get 12V on the B+ output of my Bosch altenator. It draws just 10mA but the altenator needs to see the 12V in order to work. So I hooked up a wire from the engine ‘contact’ key-switch to the energize input of the FET isolator.

  54. I’m with RC on this one – the voltage-sensitive combining relay is reliable and doesn’t require a work-around to start the alternator. I have two on ATSA – a bit Blue Sky between the house #1 and #2 and a small West Marine one between the House #1 and the Start battery.
    The FET device has an advantage over the older diode technology because it doesn’t have the .5 to.6 volt drop across it, with a 20-30 millivolt drop instead – a bit deal when you are trying to keep your batteries properly charged.
    What I don’t understand is why the two technologies couldn’t be combined – use an FET switch (instead of an electro-mechanical relay)in a voltage-sensitive combiner? The low voltage drop means you could connect your house primary battery direct to the alternator (eliminating the kludge of “ignition bypassing”) and having the combiner add in the other batteries when the voltage on the primary gets high enough.
    Alternatively, just use the “FET distributor” between your primary battery and the secondary ones, and connect the alternator, AC charger, solar, etc. to the primary directly.

  55. Compass Marine says:

    QUOTE: “Alternatively, just use the “FET distributor” between your primary battery and the secondary ones, and connect the alternator, AC charger, solar, etc. to the primary directly.”
    Because it would not isolate the house battery. Any load applied to the start, thruster etc. bank would place a load on the house bank.
    Wired properly (many are not) VSR’s work tremendously well, are incredibly reliable and don’t really have any voltage drop to speak of..

  56. Hi RC,
    I see what you mean – for the start or thruster battery I don’t see the harm, but with two house batteries, that would be bad.
    I still think my original idea of using the FETs in place of the electromagnetic relay in a VSR would be useful, though. 🙂

  57. Greg Davids says:

    I am wondering if anyone has been able to get the Balmar Smartgauge to work properly with the Firefly batteries? So far I have had no luck using program #5 Carbon Fiber Lead Acid (initially recommended by Balmar) or program #1 Deep Cycle wet cell recommended by Mr. Compass Marine himself. Both end up giving a way overly optimistic percentage for remaining battery life.
    Luckily I already had an amp hour counter type of meter, but I was hoping this would be a good addition to give a more true battery life percentage as the ah counter inevitably gets out of whack.
    Balmar admitted to me in an email that this was designed before Carbon Foam AGM’s were around. They have plans to include a specific program for Firefly batteries in the next version – which is at least a year and a half away. At this point it is looking like I purchased a fairly expensive useless meter.

  58. Greg,
    I have a Firefly setup with the Balmar meter as well. I have been doing a ton of testing the last 2 months in preparation for a couple of long trips where I will need the extra capacity.
    I have also seen some very odd results from the Balmar meter. I don’t remember what I have it set to battery-type-wise, but I have seen it get “stuck” at 91% for a long while even though the bank is completely charged. After killing the charger for a couple of hours and letting it re-charge again, the Balmar meter catches back up.
    I also have seen it give me a far more optimistic reading than I expected. How are you calculating it outside of the Balmar? Purely based on voltage levels?
    Anyone know other meters that could give more accurate readings?

  59. Greg Davids says:

    I have a preexisting Xantrex Link 20 battery monitor which counts amp hours. I have 4 of the Firefly batteries on the house bank – should be 440 amp hour capability. I have allowed the batteries to drain to drain a bit over 50% a few times now based on counting amp hours and the Smartgauge is showing itself to be not so smart. I have not seen it show lower than the low 70% range. As you mention, it does seem to like staying in the 90% range even when clearly much more capability has been used.
    I have not logged the specific numbers as scientifically as I should, but the basic fact is it reads much higher in percentage than the actual value could possibly be. This is at least the case for program 1 & 5.

