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Go Back   SailNet Community > On Board > Gear & Maintenance
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Old 03-31-2008
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Battery Ratings

I apologize if my questions dip into the realm of dumb for I am very new to the sailing.

I am trying to determine what battery(s) are needed for my boat. I used West Marine's Electrical Budget Worksheet to create an excel sheet and estimate usage numbers. From that it seems I need somewhere from 85 - 95 AH.

I thought everything was all good until I called a local battery distributor. They say that the RCM is the stat to look at. From what I've read, RCM is the time, in minutes, that a battery will deliver 25 amps. So does that mean that a battery rated at 120RCM would support 20A for 150 minutes? How do I go about figuring my battery needs based on RCM? Or just in general?
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Most deep-cycle batteries are rated in amp-hours (AH) at the 20-hour rate. For example, a "100AH deep-cycle battery" will deliver approximately 5 amps for a 20-hour period before reaching 10.5 volts, at which point it is effectively dead.

As a rule-of-thumb, you should not draw a battery down more than 50% of its rated capacity, thus a 100AH battery really only provides 50AH before reaching 50% depletion. Actually, because of numerous inefficiencies in the way batteries are used -- and recharged -- on boats, it's better to use a more conservative figure, say only 40% of the battery's rated capacity is really usable on a continuing basis.

If you believe that you will need to draw 100 amp-hours from your battery between charges, then you will need 100/.40 or 250AH total capacity. One way to get this kind of capacity rather easily and relatively inexpensively is to use two 6V golf-cart batteries in series. These are rated at about 225AH each, so together in series will give you 225AH total capacity @ 12 volts.

Charging is a very important consideration, too, both in terms of practical solutions and in terms of keeping your batteries healthy. Undercharging is one of the key reasons why batteries fail early, so you'll need some way to replenish the power you take from them efficiently.

Bill
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Forget RCM...high amp draws usually occur on boats ONLY with windlasses and starters and for brief times. You get MUCH less total power out of a deep cycle battery when you put a heavy load on one so the 20 hours rate is what we use in sailing applications as Bill clearly illustrates. I concur with the rest of his post but would also offer some other thoughts.

225 AH's effectively requires you to recharge daily based on your use estimate. 500 amphours would string that out over more than a weekend...so think about how LONG you wish to be self sustaining without a re-charge.
The Golf cart batteries in series may be suitable solution bt te space available in your boat may suggest a different approach.
Group 31 batteries (12V) generally deliver around 100AH's each
Group 4D batteries ...about 155Ah's
Group 8d batteries...about 250
Here are 31/4D and 8D approximate dimensions for reference:
.................................................. ..........31.........4D............8D............. ..........

Length



13.0...
20.75...20.62


inches
Width



6.72...8.75....10.95


inches
Height



9.44...9.88...10.17


inches
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Awesome. Thanks guys.

So if I'm pulling around 18-20Ah, a 100Ah battery would be sufficient for a weekend sailer?
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What do you mean by "pulling around 18-20ah"? Do you mean that, in total, you intend to pull 18-20AH during the entire weekend? If so, 100AH capacity is more than enough.

Better go back to that spreadsheet. AH = draw in amperes X time.

So, a draw of 5 amps for 10 hours total = 50 AH total draw.

Bill
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Quote:
Originally Posted by btrayfors View Post
What do you mean by "pulling around 18-20ah"? Do you mean that, in total, you intend to pull 18-20AH during the entire weekend? If so, 100AH capacity is more than enough.

Better go back to that spreadsheet. AH = draw in amperes X time.

So, a draw of 5 amps for 10 hours total = 50 AH total draw.

Bill
Sorry, that is 18-20Ah/day.
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Yep, that'll work. And, given the size of your vessel, that's very doable.

Be sure to get a good marine battery charger, if you don't have one. One of the most cost-effective alternatives is the Iota line of battery chargers. These are robust, smart chargers which come in various sizes. For a 100AH battery, I'd select one of about 30-amp capacity. Be sure to get the IQ-4 smart charge option, either external or built-in. These chargers use pulse width modulation technology, and do a very good job of prolonging the life of your batteries. You can leave them on all the time when at dockside.

Bill
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Thanks again.

As for charging, the boat is going to be moored. What I was originally planning was to charge the battery with a solar panel. The boat will be stationary during the week and should have 4 - 5 days to charge. Would a 7.5W panel be enough for this? Is there any other option for charging a moored boat other than a generator?
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A moored boat has two choices for charging: solar panel(s) and a wind generator.

To be effective, both need some sort of regulation. You don't want to overcharge the batteries, and you don't want to undercharge them.

The output of both types of charging sources depends on where you are: how much sunlight, how much wind.

For a boat your size, I'd rule out a wind generator. That leaves solar power.

Suggest you seek some advice from those local to your boat.

Good luck,

Bill
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Waugh...I am a bit confused about your amp usage but based on your latest post I assume that you will be using 20 amphours per day and that you therefore might be using 40 amphours for the weekend and you need to find a way to put 40 amp hours back into your batteries during the week on the mooring so you are all set to go from week to week. I further assume that you have an outboard and no way to charge the battery except for the solar. Correct?

Now, you have 5 days to recharge and given some charging losses you need to average about 10 amps a day to get back to 100%...BUT...it will not be sunny all the time so lets say you need to achieve a full charge in 3 days...so you need a panel that will put in 16-18 amphours a day over three sunny days.
Using the 1/3 rule...amphours needed times 3 (minimum) equals your needed panel wattage...so you need a panel (and regulator) on the order of 50 watts to be reasonably sure of a charged up battery on the weekends if my assumptions about your use are correct. A little more wattage would be even better...and a good group31 deep cycle battery should be sufficient as your house battery.
Does this make sense to you?
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