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Old 22-09-2010, 12:47   #1
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240 Watts Solar - How Big Battery ?

Hi

I have 240 watts of solar in 2 panels.

I need new batteries at some point in the next 12 months.

Could I have your advice on this problem both technically and in your experience practically:

If I only ever drop the batteries to 50% what is the most amp hours I could have in a house bank

Given: The boat will be in the tropics

Given: All the losses etc in reality.


(You will note I have not said how many amp hours I would be/ or do use per day. Thats not the question)


Thanks guys (and dolls)



Mark
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Old 22-09-2010, 12:59   #2
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I have 2x 135 watt panels (270 watts) showing up today. Trojan 6v batteries; I'm sizing at 450AH (house side). It's the first time I've ever actually sized out an electrical system so I'm hoping I get better results than just randomly jamming batteries in and adding panels sans controllers.
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Old 22-09-2010, 13:05   #3
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I have two 130-watt panels (plus a 200-watt wind generator) running into two 4D AGM house batteries rated at 210 AH apiece. That gives me plenty of storage.
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Old 22-09-2010, 13:22   #4
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Along the same lines, I'm also looking at capacity planning. I currently have a couple of group 31 AGM's. I was looking at upgrading to 4D or even an 8D. I've seen a lot of other's doing like Rebel Heart and going with 6V batteries in a series.

Anyone know the pros/con for mult. 12v in parrallel, a larger 12v, or 6v in a series?

BTW- I'm hoping this is inline with the thread and didn't intend on hijacking it. My apologies if you feel I'm trying to hijack the thread Mark.
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Old 22-09-2010, 13:49   #5
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You don’t make it easy Mark
The required battery capacity is partially dependant on the amp hours used, but as this is classified information (secret men’s business) I will try to give an answer anyway.

240w of solar in the tropics will give you about 90Ahrs to play with when at anchor. You cannot use more than this or your batteries will go flat no matter how large their capacity.
Battery capacity will only allow you to even out the high and low output days.
If the solar panel output is consistent (like the Med in summer) you can get by with a very small battery capacity. A single 200AHr capacity battery would be plenty.
On the other hand take somewhere like the Whitsunday’s with its sunny and cloudy days and a larger battery capacity is needed to even out the highs and lows. Somewhere around 500-600 AHrs would be much better.
As a compromise 400AHr would be a good solution.

Note I have ignored the output from the alternator, but this will usually nicely compensate for the higher consumption when sailing (to run GPS chart plotter and nav lights)
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Old 22-09-2010, 13:54   #6
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What contorller are you going to use??
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Old 22-09-2010, 14:06   #7
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Quote:
Originally Posted by Shrew View Post

Anyone know the pros/con for mult. 12v in parrallel, a larger 12v, or 6v in a series?
Not much practical difference. A lot (but not all, I use Sonencen solar bloc batteries which come in 12v form) of good deep cycle batteries are only available in 6V or 2v form hence the popularity of using these lower voltages in series.
Decide the battery you would like and it really doesn’t matter if its 12v, 6v or 2v it only need slightly different cabling to make up the required 12v bank. More important is the battery quality verses price.

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Old 22-09-2010, 14:46   #8
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To me you should have a min 400 AH bank. This allows to use the 25% of capacity rule and rounding the hoped for 90AH from the panels/day. Seems everyone has the same answer using various methods to get there.
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Old 22-09-2010, 14:48   #9
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What contorller are you going to use??
Dunno yet, Rick. None at the moment but I will get one. A low cost one.

Thanks John, your info is always great


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Old 22-09-2010, 15:06   #10
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Quote:
Originally Posted by noelex 77 View Post
240w of solar in the tropics will give you about 90Ahrs to play with when at anchor. You cannot use
I don't think in real life you'll get anywhere near that many amp hours. I know you are in the tropics, but around here you would get less than 50 Amp hours. (On a good day)
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Old 22-09-2010, 15:10   #11
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Considering a low cost Charge controller is not the best option. It is the Charge controller who will make the difference in your daily production.
240 W of panels will produce as an average, each day :
240 (Watt/h of you 12V panels
X 0,8 (for cable losses and heat)
= 192 W/H
X ? 5 hours a day of peak sun = 960 W/H per day .
* The number of hours of peak sun on a boat is very difficult to evaluate:
1/ If you are in a marina and your panels are not directly facing the sun, you will produce very little
2/ If part of your panels are shaded, you could loose up to 75 % of your panel capacity, depending on the type of panels, ....and the type of charge controler used.