  60. Compass Marine says:

    This is all good information but we must keep in mind that there are a lot of variables at play such as charge devices moving to float prematurely, Peukert etc.
    In testing I have been able to get the SmartGauge to track pretty accurately to the Firefly but the bulk of my testing was done before the Firefly hit my bench. I do know that Carbon fiber is not the setting to use.
    I have been using either #1, #2 or #3. It is possible that #1 has too high a full charge voltage expectation (14.6V) outside of a lab like environment where absorption can be held as long as it takes. I have been trying to get an optimal setting list, for batteries listed by brand & type, out of Balmar now for approx one year. Apparently the guys at Merlin have not responded and I know the owner of Balmar has been pushing the issue. I actually sent Tim a list with the batteries I wanted the optimal setting types for, seems easy, apparently not….
    With the SmartGauge you do however need numerous deep cycles and recharge events for it to “learn” the bank. You should ideally not pay much attention to it during charging and should wait until you are discharging again and it again finds its place. The longer it remains connected the faster it seems to narrow in on SOC upon discharge.
    If you have changed settings for battery type you will ideally want to do a full factory reset and start the deep cycles over again. The longer it remains connected the more accurate it becomes. I can tell you that my own Firefly has remained uncharged now for over 8 weeks and the resting OCV, as of yesterday, was 12.96V. A full Firefly will rest at over 13.0V yet with many of the charging systems installed on boats getting to 100% SOC may be a much rarer event than most assume. Hanging at 91% SOC for example could be because the charge source dropped to float far too early, not at all uncommon. At a premature 13.2V float the charge rate will be a snails pace.
    Ah’s out do not always equal SOC unless they were drawn out at the 20 hour rate at 77F. If you are discharging at say 25% of the 20 hour rate (5.5A vs. 22A) the battery bank will actually have slightly more capacity than 440Ah. Most Ah counters DO NOT calculate for any overages in Peukert only high discharge impacts on the negative side but the SmartGauge can more accurately account for this.
    The best correlation is to let the battery sit for 24 hours at a temp between 70F and 80F and look at resting open circuit voltage and compare it to the SmartGauge’s SOC.
    If you want to see a real world example of how well the SmartGauge can track (though these are not Firefly however) this one is interesting (Smart Gauge part at end):
    How to Murder Batteries in Half a Year
    I would be curious to see how it tracks on setting 2 & 3 compared to #1. Unfortunately the Firefly I have been using for testing is in the middle of more testing and can’t be used for SG testing right now…

  61. Greg Davids says:

    I have now changed over to the program #3 AGM setting. I will report back after it has gone through a couple cycles. Unfortunately, since I live on the boat, doing a 24 hour open circuit test just isn’t gonna happen.
    I am using a Victron battery charger with a separate voltage sensing lead attached to the batteries. I would think this would help with avoiding a premature float. My sense is that this is not happening. I did program the Link 20 with the Firefly recommended Peukert value – though it is interesting you say this is only factored in for extra high loads and not smaller loads. We definitely spend most of our time with loads less than 22amps.

  62. Compass Marine says:

    For a little more depth on Ah counters:
    Keeping Your Ah Counter More Accurate:

  63. Greg Davids says:

    After just one day I can tell that program #3 is much more accurate for the Firefly batteries. My amp hour meter tells me that I have consumed 120.7 amp hours. This should theoretically leave me with 73% remaining battery life. The Smartgauge is now reporting…. drumroll…. 74! Pretty darn close.
    Given that this is just the first cycle, and it is supposed to get more accurate over time, I would say this is a winner.

  64. Greg,
    I was actually using battery program #1 and had very good results. I switched to #3 as you are using, and I am getting weird results when the batteries are fully charged and in float.
    The Balmar is showing 82-89% full even when the batteries are fully charged and in float.
    If I kick off the charger, and let the batteries drain down a bit, things get better, but not much.
    I think I may go back to program #1 – have a week long trip starting this Friday and want this to be as accurate as possible.

  65. Greg Davids says:

    My further experience with program #3 is not quite as good as the first cycle I reported on, bit it’s still definitely better than Program #1 or #5.
    Currently with the battery fully charged and in float mode at 13.2 volts, it is showing 99%. Fairly quickly after removing shore power it does drop down to the low 90’s. By the time it is in the 80 % range it is starting to be reasonably accurate. When it was closer to 75% I would say it was reading only about 3 or 4% lower than it should – according to my amp hour meter. That is close enough for me.
    The other programs were reading much higher than was possible. I guess I would rather have it give me a slightly pessimistic figure than a wildly optimistic figure. Of course it would be best to simply be accurate.
    It sure would be nice if we could load an updated firmware that included a proper program for these batteries. I am guessing that based upon all the recent positive press (and the manufacturer feeling they can get away with a decent price jump), sales of these batteries must be doing well.

  66. Leo Starrenburg says:

    I’m thinking about buying a PanelPilot and configuring it to a 24h graph to show the trend of the service batteries’ voltage. It’s not as sophisticated as an intelligent capacity monitor, but you’ll see what’s going on at a glance.
    cheers, Leo

  67. Leo Starrenburg says:

    The PanelPilot has arrived, as has a Hall-effect split core current sensor. Next week I’ll make a test setup, the display has two voltage inputs, I’ll use one for the battery voltage and the other for the current sensor, my version gives 0 to 4Vdc out for 0 to 100 Amps. I have to see which is most practical, a voltage only display with the 48h graph, or a voltage and current display for the charging current. The nice feature of the PanelPilot is the easy configuring of a lot of display types and voltage (conversion) ranges. Will be back with the results of the test.

