Darn AGM Batteries

Discussion in 'Musings With Maine Sail' started by Maine Sail, Sep 12, 2011. Add this thread to a FAQ

  1. Maine Sail

    Maine Sail Moderator

    Joined Feb 6, 1998
    10,950 posts, 747 likes
    Canadian Sailcraft 36T
    US Casco Bay, ME
    Warning, rant...

    For the second time is a week I have had to explain to a customer that their $1000.00 + bank of AGM batteries is dead in under four years. I have asked Lifeline to publish their tome on how to properly care for them on their web site so people will at least have a skating chance if their boats are cruised or left on a mooring.. Hopefully they will...

    The boat today has a very knowledgeable owner, decent charging system, battery voltage meter and at the beginning of the season his batteries tested well (for cranking ability) with a Midtronics EXP-1000. When I see AGM batteries fail sometimes they fail, suddenly and without warning.Why? I don't have a good answer for that. AGM batteries are great batteries but if abused, as they often are, they are no better than any other lead acid battery, just more expensive.

    Dual Circuit Plus Battery Switch Rant:

    This is also a rant or vent about the Blue Seas Dual Circuit Plus battery switch. This owner installed one, and when his house bank failed internally last week, he had no way to switch to only his starting bank, in an isolated manner. They key here is; "in an isolated manner".

    to get any sort of VHF, instruments etc. without combining a good battery with a completely failed, internally shorted battery/bank.

    The start battery still likely had a little life in it, about 110 cranking amps (Midtronics) before I tried to start the motor, vs. the 1000+ it should have, but due to the Blue Seas DCP switch he was unable to take any advantage of his second bank, in an isolated manner, as a safety back up because the failure in the house bank bled it down to the point where he could not start his motor at all after using the "COMBINE" position of the DCP switch.

    The owner did not realize the house bank had failed internally so when he got to his destination, and could not make a VHF call or turn on any of his electronics, he did what he thought he should do, he turned the battery switch to COMBINE.....

    When he did combine the start battery with the house bank it was apparently into an internally shorted house bank. He left it in combine/parallel long enough, while using the VHF radio & trying to figure stuff out, that it sucked the remaining useful life out of his starting battery and he was left dead in the water and needed a tow to get home.

    • If buying AGM's, please charge & care for them correctly..
    • If buying a Blue Seas Dual Circuit Plus Switch please do your home work and make sure it is the right fit for you.
    CONSIDERATION: The Blue Sea DCP switch can not provide isolation of a bad bank while using the other bank. Your only course of action is the COMBINE or PARALLEL feature. In a case like this it forced the owner to combine his marginal condition starting battery with an internally failed house bank that was hoovering in the 7-8V range. A marginally healthy 12V battery can't charge an 8V battery for very long without being drained to the point of not being able to start the motor, especially one that has already been abused from improper practices...

    This is what Lifeline suggests for expected life, based on care.

    • #1 Fully charge after each discharge. Estimated life: 6-9 Years

    • #2 Fully Recharge at least once a week and equalize once a month. Estimated life: 4-6 Years..

    • #3 Only recharge to 85% and equalize once a month. Estimated life: 2-4 years.

    • #4 Only charge to 85% and never equalize. Estimated life: 1 year.

    How many of us fall into scenario #4?

    Bottom Line:
    • He never equalized his Lifeline batteries
    • Mooring sailed the boat with no solar
    • Only used an alternator for charging (except in the off season)
    • Used the COMBINE feature of the DCP switch, with a bank failure, and it left him dead in the water

    Dual Circuit Plus Switch Options:

    If you want to use a DCP switch the wiring diagram below makes it much more flexible and provides both isolation and redundancy.

    With the Blue Sea 5511e Dual Circuit switch your only choice in an emergency is to combine a perfectly good bank with a potentially bad one. Not really the best practice. The switch offers no way to fully isolate a bad bank.

    You would also need to wire the SI feature of a Blue Sea ACR to get the benefit of "isolated" banks for starting, this is something most boaters with a DCP switch almost never do. Without the SI (start isolation) feature you may not be "isolating" start from house loads like you think you are.