960 W/H per day in 12V is the equivalent of 80 Amps . (960 divided by 12)

This means you should produce 80 Amp of current each sunny day.

If you plan on using these 80 amps each day, and you want to discharge your batteries at a maximum of 50% of their capacity (25% is too low , Deep cycle batteries can easily handle 50%) ,plus you want to have a minimum of 2 days autonomy, this means you need to have a battery bank of :

80 divide by 0,5 = 160 Amp/h
2 days = 160 X 2 = 320 Ah...... in 12V

The best option for you will be a set of 2 real good quality 6V battery of 350 Ah.

This is not the correct place to advertise but if you need any additional information you can send me a message with your email address and I will give you more details.

Best
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Old 22-09-2010, 15:46   #12
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G'day Mark,
Making beer is easy & works every time. Making electricity from solar not so easy. The first point is boats require compromises by their very being. So if you like most of us have flat panels then you don't get all the 240watts because of the sun angles involved. So work on 5hrs @ 140watts & it will be closer. Think of batteries like a dam collecting water & work out what is good for you. 12 mths is a good lead time to do that!! Who can tell LIFEPO4's might be the answer in that time. Also LIFEPO4's use BMS & the regulator would not be needed.
(ps I sent a email to Nic to contact us since she is in Bris)
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Old 22-09-2010, 15:54   #13
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We have 480W of solar in 4 panels mounted horizontally on the bimini using a regular PWM controller. Unshaded most of the time. In the South Caribbean here, we see 140-150Ah/day on sunny days and ~100-120Ah on partially cloudy days. I would assume you would see half of this output using your panels. You might see a bit more at your higher latitudes during the summer. We used to get 160-180Ah/day in the Northeast US during the summer (longer days, cooler temps).

You should use your expected Ah/day as your 50% battery capacity number. I would fudge it lower to take into account partially cloudy days so that you easily make the recharge each sunny day. You will need an alternate energy source (alternator?) for those cloudy days.

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Old 22-09-2010, 16:08   #14
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Quote:
Originally Posted by PamlicoTraveler View Post
I don't think in real life you'll get anywhere near that many amp hours. I know you are in the tropics, but around here you would get less than 50 Amp hours. (On a good day)
You are certainly correct in that the output from solar panels varies enormously depending on location and time of year, but 20 AHrs is very low for 240w (@ 12v).
And is not typical of the output in tropical conditions.
I see you have edited the output in your post to less than 50AHrs a day. This is still a bit below what would be typical for tropical conditions, but this sort of output is certainly possible for some tropical locations, particularly with some shading and a non MPPT regulator.
Unfortunately solar power production and electricity usage has a high standard deviation based on many variables- hence my vague answer in battery capacity from 200 to 600 AHrs


I think overall the answers are converging on a recomendation of around 400AHrs
which I agree is about right.

I want to make sure Mark has enough electricity to offer me a cold beer when we finally meet up.
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Old 22-09-2010, 16:15   #15
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There's some good info in this thread, but in my humble opinion, I would approach the problem differently. As Noelexx 77 suggests, the size of the battery bank is more closely related to your daily consumption (in amp/hrs) than the size of your solar array. Imagine a cloudy 48 hour period in which you barely produce any solar power. How many amp/hrs will you consume from your batteries using lights, pumps, electronics, etc? Go ahead and take inventory, and add it up. Now install a battery bank twice that size (or a bit larger). Once you install your new batteries, they won't care how you recharge them, whether with your alternator or your solar panels (actually, that's not quite true - if you're going to use the alternator, then do so when the batteries are low and the alternator charge controller will charge in bulk mode, then top your batteries off with your panels). Anyway, the point is that your solar panels will merely replace what you consume, so unless we're talking about a ridiculously small or large bank, they don't care how much untapped battery capacity you have. Size your battery bank for the times when the sun ain't shining. Here's wishing you lots of sun!

FWIW, our 216 watt array with an MMPT controller will throw 12.5 amps for about 6 hrs a day in the Bahamas in winter, and about 7-8 hours in Maine in early summer (plus additional charging, at proportionally lower output, in early morning or evening). That's with clear skies, so you can figure the effects of weather based on your locale. Shading isn't much of a factor unless sailing at an angle that puts the main between the sun and the panels.
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