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  #1  
Old 02-18-2009
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Cost Per Amp Hour

I need to upgrade my house battery bank. I am looking for the most capacity for my money so I am pretty well set on Wet-cell batteries. Can I comparison shop by looking at amp hours per dollar, or is there another spec I should also pay attention to?

Right now I am looking at getting 2 or 3 sportsman group 31 12volt batteries from defender. Each is rated at 140 ah for $121 (86 cents per amp hour.) Is there a better solution for me?

My other dilemma is that I have an alternator on my 1976 Westerbeak that is rated at 55 amps. West Marine advises that the alternator be no less than 25% of the total amp hour capacity. With two group 31s my alternator will be about 20 percent of the amp hour capacity. Is it worth it to add a third battery. How long would I have to run my engine to charge them?

I will be cruising throughout the Caribbean for a year spending almost all of the time on anchor. The boat is a Pearson 365 and will have all of the typical power demands + laptop + radar + occasional use of a small TV. I have about 1000 dollars to increase our power supply do you have any other ideas?
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Old 02-18-2009
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a lot of people like the walmart deep cycles, for 61 bucks for 115 amp hours they are cheaper, infact i plan on 3 new ones for my boat in the spring.

as for charging time, the old saying is you will get 75% charged in 25 % of the time. say with a 230 amp hour battery at 50% discharge about 115 amps ( ie 2 walmart batteries, at 50 % discharge for max battery life), which a 55 amp alt will put in 55 amps or so the first hour, so a 50 % charge of 115 amps. the second hour you will put about another 20 to 30 amps in, so you will be up to about 70 % charge or so, then the extra 30 % might take 6 hours. the reason is as the battery gets closer to 100% it accepts less charge amperage.

if you go to 3 batteries at about 350 amps you will want a bigger alt or lots of wind and solar charge. face it at 350 amps ( 175 useable amps ) a 90 amp alt after 2 hours will put in about about a 140 or so amps. thats why 10 amps of solar ( about 150 watts of panel ) is so needed in 8 hours of day light you will get about half of of your charging done with no fuel use

Last edited by scottyt; 02-18-2009 at 01:38 AM.
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second post i would recommend at least 350 amps of battery, so your useable amount is 175 amps ( 50 % of capacity ). that should get you two days of normal use before any charging , then 150 watts of solar, plus a honda genny for weekly topping of the batteries with out using the main engine. this should have you only needing to use fuel once a week if you are frugal, because of the solar
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Old 02-18-2009
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Just curious as how you determined he could get two days of use out of 175 amp-hours of bank capacity, since he hasn't said a thing about what his electrical budget is...

Also, with a budget of $1000, getting 150 watts of solar, four batteries and a Honda generator is not going to be possible, unless he steals some of it.

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Originally Posted by scottyt View Post
second post i would recommend at least 350 amps of battery, so your useable amount is 175 amps ( 50 % of capacity ). that should get you two days of normal use before any charging , then 150 watts of solar, plus a honda genny for weekly topping of the batteries with out using the main engine. this should have you only needing to use fuel once a week if you are frugal, because of the solar
Snboard—

You really need to compare amp-hours/$, but make sure you're comparing apples to apples. Some batteries give the amp-hour rating for a 5-hour use rate, others for 20-hour, others for 25-hour... so make sure the amp-hour ratings are for the same time period.

Personally, I like the Trojan golf cart batteries, as I believe they're probably more durable than the wal-mart batteries. Four of them would give you 450 amp-hours at 12VDC, with two-pairs hooked in series, then connected in parallel. With batteries, you often do get what you pay for.
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Trojans are available everywhere. They have many different configurations to fit the space available and are well respected for durability

how much will it cost to ship those batteries from defender?

you will have longer charge times with a smaller alternator. You might be able to swap out for a bigger 'small frame' altermator, up to about 100 amps.
What about the regulator? Do you have an external regulator? it just keeps adding up!!
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The regulator you use will be important. A fixed voltage regulator is likely to load that alternator heavily as the fixed charging voltage does not really take into account the battery charge state.

The so-called 3-step regulators are a better product, but they can be expensive.

Also, be wary exactly what they mean by a 55 A alternator. Is that continuous rated load? If so, at what cooling air temperature?

16 years ago I bought a small frame 100 A alternator from Ample Power and a 3 step regulator. The alternator, though expensive (then perhaps $300, and the 3-step regulator perhaps $100?) was rated 100A CONTINUOUS at a max cooling air temperature of 91 degC ( 196 degF !!!). I was sceptical at the time, but it is still running. I have seen it hold 60-80A for perhaps 15 minutes and manage it. The alternator output steadily drops as the charge of the batteries recovers, so I can't state a single figure. As a rule, I let this alternator breathe when it is working hard.... I leave the engine cover off the front of the motor and don the ear defenders.

I had experience of another manufacturer's alternator, seemingly rated to 100 A, and it blew in less than an hour, pulling perhaps 1/3 of that.

I have no interest in Ample Power other than saying that the alternator worked for me, and it still works.
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Last edited by Rockter; 02-18-2009 at 09:38 AM.
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Quote:
Originally Posted by sailingdog View Post
Just curious as how you determined he could get two days of use out of 175 amp-hours of bank capacity, since he hasn't said a thing about what his electrical budget is...

Also, with a budget of $1000, getting 150 watts of solar, four batteries and a Honda generator is not going to be possible, unless he steals some of it.


i said normal use, which in his post he said lap top, radar, small tv occasionally. i also expect things like lights radio etc. he did not say fridge or water maker, so his use will be slightly above minimal but not extreme IMHO. if he was using just lights and radio his use would be around 50 amp hours a day from my experience, the tv would not use much becuase as he said it would be occasional. the radar and laptop are the only things above minimal use i saw, and the laptop if a newer one would only draw about 10 amps a day ( based on 2 amps 5 hours a day ).

basicly i said what i said ignoring budget to show what he may need. also the recharge every 2 days thing is a slightly educated guess, based on what he said and what i know.

SD what would you calculate based on lights ( on anchor not running at night ) small tv occasionally, radio and a newer laptop, but no fridge and no watermaker?

edit if people read around lots of people use the walmart batteries, and at 50 % of the cost if they last 3 years it is a break even or slightly ahead, but a savings of about 200 bucks now when he needs the cash

Last edited by scottyt; 02-18-2009 at 12:50 PM.
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Old 02-18-2009
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how does one start a new thread?
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SD was very correct but you must consider the cycles the battery can last. A good deep cycle battery can go fully dead to fully charged about 300 times. Cheaper batteries may only last 100 or less. This is where the cost of amp hours comes in. The batteries may cost less but not last as long.
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Old 02-18-2009
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We went through the math on battery amps and cycles and the best value were 2 volt industrial batteries. The downside, a hugh initial investment and they're big. Our 600 amp bank was over $1000 back in 1999, but they're ten years old and still going. I don't expect to get another ten years out of them though.
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