  68. Gary Warner says:

    Ben – can you post the custom settings for the Balmar MC-614 for your Firefly batteries?

  69. Ben Ellison Ben Ellison says:

    Sorry, Gary, I’m headed to NMEA conference and the Balmer notes are on the boat. But I do know that the major change Rod Collins recommended for my Firefly setup was setting the adsorption time period to 4 hours.
    Actually it would be great if Rod chimed in as he understands the issues MUCH better than I. Otherwise it will have wait until next week.

  70. After a long road of install, configuration, reconfiguration, yelling, and testing, I’ve finally written up part 1 of my MasterVolt+Firefly setup on Grace at
    More to come in terms of the actual installation, configuration and of course actual use over the last few months.

  71. I followed this thread with interest and am considering Firefly batteries for my next upgrade. It would be great to hear what your experience has been now that you’ve had them onboard for a while

  72. James,
    I’ve had mine since Feb 2016 and absolutely love them. I’ve been on a number of trips this last summer away from the dock and have had great results with how far down they discharge and how quickly they charge in general.
    I am 100% satisfied and I haven’t said that about batteries in a long time.

  73. Chuck Lanter says:

    I just replaced my house bank with Fireflys and am planning on installing a SmartGauge. I’m wondering if there has been any update to how best to configure the SmartGuage for this battery type? Also, is the SmartGuage firmware user updateable so that it can be reconfigured as technology changes? Thanks for any information.

  74. Ben Ellison Ben Ellison says:

    Hi Chuck,
    Last July I wrote a bit about how a SmartGauge is not working well with my Firefly bank…
    …and the situation is about the same though I did try a different battery profile in the SmartGauge. The Victron BMV600S is doing better at tracking State of Charge, but it’s not perfect either.
    In both cases, there may be more I can do to adjust the gauge to the batteries, and researching that is getting to the top of my winter to-do list.
    PS I don’t think the Smartgauge can get a firmware update in the field.

  75. Greg Davids says:

    I also find my Smartgauge does not work well with the Firefly batteries. In retrospect that was a bad purchase – the monitor, not the batteries. I think you are better off with a monitor that counts amps in and out. The manufacturer of the Smartgauge said they are working on a newer version that will probably have an appropriate setting for the Firefly batteries, but that will take a while.
    Greg Davids

  76. Compass Marine says:

    Balmar finally updated the settings in their manual.
    1 Deep-cycle flooded lead acid: (Examples: Trojan, US Battery, Rolls, Interstate, Exide, Deka/East Penn, Dyno, Crown, Superior)
    2 Gel Cell: (Examples: Deka/East Penn GEL, Sonnenschein Prevalier GEL)
    3 AGM – Absorbed Glass Mat: (Examples: Lifeline AGM, Firefly AGM, Deka/East Penn AGM, US Battery AGM, Rolls AGM, Optima AGM)
    4 Dual-purpose lead acid
    5 Carbon fiber lead acid
    6 Sealed, maintainance-free lead acid
    I have finally had a chance to test the SmartGauge (setting 3) against two banks of Firefly’s and in each occasion it has been within 2-3% of where the SmartGauge said it was before I removed the batteries from the vessel, to test them in my shop.

  77. I’m patiently waiting for an answer to the Balmar MC-614 settings question.

  78. Ben Ellison Ben Ellison says:

    Thank you Rodd (Compass Marine); I will switch my Smartgauge to setting #3 today.
    Norse, yesterday I did manage to bring home my Balmar 614 manual, which I annotated with Rodd’s Firefly recommendations (for my use case) last summer. But here’s hoping that Rodd himself will weigh in with more general (and better informed) guidance about how to set the regulator for these batteries.

  79. Chuck Lanter says:

    Thank you Rodd and everyone else for the updates on the SmartGuage. I’ll start with setting #3 and report back later this summer. As I’m also updating my alternators (dual engine cat) and hoping to go to Balmer MC-614s I’m also looking forward to hearing about any update to those settings as well. Thanks again.

  80. Dan Corcoran (b393capt) says:

    What is the difference between the firefly E31 and G31 batteries? The brochures show identical dimensions and other specs.

  81. Just noticed your question, Dan. I’m looking at new batteries for Atsa, and I noted that the E31 has slightly lower capacity and CCA specs, for whatever reason. Otherwise, they appear to be identical (and the picture on the E31 brochure is of a G31!)