    When I "modify" DCP switch wiring on customers boats, who finally relaize they do want the ability to isolate a bad bank, while using the good one to power everything, I add two mSeries ON/OFF switches which are are out of plain sight, usually in the battery compartment.

    All the guests and owner see is the single DCP ON/OFF switch. The DCP label already has a yellow WARNING placard with an ! for "combine batteries". This should be enough to prevent guests from messing with it. The general wiring would usually look like the diagram below.. The hidden emergency ON/OFF switches are usually within about 7-10" of each bank after the fuse and their normal position is ON. This switching gives you the best of all worlds plus 100% bank isolation in the event of a failure. In the form of redundancy now either bank can be used for powering the entire vessel without having to be combined with a potentially bad battery.

    If your alternator is direct wired to the house bank, for optimal charging performance, you'd already want an alternator service disconnect switch for service techs anyway.


  2. tommays


    Joined Oct 2, 2006
    1,517 posts, 2 likes
    Jboat J24
    US commack
    I like my Blue Seas Dual Circuit Plus Switch if its the one they package with there ACR BUT i also understand the purpose is to keep the house and start separate and use ONE switch to turn the boat on and off its NOT for bringing a stone dead battery back to life :)

    I am also smart enough to remove the cable from the dead battery IF i did need to use the combine setting to supply the house loads from the cranking battery

  3. Stu Jackson

    Stu Jackson

    Joined Feb 26, 2004
    20,531 posts, 949 likes
    Catalina 34
    224 CA Maple Bay, BC, Canada
    You may be but most boaters aren't.

    The ability of any boat electrical system to have the reserve bank be able to power limited house loads if, when or maybe never the house bank completely dies, is critical.

    That's where the stupid BS Dual Circuit switch fails miserably.

    That's also why a simple 1-2-B switch, with the AO going to the house bank, and with an ACR or echo charger, etc. is far superior.

    Ya want one simple switch? Use a 1-2-B switch.

    Most people who have the dual circuit nonsense don't know that, 'cuz they're still in the "combining batteries gives me more juice" "Kool Aide" nonsense from Blue Seas.

    What a dumb idea.

    Rant over, not like you haven't heard it from me before, right?:naughty:

  4. Fred Villiard

    Fred Villiard

    Joined Dec 28, 2009
    397 posts, 22 likes
    Macgregor M25
    US trailer
    But do not get one of the switches that you have to go thru the both position to go from one to the other.

    Twice I have had problems because of that, once was my pure stupidity.

    I was working on my Grady relocating my second battery bank to a location out of the engine compartment. I tied the two 4/0 together so that they could be fished under deck. Then had to cut a 2 1/2 inch hole thru 3/4 inch fiberglass, and my cordless drill wouldn't pull the hole saw.

    I didn't have a long enough extension to get to the shore power with me, but I had a 2 kw inverter so I went to switch the battery bank on and it was on position 2. When the switch hit the both smoke flew and it welded the contacts, luckly I had a 350 amp fuse on the bank and it blew before any damage.

    Never gave it a thought that I still had the cables for bank 1 one the switch and tied together, dumb.

    The other time, I was out off Hatteras on a friend of mines 45' sport fisherman, trolling. It was about midnight when we suddenly lost power on the house bank.

    Tom started to switch to the engine bank and thw switch welded in the both position, smoke and fire started flying, the starting bank wasn't fused the engines were twin 700 hp diesels.

    As luck would have it I had my electrical tool kit, on board, the day before I'd installed a new radar for Tom. I took my heavy cable cutters and cut the hot wire from the starting battery, then we put the fire out.

    What had happened was the positive wire from the house bank, the plastic cable clamps had broken and the wire fell down on the transmission coupling and rubbed thru
    shorted out to the transmission and blew the fuse. when Tom tried to switch to the other bank he had to go thru both.


