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Bigdtown
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# Posted: 24 Sep 2011 08:26
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Hello everyone. I just got my first deep cycle marine 12v battery for my small solar setup. It is a autocraft 27dc-2 battery with 600 cold cranking amps, 750 cranking amps at 32 degrees and a reserve capacity of 160. This is all from the plate on top. Can you tell me how I can get the amp hours from this info or if that is even possible?
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razmichael
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# Posted: 24 Sep 2011 09:43
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There is no direct conversion from reserve capacity to amp-hrs as they are measured very differently. I've added some good links just below to provide more details (rather than try to reproduce the facts here). You may even find some difference of opinions within the links. Firstly though, keep in mind that it is very unlikely that you have a "true" deep cycle battery. Almost all marine batteries are hybrid compromises between starter batteries and deep cycle. Generally any battery that provides a cranking rating is a starter battery or a hybrid. Depending on how you plan to use your battery this may not be an issue but you need to understand the limitations (power, amount of discharge it can handle, number of cycles it can handle etc). Generally a hybrid marine battery is not the best choice for a solar system but they can be cheaper so many will use them for systems with minimal demands. Some estimates on RC vs amp-hrs will say that the amp-hrs is about 1/2 of the RC but this, again, is only an estimate and does not really tell you how you can use the battery. For example, the minimum you would want to discharge a standard battery is 80% while a true deep cycle can go done to 50% over many cycles without major problems. RC measures at a 25 amp discharge rate which is applicable to a car battery running an engine (without the alternator working) but rarely a use case for a cabin or rec vehicle.
You already have your battery so this information should help you understand the limitations of your battery and whether they really matter. I have had people put in very expensive solar systems but "save" money on cheaper batteries. This is generally false economy as the cheaper batteries will have a much shorter life (since they are not optimized for repeated discharge, may not provide the power you really need or just die prematurely - can be a bit like buying an expensive stereo but getting a great deal on cheap speakers.
-None of this is to say that your battery will not do the job for you!
Some good links:
12 volt side of life - slightly old but still used by many as the best source of general information on batteries and solar systems (part one and two)
URL
A link of a discussion a sailing forum: URL
Another site with some information: URL
Hopefully this helps.
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Rob_O
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# Posted: 24 Sep 2011 10:41
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A quick google search tells me other sellers rate their group 27 marine batteries around 85-90 AH. Yours will be the same, not much difference between manufacturers for typical consumer batteries
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Bigdtown
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# Posted: 24 Sep 2011 11:12
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thanks for the help everyone. I also found this link that has tons of great info on batteries: http://www.windsun.com/Batteries/Battery_FAQ.htm
Razmicheal- yeah, for now my system needs very little as I am not really using it much, but once I get more batteries in I will be asking more and more of it so we'll see. Thanks again for the info.
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MtnDon
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# Posted: 24 Sep 2011 11:57
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I look upon RV or marine batteries as "learner" batteries. Don't spend much more money buying more of them. At a minimum 6 volt golf cart batteries like the Trojan T-105 series should be used for PV power systems. Some Sam's Clubs and Costco's sell them. Where available they are usually the best price you can find. They will not be Trojan's, but the ones I have used over the years have stood up well. 6 volts and 210 to 225 amp hours per battery.
For PV or alternative energy use look for the "20 hour rate" amp hours figure.
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Bigdtown
Member
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# Posted: 24 Sep 2011 13:50
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so here is another question for you. I have my new marine battery but until now I have had just an automotive 12v battery that I was using and a friend of mine just bought a fourth wheel that came with two 6v golf cart batteries that he donated to me. All batteries seem to be holding a charge just fine and I was thinking of putting them all in my bank since I wired the two 6v batteries in series they are all 12 volts and I don't see how this could do anything but make my bank bigger and better all around. What do you all think?
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razmichael
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# Posted: 24 Sep 2011 18:39
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Mixing battery types, even the same type but different ages (more than 6 months generally), is not a good idea. They will all have different profiles and you get some bad situations where one type is over charging another or the draw is not even etc. Charging them is very difficult because they will all have different voltage points for float, charge and boost. In addition, newer/bigger batteries will be cycled more often (based on the weakest link) which will reduce the life of the expensive ones. Without knowing the type/condition of the 6 volt batteries it is hard to say for sure but I would strongly suspect you would be better off using these two for your system and keep the 12v as a backup (depending on your solar controller you could have both being kept charged at the same time (some controllers will charge two banks separately).
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Bigdtown
Member
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# Posted: 24 Sep 2011 19:11
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Thanks for the response raz, I actually think I will keep them seperate. After further review the two 6 volters are not any good I think that for now I am going to add another marine battery like the one I bought today and then concentrate on adding more panels. Like I said for now this is just a hobby so I can't justify sinking real money into batterys yet, but I can beef up my solar panels in a real way.
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razmichael
Member
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# Posted: 24 Sep 2011 22:44
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Assuming your batteries are about 80 amp-hours then two will give you 80 amp-hrs of power (stretching them to 50% discharge). There are lots of sites that will help you calculate your usage requirements and then your panel capacity to maintain the batteries. Good luck with your system.
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MtnDon
Member
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# Posted: 24 Sep 2011 23:16
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one calculator may be D/L HERE
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Bigdtown
Member
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# Posted: 25 Sep 2011 07:07
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Thanks guys. It will be nice to have all this info and all of these sites when we make our move. I am eyeing some property (an acre) in Western NC and if I get it I will then be building my small place. I was a framer for about 6 years and into remodel for about 5 after that. Now I am an HVAC technician so I don't think the building or the wiring of the place will be any trouble at all, it's just knowing what I need and being able to afford it that will be the challenge. It will be nice to have all of the knowledge though, which is why I have enjoyed this site so much. Really helps me out.
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