  82. Dan Corcoran (b393capt) says:

    Lithium anyone? Are there any vendors yet offering a lithium battery suitable as a house battery that fits in the form factor of a E31/G31 with built in protections so that there are no additional requirements of the electrical system other than maybe a smart regulator for the alternator?
    If someone has a product, not ready for market, I would be willing to test and report back on installation efforts and results. I am looking for something along the lines of
    * E31/G31 size or smaller with BMS
    * Has the benefits of lithium, e.g. fast charging even as the battery approaches 95% charged, deep discharges to say 30%, so engine run time for charging is limited.
    * It can support house loads including inverters.
    * Two 12volts in parallel (or 2 6volts series)
    * If the battery discharges beyond a dangerous percentage, it automatically disconnects.
    * If it disconnects automatically, ideally it will reconnect automatically if charging current is applied, or have a simple reset button if manual reconnect required.
    * Ideally someone can choose to have 2 for house batteries and 1 for starter, so that the same chemistry is used across battery switches should user need to use the combiner option on the battery switch (e.g. one or the other set of battery’s has cut off).

  83. Compass Marine says:

    There are “drop-in” LFP batteries out there however I would strongly urge staying far away from them unless they also offer external communication to the rest of the DC system for warnings and disconnect events.
    External system communication is critical so an HVC (high voltage cut), LVC (low voltage cut) or HTC (high temp cut) does not cause a damaging transient on the system bus or destroy the charging source.
    A good LFP system system design will almost always be a dual bus design with a charge bus and a loads bus not just a single contactor..
    Another short coming of most “drop-ins” is the wimpy low current contactors (usually light weight mosfet switches)inside these batteries. I’ve done done far too many consults on “drop-in” LFP failures to recommend anyone using them at this point in time.

  84. Dan Corcoran (b393capt) says:

    Destroy the charging source? That’s extremely undesirable on a distance race or sailing cruise. No battery and no alternator! Is there a potential drop in design (multiple battery posts? Quality Contactor? BMS controlled fake voltage sense output to sources) that would be a solution for a future drop in LFP design that would potentially work without having to upgrade the other components in a charging system?

  85. Compass Marine says:

    The installation of an LFP bank should always be treated in a systems approach manner. Unfortunately there is no such thing as “drop-in” LiFePO4, unless you desire a big fat hole in your pants where your wallet used to live.
    AGM and GEL batteries should also be treated this way and far too often they are not. I then get the call from an irate owner who “Bought the best batteries money could buy!” and then charged them with a ProSport bass boat charger and a 60A Hitachi factory wired alternator. He’s irate and wondering why they died in less than two years and he now has a $2000.00 pile of scrap lead and needs to start all over. The answer is because the rest of the system could not treat the batteries as needed.
    Any cruising boat battery bank upgrade should be treated as a system level installation. If the existing system & use can’t support it, don’t upgrade it until it can.
    I get emails almost weekly from folks who have destroyed thousands of dollars in LFP cells or entire banks by taking shortcuts.
    The closest thing to a drop-in, in the world of LFP, will be Bruce Schwab’s OPE-Li3 system, but it is still pretty far from “drop-in”…

  86. John says:

    I basically agree with Dan, but I must add that I purchased some of the original Sonnenschein gel batteries for use on a catamaran because I wanted sealed, with no gassing, and no problems if for some crazy reason we capsized. I absolutely abused those batteries for 12 years, with no real charge regulation, frequently flattening them until dead, and then recharging using nothing but solar and wind. Those batteries lasted 12 years on a boat, which is by far the best service I have ever had from marine batteries. And, I still, years later, periodically checked one out in my basement, stored completely uncharged and it would show 12 volts. I retired them due to capacity decline, but they were still working fine after 12 years onboard, much of it spent in the tropics, and often discharged below 12 volts. I wonder if battery companies have no incentive to make long-lasting batteries because the upfront cost will reduce sales, and the lack of failure will reduce replacement. Finicky batteries that require special charging and care add to sales of battery monitoring gear and almost guarantee expensive and frequent replacement.

  87. Dan Corcoran (b393capt) says:

    Thank you for pointing out the OPE-Li3 system. Have you come across folks using this battery damaging their boats or batteries?

  88. waren says:

    Hello, just wondering if the accuracy of the BALMAR gauge improved with the Firefly battery when set to “type 3”?
    Also, any update on the battery performance?
    I have the balmar gauge and considering a firefly battery.

