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Old 25-11-2006, 16:25   #1
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AGM Batteries and Battery Box

I am considering replacing existing wet cell batteries with AGM batteries and would be interested in opinions on the following scenario:
  • I am going to relocate battery location to under a seat that has limited space because of hull curvature
  • This location is closer to engine and means minimal cable run to starter.
  • Batteries will fit OK, but battery boxes to suit will not fit without further modification
  • Given that the design of AGM batteries is to NOT include liquid (acid), and the purpose of the battery box is to contain liquid spills, surely the battery box becomes redundant.
  • I am able to securely tie batteries down.
Is this all to simple and straightforward, am I missing something, or does this seem like an idea that will work.

Purpose of my boat is to cruise and relax, so strict survey requirements are not an issue.

Your thoughts and suggestions will be appreciated.

Fair winds

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Old 25-11-2006, 16:33   #2
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On our previous boat which we lived aboard and cruised, we had 2 8D Gel cell batteries which were strapped down but without boxes, and they gave zero problems.

I haven't had any personal experiences with AGM, but if they're more or less sealed, I wouldn't hesitate to at least give them a chance without boxes. The "necessity" of battery boxes is probably tradition not having caught up to reality.

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Old 25-11-2006, 18:10   #3
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Sorry Steve Flood batteries require a a box - even if you don't have problems. They can leak. AGM's don't require a box at all (they are 100% sealed) but you need to secure them as you really don't want anything heavy bouncing around. Almost any way is fine but it would be nice if they were easy to access and inspect. Making the run as short as possible just saves money and potential problems. This would be a good thing.

When you switch from flood to AGM all the voltage regulators have to be readjusted. AGM's are different than floods and you can't just swap them out and forget about it just yet. Smart 3 stage chargers (shore and alternator) will (I hope) have an AGM setting. If you don't reset them you will end up being one of those people that know first hand AGM"s were the worst thing they ever did. They don't charge the same as flood batteries! If you charge them like flood batteries you will lose most all of the advantages they bring.
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Old 27-11-2006, 12:33   #4
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VRLA batteries and venting

Sealed VRLA (valve-regulated-lead-acid) batteries like gel-cel and AGM lead-acid batteries still follow the classic electrochemical conversion formulas which show the generation of hydrogen and oxygen during charging. To be sure, the gasses are normally recombined UNLESS something goes wrong with the charging system (catostrophic failure) and the valves are forced to release the gasses.

In this case the batteries need to be vented ultimately to the outside and here is a case for being careful when you visualize just where the gasses may go. Recreational vehicle manufacturers have for some time been required to vent ALL battery boxes to the outside of the vehicle. I believe that it is just a matter of time (if not already) that ABYC requires similar venting which may, at least for us boaters, pose some difficulties.

I am familiar with a few cases of such catostrophic failures where fortunately no spark caused any explosion. What DID happen, however, was a spewing of some of the electrolyte sulpheric acid as a fine mist which covered many items ultimately destroying them or at least contaminating the surfaces in the immediate vicinity of the batteries. Now I realize that this is not likely to happen, especially if when the battery vents it is not overly excessive and you don't even smell much sulpher, if at all, in those cases.

Just be aware that no one recommends placing sealed VRLA batteries in sealed containers or confinements, not that YOU would, would you?
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Old 27-11-2006, 14:34   #5
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I agree with Paul that AGMs need proper charging. You need to be able to accurately monitor, charge when needed and periodically get them fully charged to make them last. Having said that, I did the change over last year and they have been great.
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Old 16-05-2010, 18:51   #6
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Despite the advertising claims...I have been in the Avionics and Marine electrical/electronics field for 50 years......remember Murphies first law...if something can go wrong, it will. As previously noted AGM batteries have advantages, but there are rules. To have those advantages they must be properly charged, and vented, and although boxes are not required and they may operate in any position, they should be SECURED...and secured well.
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Old 16-05-2010, 20:29   #7
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Most everybody here has it right, and your own analysis was good. No, you do not need a battery box. It is an all to common myth that they are required by somebody, USCG or ABYC, or.. Actually they are not. Batteries are required to be vented to the atmosphere, secured so that a 90 lb force can not budge them or knock them over. ABYC does require some means of containing an acid spill which is possible, but of minimal risk with AGms and Gel Cels. Get a tray about one - two inches deep to put under the batteries, secure them properly, make sure they are vented, and go sailing.
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Old 16-05-2010, 20:47   #8
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Steve,

The principal thing to think/worry about when switching from flooded to AGM lead-acid batteries is the difference in charge acceptance. Why? Because AGMs can take a charge up to and even larger than their AH capacity, while flooded batteries are self-limiting to about 25% of their AH capacity.

This means that a 100AH AGM battery can easily take a 100 amp charging current during the bulk charging stage. A typical 400-600AH house battery bank on a 40-footer can take 400-600 amps charging current...easily.

In practice, what happens is that unless you have a humongous alternator and a programmable external regulator you run the risk of burning up your alternator. This happens often. Typical alternators with internal regulators do OK with flooded batteries, but when faced with a battery bank which can really take a large charging current they try to deliver....but burn up because they weren't designed to deliver so much current for so long.