  5. tommays


    Joined Oct 2, 2006
    1,517 posts, 2 likes
    Jboat J24
    US commack
    It's a good idea that can't fix stupid users

  6. Benny17441


    Joined May 24, 2004
    5,764 posts, 420 likes
    CC 30
    US South Florida
    Batteries that need to be always fully charged after discharge are not in tune with the realities of boating. For many different reasons the power to recharge batteries can be considered to be not always accesible or the time required not always available when boating. A cruiser on the go is lucky if he recharges to 85% day after day. If I can get 3 years out of a lead acid battery that costs $80 what would motivate me to spend $250 for an AGM equivalent that may not last as long? I like a battery that if in an emercency I need to run it flat down I know that I can without feeling that pain all the way down to my wallet.

  7. Joe


    Joined Jun 1, 2004
    6,683 posts, 436 likes
    Catalina 27
    US Mission Bay, San Diego
    Roger that, Benny. 2 $60 group 27 batteries every 36 mos... is better than 2 $250 batteries every 72 mos. Spend the money you save on a smart charger, solar panel and a bunch of other cool stuff.

  8. shemandr


    Joined Jan 1, 2006
    3,980 posts, 919 likes
    Marblehead Skiff 14'
    US Greenport, NY
    Equalize AGM's?? I thought we weren't supposed to. Should I be doing this?

  9. jsalley


    Joined Mar 20, 2007
    494 posts, 4 likes
    Catalina 355
    US Kilmarnock, VA
    Lifeline batteries can be equalized; most others should not.

  10. mitiempo


    Joined Sep 28, 2008
    922 posts, 8 likes
    Canadian Sailcraft CS27
    Ca Victoria B.C.
    Lifeline recommends equalizing. I don't think any other manufacturers do. According to the post by Maine above, Lifeline doesn't suggest equalizing if the batteries are fully charged after each discharge.

  11. Stu Jackson

    Stu Jackson

    Joined Feb 26, 2004
    20,531 posts, 949 likes
    Catalina 34
    224 CA Maple Bay, BC, Canada
  12. Merlin Clark

    Merlin Clark

    Joined Oct 26, 2005
    2,057 posts, 3 likes
    - -
    US Satellite Beach, FL.
    Never saw that before and had to look it up. My new favorite acronym to be added to BOHICA, SNAFU and FUBAR!!!

  13. jviss


    Joined Feb 5, 2004
    3,125 posts, 412 likes
    Tartan 3800
    US Westport, MA
    I don't think it's necessary to resort to calling users stupid to justify this. I happen to think it's a pretty stupid switch concept, and, in fact, the Blue Seas datasheet for this is a pretty stupid, lame piece of marketing.


    For example, note that there are, miraculously, no disadvantages to the Dual Circuit Plus Battery Switch (DCPBS), and there are no advantages to the other two illustrated approaches. Try to reconcile that with the fact that one of the configurations of the DCPBS that's an advantage (3) is also two disadvantage with a conventional selector switch (2,3)

    Finally, what is in my opinion the preferred solution isn't even shown, which is two OFF/1/2/BOTH switches, one for the house loads, and one for the engine.

    They mention one disadvantage as "most expensive option;" the solution not shown, while superior, is less money that the DCPBS.

  14. Stu Jackson

    Stu Jackson

    Joined Feb 26, 2004
    20,531 posts, 949 likes
    Catalina 34
    224 CA Maple Bay, BC, Canada

    You can do it with one on/off switch and a 1-2-B, don't need two 1-2-Bs.

    Anyway, we recently had a skipper who had one of these stupid switches (Can I call the switch stupid? It's not meant against a person!!!) and he didn't know what it was and kept his batteries combined for six months until he started doing some reading on our C34 'board.

    These switches should be outlawed, you know, how I feel about 5200, for instance...:):):)

    But I have said, repeatedly, that IF, just IF, someone understands the limitations of it, and his house bank dies, he's perfectly free to go and DISCONNECT his dead house bank before he flips the switch.

    It's a boat, so it must be a compromise, right? :doh:

    But why ANYONE would actually CHOOSE a system that would REQUIRE physically disconnecting batteries INSTEAD of SIMPLY turning a switch is completely beyond me.

    PS - to add to jvss: The literature says "in the evnt of a low start battery." What they completely FAIL to mention is that the start battery rarely goes flat, the house bank does, and when that happens and you combine the banks, you're screwed.