  89. Ben Ellison Ben Ellison says:

    The Firefly batteries are amazing, Waren, and the Balmar SmartGauge is also impressive. But it took me quite a while to come to those conclusions because I managed to screw the system up in such a way that the problem came on slowly and was quite hard to figure out (at least for me).
    I plan a full entry about what happened, mainly because I learned a lot about troubleshooting power issues and also about adjusting the Balmar MC-614 and Victron Multi to best use the Firefly bank. But here’s a short version:
    By late last September I was convinced that the Firefly batteries were failing and also that the often low Smartgauge SOC readings were bogus. I had lots of evidence. In fact, Bruce Schwab even replaced one Firefly after it seemed to fail a properly done open circuit voltage (OCV) test (by me).
    What I had not yet figured out was that several of the battery grounds had worked themselves slightly loose, loose enough to lose current to resistance. I know it sounds obvious but those grounds went to a bus bar on the battery side of the Victron shunt, so the 12v current loss did not show on the Victron BMV monitor. Also the bus bar is somewhat hard to reach and it’s in the engine room where the heat/current being wasted wasn’t easily noticed.
    I now have a lot of usage on the repaired system — including some rough capacity testing, and also what Firefly calls “Restoration Charges” — and it appears that the three original Fireflys which endured quite a lot of very low SOC time have come back nicely. For instance, the BMV counted 380 amp hours used after a fairly controlled discharge (at around 20a), and the bank voltage was still 11.1.
    In short, I think I accidently tested the Firefly bank harder than normal, it survived and can still perform quite well when properly cabled. It’s also accepting fairly high charge rates from my alternator or charger when needed.

  90. Ben Ellison Ben Ellison says:

    Happy PS: Writing the above comment to Waren inspired me to finally fess up to Bruce Schwab. When I last spoke to him — at the Annapolis Sail show in early October, before I discovered the problem with battery ground cables — he told me that the “bad” Firefly he had graciously and quickly replaced was showing 100 amp hour capacity in his shop testing (though the testing wasn’t finished).
    So I’ve been thinking that the battery OCV tested bad on Gizmo because it had not fully charged due to a bad ground connection, and that I owed Bruce an apology and some money.
    But, surprise, here is Bruce’s email response this afternoon:
    “Actually that battery did indeed have an internal problem. It tested ok to start with, however after some time it went dead as a doornail. Must have been an internal weld or something, that was intermittent to start with then failed completely.
    We’ve tried to revive it; no go.​ ​It’s a goner. You don’t owe us a thing!​”
    So now I don’t feel as dumb as I did — always a good day — and the Firefly bank problems I experienced late last summer were possibly caused by a bad battery, maybe intermittently bad, and maybe also failing ground connections (that definitely were a problem after the battery replacement). Troubleshooting can be quite mysterious sometimes, and explaining all the symptoms, testing, and attempted solutions will make a good entry I suspect.
    Incidentally, I had heard that occasional Firefly batteries have failed, but that Firefly and Bruce’s OceanPlanet Energy company were very good about replacements when merited. I certainly had that eperience. Thanks, Bruce!

  91. I have been using my Firefly bank now for about 20 months (just shy of 2 years) with the Balmar gauge and MasterVolt stuff around it, and absolutely love them. I have had no problems whatsoever from the bank, and I have used it quite heavily with the addition of a lot of electrical loads, and inverter time away from the dock.
    That is fantastic to hear that they stand behind their product. They have a lot to be proud of with this one!

  92. evan says:

    This very helpful thread got me started on revising my House Bank.
    We have a Nordic Tug 37 that we use a lot in the winter months. We only see limited low angle sunlight to aid in charging. (550W of solar on the pilothouse roof is great in summer but not so potent in winter) With the need for our Wallas heater to be running 24/7, and longer nights requiring more lighting, our daily usage easily gets up into the 300 A/h range. We are usually at anchor and so the house bank gets a workout every time we go out
    I prefer running our generator for as short a run as possible and if we can get a good bulk charge without running an extra hour for the tapering off charging, that’s great in my book.
    Thus, the Firefly batteries make a lot of sense as they accept high charging currents and don’t have to be returned to >95% SOC to be kept safe. They can also accept the same charging profile as all the other AGM batteries we have on the boat (Bow Thruster / windlass, Rear Thruster, Start Battery etc.
    So, I have gone ahead and installed 6 Firefly batteries.
    Your balanced battery wiring was very helpful but I struggled to find a schematic showing how to wire 6 batteries in a balanced fashion. I eventually figured it out. The solution is fairly simple. see:
    The new bank looks like this:
    At first I wasn’t too impressed with the overall performance of the new battery bank as it seemed to drop into the 12.65V range fairly quickly but I did note that it had a relatively flat discharge to Voltage curve from there.
    The New Year’s weekend with subfreezing temperatures (-1 to -2C) for three days demonstrated pretty good performance. (311 Ah from this 660 Ah bank drew the voltage down to 12.30V) My impression is that the system is improving with each use.
    Anyways, thank you for this article and the great resource it represents.
    I may have to look to improving the battery hold-down system…

