12 volt AGM batts. worth it ? - SailNet Community
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post #1 of 15 Old 08-05-2013 Thread Starter
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12 volt AGM batts. worth it ?

Hi , 8 yrs. ago I installed 2, starter/deep cycle Trojans (12volt) I forget the group size, but they were 12"x6" and 8"tall . My batt. charger is a Guest 10 amp. 3 stage type . This set up works very well . I hydrometer check and water check within reason . My power requirements are small , when I'm at my little spot over at Catalina the biggest draw is the TV (runs on a 200 watt inverter) We watch it very little. Other draws are the boom box that powers a MP3 Nano . And the overhead lights (florescent) . And that's it . Of coarse we have a Jabsco macer pump, but we only use that when we are motoring. I was really thinking about replacing with the AGMs. I think they charge faster and drain slower , however by a small % I think, lighter? And spill proof. Anyhow I was at Costco and saw a really good deal on some exact replacement batts. Lead / flood , for about $190. that's for two. So I bought them . AGMs would have been over $400. Sorry if this question has been asked, I did a hist. search and didn't see anything . I think this post will have some interest to people deciding on what type of batts. to get . I would still like to hear from AGM users . Thanks . Sail on Bors. and Broetts.
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post #2 of 15 Old 08-05-2013
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Re: 12 volt AGM batts. worth it ?

I bought AGMs a few years ago. I had been in the habit of bringing the batteries home during the winter to prevent them from freezing if they discharged. (I had repeatedly killed several batteries by not attending them and not keeping them charged over winter) After a few seasons of lugging them home, I switched to the AGMs. I've left them on the boat through winter and had no problems in the spring with almost no self discharge over winter. I haven't weighed them, but my sense is that they're heavier than regular batteries. You're right, there are no vents (at least on mine) and nothing to spill. They said they could even be mounted upside down if I wanted. They are pricey, but to me it was worth it.
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post #3 of 15 Old 08-06-2013
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Re: 12 volt AGM batts. worth it ?

AGM"s IIRC the specs, discharge at about 3% a month, vs 6% for lead cell batteries. SO if you leave them with out a charger hooked up for longer periods of time, they could be a good thing to have. An issue with them, good or bad if you will. They will take up to 40% of the discharge at a time when charging, vs lead at 25% and gels at 20%. So they will charge quicker.......IF you have the setup to take that high power of charging. Land line electricity is not an issue if you have the correct charger. You alternator on the other hand, may not like AGM's vs lead or gel, as the AGM's may over use the alternator, and destroy it in short order. Many have had to upgrade alternators to suit the AGM use.

My personal feeling, either work. 6V lead cells do get the most discharges out of a given set at upwards of 1000 cycles, AGM's are like most others including 12V lead at around 300-500 discharges. Kinds of expensive for what they are to a degree. BUT, as noted, being able to set them on their side, can be a good thing in some cases!

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post #4 of 15 Old 08-06-2013
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Re: 12 volt AGM batts. worth it ?

I'm currently using two regular marine 6V GC2 215 AH deep cycle batteries in series. Given local stock in marine batteries, that was simply the most cost-efficient tradeoff, and the largest AH that I could get to fix into the available space on my 27' boat without significant modifications.

I thought long and seriously about gel cell and AGM, and I basically concluded that the perceived benefits just couldn't match the fairly massive price hit. I strongly suspect that there's no way that any of the life etc benefits will outweigh the cost.

The only way I can see gel or AGM making financial sense is where there is some specialized use where some other variable is much more important, such as you're trying to maximize AH within a fixed space, or you specifically need the batteries to be able to last for a longer number of cycles without refit, or not requiring "maintenance" for some reason, such as them being hard to get at or something.
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post #5 of 15 Old 08-06-2013
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From all I have seen over the last 10 years, agm are fine IF they are set up properly and taken care of. Meaning they are more likely to fail from less than perfect setups. Wet cells are much more forgiving. At half the price, usually lasting twice as long...
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post #6 of 15 Old 08-06-2013
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Re: 12 volt AGM batts. worth it ?

Mainesail has great data on AGM's, he is not a fan, let's see if we can raise him

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post #7 of 15 Old 08-06-2013 Thread Starter
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Re: 12 volt AGM batts. worth it ?

Hi, thanks for all the reply's . I think this turned out to be a informative thread . I agree with all , plus I learned some stuff . I never heard about the alternator needing a rework for a AGM in fact I read that it needed a rework for a gel ? Well anyhow I'm happy to report that the new Lead/acid batts. are installed and took a charge over last night and the charger says all charged up , so it's in float mode . And that is how I leave it when I'm in the slip. Thanks again.
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post #8 of 15 Old 08-06-2013
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Re: 12 volt AGM batts. worth it ?

I never heard of the alternator needing to be beefed up for AGMs.. I have run AGMs in my cars for the past 10 years without a single case of alternator failure.. but granted, I am not running them "dry" between charges either

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post #9 of 15 Old 08-07-2013
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Re: 12 volt AGM batts. worth it ?

AGM's won't run your alternator to death unless you deep cycle them. That's something that doesn't happen if you use them in your car, or just for daysailing on the lake or bay (unless you run a lot of power hogs, or can't charge them with shore power). But if you aren't using most of the deep cycle capacity, then there is very little benefit from the AGM's unless you just can't fit a wet cell into the space available.

The AGM's are a strain on your alternator if you deep cycle them because they will have such a high charge acceptance rate. I replaced two 4D wet cells about a year and a half ago, and was planning to "upgrade" to AGM's at the time. However, I realized that my existing alternator would not be able to keep up with the charge acceptance rate of the batteries any time I drew them down any more than I could have drawn down a comparable set of wet cells. I also realized that an alternator can't survive running at full capacity for long stretches, and my existing alternator would likely burn up if I tried to make it keep up with the AGM's. New alternators aren't cheap, but I am. So I bought new wet cells, saved several hundred dollars, and have had no problems since.

Based on what you described in your original post, you don't sound like you use much electric power, and wouldn't have gotten much bang for your buck if you had spent more for AGM's. Bottom line is, you made the right call.

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post #10 of 15 Old 08-07-2013
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Re: 12 volt AGM batts. worth it ?

I use a 12V 50Ah AGM as a "reserve bank" on my boat. It's a Diehard "wheelchair" battery. The "main bank" is two 6V flooded golf cart batteries. The AGM has enough power to start the 2GM20 and hardly loses any voltage if the boat has been sitting "unloved" for a couple-three of months. It's nice to know that even if the main bank fails for some reason, the AGM reserve should get the engine running without a problem.

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