Bottom line: for you and for anyone contemplating a switch from flooded to AGM batteries, carefully consider your charging situation. You may need to make some changes to take advantage of the much higher charge acceptance rate of the AGMs and to avoid destroying your alternator.

FWIW,

Bill
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Old 16-05-2010, 20:51   #9
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I really wish to emphasize SECURED. If any part of the battery or the power leads to/from the battery can swing or move they may develop fatigue over some time an separate and now you have a live wire in the bilge....the other possibility is that in severe pounding the battery may break free and cause a tremendous amount of damage....when I think of damage in bad weather, I tend to thing of a battery in a box, with the lid and security strap off and had been left that way for a couple of days....coming across the Indian ocean on way to Madagascar the electrics got rather erratic, I opened the hold and started to drop in, feet hit the deck.sole down below, boat lurched, and I ended up with a fractured kneecap. It hurt. It more than hurt....it really, really smarted. And days like today when the humidity is up, there's a certain fellow from Inner Njardvik Iceland that I have dreams of, with a hangmans noose around his neck being towed behind the boat. Secure everything. Plastic tie wraps are another pet peave. My hands have hundreds of cuts from the tie wraps......if you use them, make sure they can be seen and have a pair of sidecutters handy to start removing them in the event of trouble. Waxed doubled dacron line is faster and neater and less expensive. It looks like a professional wired the craft.
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Old 16-05-2010, 21:08   #10
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You will need to check your battery charging equipment to see if there are settings to allow switching battery chemistry, Most modern systems are sep this way. I have the Pro Mariner charger and it allows to program the battery system that you are using. I have AGM for house battery and a lead acid for the engine starting battery. The advantage of the Pro Mariner is that it will operate from U.S. volt/current and foreign voltages and currents as well with no switches or changes. When I wired Tana Mari I have no a.c. fed thru anywhere. All a.c. entering the boat goes directly to the battery bank and chargers. If I need a.c. then I convert it from DC as needed by a 2500 watt inverter. The only thing I normally used it for was the microwave machine, and battery chargers for my sabre say and battery skil saw.....Everything was d.c. operated.
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Old 17-05-2010, 20:34   #11
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The other part of this that I have not seen anyone mention is protection of the terminals. A properly designed box with a lid that can be secured will prevent metal objects from contacting bare terminals or battery posts.

By putting the batteries under the seat, are you sure you will never have an instance of loose objects causing an electrical short?
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Old 17-05-2010, 22:04   #12
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Absolutely right Steve. The positive terminal on the battery should have a rubber boot on it to prevent inadvertent contact with metal objects such as tools.
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Old 18-05-2010, 05:46   #13
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I never have been very impressed by those little rubber boots. Over time they tend either not fit well or else migrate up the cable a few inches so they don't do a good job of protecting the terminal.

I'm a big believer in everyone making their own decisions on equipment installation on their boat. For me, one of the primary requirements of battery installation is that all electrical terminals are protected from inadvertent faults, and an excellent way to do that is with a cover well secured over a battery box, which is also well secured.

If you do go with the rubber boots, I would cover the negative terminals as well. This way it takes two misplaced covers to result in the possibility of a positive to negative fault.

Like the old joke of taking two paper grocery bags to cover the head of an ugly date - one for her and one for you in case hers falls off.

Cheers.
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Old 18-05-2010, 06:06   #14
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Originally Posted by steve77 View Post

If you do go with the rubber boots, I would cover the negative terminals as well. This way it takes two misplaced covers to result in the possibility of a positive to negative fault.
Not sure if this makes sense. If the positive cover comes off any short to ground will be a problem regardless of a boot on the negative terminals. The main reason you remove the negative leads first (and install last) is so an accidental wrench touch to metal won't result in a welding experiment.

We have our three batteries on a platform behind the saildrive in the bilge securely strapped down by web straps. I bought boxes but there was no way the boxes and the batteries would fit. 3+ years and counting.
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Old 18-05-2010, 07:30   #15
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Let me clarify why I recommended covering the negative terminals.

I think we all agree that an electrical fault occurs when you have an unintentional current path between the positive and the negative terminals. This path may involve wiring to some remote part of the boat, but the fault doesn't happen until there is some way for current to get from the positive to the negative terminal.

(Let's not get into an argument on electrons - convention is that current flows form positive to negative.)

In the area under the seat, I was envisioning the only exposed electical connections to be at the battery terminals. For that reason, I would think the most likely cause of an electrical fault would be a tool dropped on the battery that contacted between the positive and negative terminals. I'm not sure what else is in that area that is exposed and could result in a fault.

If there were other exposed metal terminals or objects in the area that are connected to the battery negative ("grounded"), then of course they may be a potential path for a dropped tool to contact and short.

So in the area where the batteries are installed, you would be safer protecting both terminals of the battery than just one terminal to prevent a positive-to-negative fault in the event of a dropped tool when one of the terminal covers was mispositioned.
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