  93. Ben Ellison Ben Ellison says:

    Thanks for the interesting contribution, Evan, but credit for the balanced battery cable schemes goes to the good folks at SmartGauge:
    And to clarify for myself and other readers, your bank is 3x series and 2x parallel totalling about 330 Firefly amp hours at 24v, right?
    And essentially you used two levels of SmartGauge Method 2 to achieve balance?
    It does look to me like the total positive/negative current loops from each battery to the load/charge buses are the same length and hence the same resistance.

  94. evan says:

    Hi Ben,
    Thanks for your comments.
    I apologize for the messy schematic drawing. It should be showing a 6 battery system in parallel for 660Ah at 12 V.
    All batteries should have equal total cable lengths and resistance at the final terminals.

  95. evan says:

    Oh, and yes that Smartgauge site was/is a very helpful resource.

  96. Ben Ellison Ben Ellison says:

    Thanks, Evan, but the fault is my poor analysis. At least I illustrated the struggle it can be to visualize this stuff 😉

  97. evan says:

    I totally agree. The principle is clear the execution is more difficult.
    The jump from a balanced 4 battery system to a 6 battery system was not obvious to me and I couldn’t find any examples on-line. The solution is fairly simple but it took me a while to figure it out…

  98. Evan, the wiring looks great! The hold-down straps, however, do not. I hope the plastic buckles and cable-tied strap ends are just a temporary solution?

  99. Anonymous says:

    Thanks Hartley.
    Yes the hold down straps are not up to snuff; at least in theory.
    I did a trip from Victoria to Seattle last month with 40-50 knot winds & 10 foot seas.
    I was bouncing all over the cabin but the battery Bank stayed put.
    After reading the contents above regarding securing the batteries, I will need to upgrade the hold downs.
    – evan

  100. Waren says:

    Ben, thanks for sharing your experience, really really appreciate it! It is a bit worrying that you had a battery fail (1 in 4 yikes), but the promise of deeper cycling and longer life at partial state of charge of the “good” ones seems too good to pass up.
    My application is actually a campervan that I am customizing for overland back country adventures. I have found the sailing community to be a fantastic resource for innovative ideas as we share many of the same challenges in terms of space, the need for high levels of reliability and safety. Much of the RV community is focused on moving from serviced lot to serviced lot.
    It would seem the firefly batteries have the highest usable power density of lead acid batteries currently available. Another obvious advantage is the batteries seamlessly integrates with existing alternator charging and monitoring technologies. This will allow me to use a simple bluesea charging relay to charge the house and engine banks from multiple sources (solar and alternator). Again my appreciation for the information on the site and sharing your personal experience.
    I really tried to make a Li system work due to their high usable power density. Ultimately I came to the conclusion that for my needs the added complexity and cost of unique charging and monitoring equipment for my house bank would out strip the value added. The final strike for lithium is the abysmal cold weather performance.
    My experience with Smartgauge hasn’t been overly successful to date however I should point out that my current installation was not a recommended mode of operation, monitoring a starting bank. I reasoned that because my starting bank consisted of two 100Ah+ agm batteries, the starting loads (withdrawn and quickly replaced by the alternator) would not affect Smartgauge’s ability to track SOC. The manual clearly says Smartgauge won’t work in this scenario but I hoped the warning was conservatism. It wasn’t. The longer Smartgauge was connected to the bank the further it drifted from reality. Obviously no fault of the product, it was a gamble I knew it might not work. It was worth trying I wasn’t willing to install a shunt.
    Looking forward to having a house bank and installing Smartgauge as intended, tracking soc on the house bank and voltage on the starting bank.

  101. Keith says:

    Just wondering if there was a posting or link to the Balmar 614 setups for the Fireflys that were talked about?
    I am in the process of installing a bank on out trawler and any “more informed” info on the setup of charging would be most appreciated.

  102. Mike B says:

    Has anyone used six of the L15 Firefly batteries in series/parallel to form a 12V bank? Three of the 4V batteries in series to form 12V, two groups of Three to double the amps? Visualizing the connections are difficult for me when compairing Panbo and Calder. Help?