    Last edited: Sep 23, 2011
  15. mitiempo


    Joined Sep 28, 2008
    922 posts, 8 likes
    Canadian Sailcraft CS27
    Ca Victoria B.C.
    But it sells a product.:)

    I think any switch that would require one to remove a cable to start from the other bank without combining is a dumb idea. And the worse part is that people are throwing out what are probably perfectly good 1/2/both switches to make the change.

    Blue Seas makes great products and they are my favorite but that switch makes no sense.

  16. mitiempo


    Joined Sep 28, 2008
    922 posts, 8 likes
    Canadian Sailcraft CS27
    Ca Victoria B.C.

    The number of people that leave their 1/2/ both switch in both all the time must number in the thousands. I see it all the time.

  17. Stu Jackson

    Stu Jackson

    Joined Feb 26, 2004
    20,531 posts, 949 likes
    Catalina 34
    224 CA Maple Bay, BC, Canada
    You're probably right, but those are the same group of people who still have their AO going through the switch and the PO told 'em that's the way it's supposed to work!

    One of the beauties about these kinds of forums is that we all learn from them.

    In the case of, for instance, our C34 CRITICAL UPGRADES page, I mentioned a number of times that we can make the information available, but we can't make 'em read it or perform the necessary, and safety related, corrections that are necessary. The wiring harnesses with those flimsy plastic connectors, comes to mind, along with ammeters in cockpit panels (unless you KNOW what you're doing).

  18. Maine Sail

    Maine Sail Moderator

    Joined Feb 6, 1998
    10,950 posts, 747 likes
    Canadian Sailcraft 36T
    US Casco Bay, ME
    I wouldn't necessarily consider my customer "stupid" at all. He did what the switch maker suggests you can do in a situation like that "combine" the banks. Sadly for him he let a boat yard talk him into this switch when he did not need it. He now wants me to install a 1/2/BOTH/OFF again... I talked him into a modification of the DCP where I add two ON/OFF's:doh:

    In their situation they had gone to the mooring fired up the motor and motored nearly two hours because there was zero wind, lots of good bulk charging time, or so he thought. Once moored at their destination they noticed certain devices like their small 300W inverter, used for the Netbook computer, were not powering up?

    He hit the toggle switch for the house bank volt check and it was at about 8 volts. They needed to get to shore for dinner with friends and the VHF would not power up to call the launch and the hand held had died because he uses that in the cockpit while sailing.

    Following the suggestions from Blue Seas he decided to use the "combine" feature to get the VHF working enough to call the launch. After combining the start battery with the dead bank, he did not know it had catastrophically failed, why would he, the start battery was toast and would no longer start the engine.

    In my opinion a battery switch on a sail boat, where banks are deeply cycled, should not require a user to dig into a battery compartment to physically un-wire or physically disconnect a battery bank just to use the VHF or other necessary equipment, in an emergency situation.

    When I design battery switching systems for cruisers there are three main criteria.

    • The ability for 100% isolation of a bad bank from the rest of the system
    • The ability to use either bank for house & start use in an emergency
    • Simplicity

    The DCP switch hits simplicity but fails on isolation.

    Most owners already have a 1/2/BOTH so adding a simple $22.00 ON/OFF switch to a 1/2/BOTH/OFF can give you a dedicated starting bank and the redundancy that the 1/2/BOTH/OFF switch offers. Then add an ACR and you're still below the cost of the DCP switch.

    If you already have a DCP just use the diagram in the first post.

    On our boat if a bank fails we can simply completely isolate it by switching to the other bank entirely.

    In stock form the DCP switches do however have a good use and that is for fishing boats that shut down and fire up the motor some 30-40 times per day all while running expensive fish finders etc. that take time to re-boot. I have installed them in this application and I have also installed a simple ON/OFF in addition to the factory 1/2/BOTH/OFF. If starting from scratch I will most often install a three ON/OFF configuration..


    I prefer the thee on/off switches over all others or the DCP config in the first post. When I have a clean slate to work from the three ON/OFF switch configuration is or DCP plus two ON/OFF's ultimately preferred...

