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post #1 of 23 Old 05-21-2009 Thread Starter
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AGM battery question

Does anyone know the difference between an dual-purpose AGM battery verses a deep cycle? I found what I think is a good sale this weekend on Seavolt dual-purpose AGM batteries but don't know if it's really a good deal.

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post #2 of 23 Old 05-21-2009
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Most likely the dual purpose is a deep cycle. The dual purpose means you can use it for starting and basic electrical. Most sailboats run their house bank as a starting bank too - especially in the smaller boats.

Just get the "dual purpose". What size is it?

And just a note, explained to the best of my knowledge, is that a "starting" battery is different from a deep cycle battery in design. It has to do with the plates which, on a deep cycle, are thicker and can give more ah but also require a different charging regime to replenish the plates since you have to replenish the 'inside' of the plates versus a thin exterior. The flip side is the starting battery which has thin plates meant for a quick (high amp) burst, quick discharge, and quick recharge. However, again to the best of my knowledge, there is not a set government stadard for what thickness plates have to be to be a 'starting battery' versus 'deep cycle' versus 'dual purpose'. I think it all depends on the original design focus, how reputatable the manufacturer is, and how creative their marketing team is.

My opinions.

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post #3 of 23 Old 05-21-2009 Thread Starter
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The battery is a Seavolt AGM Group 24 AH -79 MCA-735 on sale this weekend for $149.99 at West Marine.

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post #4 of 23 Old 05-21-2009
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I'm not saying I wouldn't use this battery under any circumstances, but I will quibble a bit with CD about whether this is a good battery for your purposes.

I assume you are looking to use this for a "house bank" battery? If so, I would go with a true deep cycle.

My understanding is that the dual-purpose batteries are not true deep cycles, but a reasonable compromise between a starting battery (like the one in your car) and a deep cycle. As such, they are primarily intended for motor boats, where there are many more start/stops of the engine, and lots of available charging to top them off (because they do not have the same ability to be "drawn down" like a true deep cycle).

Sailboats have a very different use profile than a motor boat. In a sailboat, I would only consider a dual-purpose battery if I was going to install a dedicated starting battery. For a straight "house bank" I would want true deep cycle batteries (whether they be AGM, gel, wet-cells).

Don't worry if you don't have a dedicated starting battery -- standard deep cycle batteries can manage a sailboat's starting loads just fine.


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post #5 of 23 Old 05-21-2009 Thread Starter
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So basically the wrong battery at the right price is still the wrong battery I already have one of these batteries for bank one but need to replace the other two in bank two. They don't hold a charge as good as they did in the past but this is a 20 yr old Cat30 and the batteries are 7 years old. The battery from bank one had the + post corrode right off of it so I replaced it with a dual purpose. My plan is bank one for starting and bank two for the house. I do need to figure out what is crossed up because when I "unplug" all but one battery, the gauge on the panel shows good voltage switched to either bank one or two. Another day of fun on this old boat .

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post #6 of 23 Old 05-21-2009
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The Seavolt is really a Deka y East Penn AGM and it is a deep cycle battery despite the dual use advertising by West. Go get your deal.

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post #7 of 23 Old 05-22-2009
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I have a pair of Seavolt 6Vs lead-acid ganged together as the house bank for my boat. They're fine but they aren't nearly as good as the 18 year old Trojan T-105s I pulled out of there in 2006. Yes, 18 years of Canadian winters and no record of service and they could still turn over the diesel...they just couldn't hold a full charge anymore.

To my knowledge, Deka/East Penn do a lot of the "house brand" batteries. Frequently a good test is to check out their website and look at the equivalent Deka/East Penn model. Often they are identical to the WM house brand, save for the colour of the plastic and the label on the top.

I just got off the phone with a buddy who has two East Penn 8D AGMs with which he is very happy. He noted how well they held their charge compared with lead/acid over our Toronto winter. Whether that's important to you is moot.

I'm likely going to 8D AGMs for world cruising shortly, but the brand is still up in the air. I like Lifeline and Discover brands so far.

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Last edited by Valiente; 05-22-2009 at 01:52 PM.
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post #8 of 23 Old 05-22-2009
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Val...Consider the Odysseys. I would go with them if I were starting from scratch and I think you will have the capabiity of taking advantage of their qualities of very fast charge and deeper discharge % levels.

I agree the T105's are better than the seavolt lead-acid or most standard lead acids. I like the Deka AGM's and I think they are as good as the Lifelines at a lower price...but I think the Odyssey is worth the premium IF you can take advantage of them since they will pay for themselves.

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post #9 of 23 Old 05-23-2009
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I'm a little worried about the durability of the thin lead mesh in the Odysseys standing up to the "dynamic" environment of a sea-going boat. Part of my hesitation is concern about the durability of this relatively new battery type and manufacturing process, and the other is about the duty cycle. Unlike most coastal cruisers, my situation is such that I don't need smallish battery banks that can be charged quickly and can be drawn down to 50% without harm. I don't even need the aspect of a battery that will hold its charge after a month idle. I have the space, and soon, the means to have a really big bank (4 x 8D) with multiple charge sources (4 x 135W solar; Air-X wind genny; Honda 2000; two 75 amp alternators) and a relatively modest draw (fridge, radar, plotter, a bunch of LED lights...meh...).

So I'm focused on the "brute force" aspects and certainly on durability and ease of replacement. I'm like the guy who just wants "a car"...not a 2009 Ford Fabuloso with DVD/hologram and 22 coffee cup holders and a seat that shouts out directions en espanol...

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post #10 of 23 Old 05-24-2009
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Try to stick to a starting battery high current capability and deep cycle battery for house if you can afford 2 banks. I installed AGM's 5 yrs ago and swear by them. Only concern is charging them. Improper charging will reduce their life as most batteries, more forgiving than the old gel cells. They have a higher acceptance level for quicker charging than wet cells and can take the discharge cycles better. Remember the deeper the cycles the less cycles there will be. AGM's were originally designed for military durability in aircraft. Nice to have them for reliability in the boat.

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