    • Ben Ellison Ben Ellison says:

      Hi, Mike. That sounds like a nice 900 ah Firefly battery bank, and I suspect that the crew at OceanPlanet Energy can show you possible install diagrams and photos. I picture bar connections for the 3x L15 serial connections and then 2x equal length cables to the busbars that parallel the two 12v sub banks.

      I also recently saw relevant diagrams when installing a Victron BMV-712 battery monitor:

      Note that these monitors can watch “midpoint voltage” on banks like yours, and that seems like a good idea, at least when the feature is so easily available and you’re still designing your install. It probably won’t happen, but a voltage difference between the two 3x L15 sides of your bank could indicate a problem in one battery.

    • Alden B Cole says:

      Hey Mike B,
      This is a really cool project here S/V Sargo. I’ll add the link With Ben E.’s help the photo is just what you asked. Check the original article and you will find the same method used on Gizmo to balance the system. I am not locked into this method; It just worked best in this situation. Ben, jump in if you have some thoughts.

      • Ben Ellison Ben Ellison says:

        Thanks, Alden, and sorry that Panbo commenters can’t yet add their own photos. Can you please explain

        Mike, the balanced battery bank cable concept is discussed up in the entry above and here’s a photo of how my four-12v-Firefly bank came out:

        (And there is a cover that holds them all down and protects the exposed hot terminal strip.)

        • Alden B Cole says:

          Sure, Ben. There are three 4 Volt batteries in series to get to 12 V. The are two banks in parallel. The key to balancing the charging and discharging is that the total length of the interconnect lengths plus the end connection lengths are equal. The equal lengths will present the same resistance to the system thus, balancing charging and discharging performance .

  103. Alden B Cole says:

    Let’s break this down a little. A battery is a collection of (individual) cells.
    In this case, the 12V system requires is 6 cells in series for a 450Ah battery pack. Two of these battery packs in parallel would give 900Ah, as mentioned.
    In order to as closely balance these 2 packs, the resistance of each battery pack ideally should be the same. As indicated in the brochure, each cell has an internal resistance of about 0.6m Ω.. Resistances in series are additive so a pack is about 3.6m Ω.. For the sake of discussion, using the internet for info, using 1/0 AWG cable, let’s construct a battery pack with 2 battery interconnects of 1 foot each and the two ends with 2 foot leads. The 6 foot total from the internet calculates it to be 1mΩ . Hence, 4.6mΩ for the one pack. So as long as the batteries are the same and the total length of the cabling is the same, in theory, the 2wo packs would in balance. This an example of the application of Kirshoff’s and Ohm’s laws in action.

  104. Ben Ellison Ben Ellison says:

    (Thanks to RickG I now know that) Bruce Schwab Energy Systems has a PDF with recommendations about how to setup a Balmar 614 regulator for Firefly batteries:



    Meanwhile, I just posted Gizmo’s more aggressive settings in a comment here:

  105. Chris G says:

    I installed a pair of Firefly Group 31 batteries in my sail boat. They were great until one of them failed within 18 months of purchase. Charge sources were Balmar 614 regulator and a Blueseas P12 shore charger all set to 14.4 bulk/absorb and 13.4 float. I have a warranty claim but have been told the company takes weeks to respond to a claim. Will have to see how this turns out…. Anyone else have QC issues with Firefly?

    • Ben Ellison Ben Ellison says:

      Hi Chris. I think there have been some QC issues with Fireflies. One of my four G31’s was replaced after about a year (see comments above) and a friend had 1 or 2 of his almost new L15’s replaced last fall. But in both cases, replacement was well and quickly handled by OPE in Bath, Maine, and I don’t know the extent of the issues.

      Also, I’m curious about how your battery failed? If the sympton is low capacity, the restoration charging described in the Firefly manual may help it. In fact, my whole bank — now mostly 4 years old and used hard — had gotten pretty wimpy, but I put them individually through several aggressive restoration charges in March and now they’re acting almost as good as new.

      • Chris G says:

        Ben, thank you for your reply. Also I appreciate your forum. Great info here in new technology!. To your question above – I had noticed the batteries were diminished in capacity so I did their first restoration charge this Spring (one battery at a time) to try to get them back up to spec. I discovered one of the Fireflys had about a 7 amp capacity and after discharge to 10.5v at a 20C rate, the voltage would then pop back up to 12.7v after the load was disconnected. It seems to me there was some kind of internal fault. The other battery was much better – but still impaired having been in parallel with a bad battery all winter. I bought the battery from Pacific Yacht Systems, a big volume Firefly dealer and they confirmed its failed condition.