    I don't find a sailboat that already has a sufficiently sized house bank to prevent voltage drop out of electronics to be the best situation for a DCP switch but give me a center console with twin 250's and it can be a decent option depending upon what's already there..

    The Lifeline's that failed my customer did so in a way I have become somewhat familiar with. They did so suddenly, and with little warning. Because they never started the motor on the house bank, and they don't have a windlass, they had little idea the bank was failing.

    Even a failing bank can support a 2-6 amp draw for a while and "appear" to be fine. This is an actual benefit of starting your motor off the house bank. By using the house bank for starting loads it lets you know early that your house banks capacity is getting weak. With a DCP some sailors don't know until it is too late. Just like my customers never knew.

    The week before they had no noticeable issues and then the bank just died. He assumed the bilge switch might have stuck, but it had not, so the combine feature was not such a "stupid" move especially given the Blue Seas suggested use.

    I attempted to charged the three Lifeline batteries yesterday and last night. The start battery went from a paltry 110 cranking amps to 183 cranking amps and the two house batteries, well.... Deader than dead.....

    After nearly 16 hours on the charger the House Bank #1 came off it and within 15 seconds was down to about 10.3 volts and House Bank #2 was in the low 8's. This indicates that at least one battery has an internal short if not both. I was charging at low current just in-case the batteries were shorted so as not to develop too much heat.

    If I want to call anyone "stupid" it would be the boat yard that talked my customer out of the perfectly adequate battery switch that he already had. Oh and it is still wired so that if he moves it through OFF the alternator is open circuited and potentially toast......:doh:

  19. tommays


    Joined Oct 2, 2006
    1,517 posts, 2 likes
    Jboat J24
    US commack

    So far all of the fancy battery monitors have not stoped the stone dead house bank issue or stone dead start bank at which point if you do not grasp what has happened its not likely your getting going on your own

    I have not yet seen anybodys diagram that would route power anyplace needed in a fail mode that did not require and understanding of what failed

    I am a simple person with a simple boat with two batterys and the switch works FINE at its MAIN purpose ONE switch to shut down the compleat boat

  20. Maine Sail

    Maine Sail Moderator

    Joined Feb 6, 1998
    10,950 posts, 747 likes
    Canadian Sailcraft 36T
    US Casco Bay, ME
    This customer does not have a battery monitor like a Victron or Xantrex just a good quality Blue Seas volt meter with a toggle switch. I am installing a Victron next week and adding a solar panel though.

    My customer knew exactly what had occurred, a very low voltage house bank, and he could tell that easily. He ideally wanted to switch to just the starting battery, like he could with his old switch and he'd had to do once before years ago, but he quickly realized he could not so he used "combine" thinking his bank was just low not failed..

    One switch shuts down the whole boat with a 1/2/BOTH as well.;) This is not the first time I have run into a situation like this with the DCP and I am sure it won't be the last.

    If you only have two batteries then the DCP will help prevent drop out. If your house bank was larger, like most sailboats today have, there would be little need for a DCP for preventing drop outs. This customer has nearly 2000 cranking amps available at 70F from the house bank. I started his 3YM today and the lowest voltage off the house bank, start neg disconnected, was 11.7V which is well above the drop out range. Our bank has about 3000 cranking amps at 70f...

    As mentioned it is less expensive to simply add an on/off to the existing 1/2/BOTH and you can still gain a dedicated starter and nearly all the redundancy of the 1/2/BOTH and not have to "customize" the interior or DC panel to fit the DCP switch..

    Still I run into just as many issues with blown diodes and switch issues because the the 1/2/BOTH/OFF comes wired from the factories it is a poor excuse and people just don't know how to use them correctly. Once educated they usually get it...

    All that matters is that you like your switch. I give my customers all the options and educate them on each of them and let them make the choice. I mentioned nothing about the DCP to my customer because it was already installed and paid for. He's the one who asked about going back to a switch like his "old one"....

    P.S. Please note that in my original post I never suggested that anyone not buy either AGM batteries or the DCP switch. I simply stated the facts as they pertained to this customers case and then said:


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