  106. Alden Cole says:

    Chris, Hi. You’ve given me a segue into alerting you and other readers of a very reasonably priced meter that will give you the internal resistance in mΩ. This figure can alert the user of the health of a battery. It is The digital battery analyzer from Harbor Freight
    20% off coupon also available. This is for 12V lead acid batteries only. Yes, a Firefly is a l-a battery. The device is not used to measure a bank. One can compare the values of individual batteries in a bank to assess what is going on. It’s a little cumbersome in that one has to reset the CCA (yes, that value must be set up to get a consistent outcome). CCA and internal resistance values are in the Firefly literature. Ideally, the internal resistance values of the individual batteries in a bank should be close. Let us know what you find.

  107. Perry Stanfield says:

    Hi, relative newbie trying to research Firefly batteries; I have room for 6 of the L15+ size; would purchasing 6 of the 2v batteries and running them in straight series have any merit or downsides?

    • Ben Ellison Ben Ellison says:

      Hi Perry. I think that will work, but also that you can get the same 900 Ah bank with slightly fewer interconnect cables by using 4v cells 3x series 2x parallel (and that maybe the L15 and L16 are the same battery anyway?).

      However, I have a friend who installed a six Firefly 900 ah bank early last year, but then had two replaced last fall, and all six this fall. It’s a liveaboard cruising boat and for the most part, the Fireflies worked fine. But the apparent failures showed up as excessive voltage drops under big loads, and reduced capacity.

      I don’t know if his experience is at all common, and I believe he’s been satisfied with the warranty service, but it’s also been an annoying saga. In your shoes, I’d be asking about the incidence of quality control issues and how they are trending.

      Meanwhile, my four G31 Fireflies are doing pretty well at close to age 5. But they do lose significant capacity from the partial charging they often suffer on my boat and it takes some rigorous restoration charges to liven them back up.

  108. Bill Bunting says:

    Hi Ben, thanks for this careful article. I was in the process of installing lithium in my boat when the issue of lightning vulnerability of the battery management systems came up. I’ve only recently learned about firefly batteries from Jeff Cote of PYS vlog. The other issue is the slow charge problem wit AGM’s. So as I already 450 amp hours of lithium and was about to a another 450 amp hours, I’ve decided to add 4 G31 firefly batteries as the house bankaandelen connected to the whole system, and use the lithium as an isolated set with a separate battery management unit dedicated to storing power from the solar panels and wind generator via MPPT’s, and charging the firefly batteries via a 100 amp Top Systems TS1600 programmable DC/DC converter which I can switch isolate at times of high lightning risk. I’m also installing a copper system that transfers lightning direct from the mast head to the water outside the envelope of the boat. I hope that the combination of approaches will minimize electrical damage from a lightning strike while leaving enough elements intact to be able to piece together a working electrical system.

    • Ben Ellison Ben Ellison says:

      Hi Bill, It sounds like a very interesting power system you’re setting up. But I’m sorry to report that my 4 G31 Firefly bank recently failed pretty badly. Two seem FUBAR and the other two only have about 75% capacity despite rigorous restoration charges. But that does not mean that you will have issues with them.

      I suggest checking out the many Firefly threads on the Cruisers Forum, like this one:

      In short, many Firefly users are quite pleased with them after many years of use, but there are also some like me who have had problems. I do have excellent Victron data on the last six months of my four G31 bank and hope to do some forensic analysis with OPE. But I’m shopping for a lithium solution now.

  109. Perry says:

    My mechanic appreciated the theoretical beauty of Fireflies, but persuaded me to stay mainstream. Currently have 1200 AH Lifeline bank with a 3000 Victron Multi. After these, will probably go lithium. Thank you for the informative thread

  110. Ben Ellison Ben Ellison says:

    I just got this reply from Bruce Schwab at Ocean Planet Energy about my pooched Firefly bank:

    “Your assessment is the same as ours; that most Firefly users get many years of great performance, however there seem to be some batches that don’t last as long or have some issues. The big 4V units have had a higher percentage of issues than the G31, and we now have some warrantied 4V heading to Peoria for analysis. We really want to get some answers so that whatever happened will be addressed by FF. We are also going to send back some G31’s. Perhaps yours would be worth looking at, since you have data that can be used to help in the process.”

    • Ben Ellison Ben Ellison says:

      I’ll add that I was quite hard on my Fireflies. With only 280 Watts of solar panel trying to keep up with refrigeration and monitoring loads at 44 North, and sometimes long pauses between short cruises, the bank was only partially charged for very long periods.

      The bank lost a fair amount of capacity due to that kind of use — as warned about in the Firefly literature — but a year ago I was quite pleased about how aggressive restoration charging got a lot of that capacity back. It didn’t work this winter, however.

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