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Old 03-09-2013, 22:46   #1
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Battery Bank Size?

Hi guys,
Looking at doing a lot of cruising shortly, including Trans Atlantic and Pacific crossings.
I have a 45ft boat with auto helm, galley fridge/freezer and portable fridge, stereo, twin chartplotters etc, so bit of demand on the Electrics.
My question is what size Battery Bank is suitable for long cruising? I have 320ah bank right now but contemplating adding to this for the Trip as dont think thats enough?
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Old 04-09-2013, 06:26   #2
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Hi. Just picked up on your thread and I have set up 4 different yachts now, my latest one a 50ft cat. The only true way of working out your supply and demand is put an amp meter in line to work out how much you are using over a 24hour period. Then you need enough amps to last 3 days at a very bear minimum. That'll give you time to fix something at sea and still give you frozen food and autopilot, the essentials. The next thing you have to look at is how you are going to charge them. With a catamaran we have large surface areas, so we have used this for solar panels. But still have a D400 wind generator and a old diesel genset which we try not to use. We run 2 freezers, 1 fridge, extensive electronic package and a water maker. So we combined all of this to work out how many amps we needed from solar and wind to keep our batteries charged. But the best thing you can do is put in an amp meter to work out your usage and then go from there.
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Old 04-09-2013, 07:09   #3
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Re: Battery Bank Size?

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Originally Posted by Falshator View Post
Hi guys,
My question is what size Battery Bank is suitable for long cruising? I have 320ah bank right now but contemplating adding to this for the Trip as dont think thats enough?

I think if you do an energy plan you are going to find you are going to use 200-250AH/day underway. Normal guidelines are that your battery bank be 4 times your daily use, which really means that if you are only able to maintain the batteries between 50-75% you are charging each day.

But the bank can be reduced if you add solar panels etc, then you can base the bank on the smaller AH/day net lost. As an example if you had 400W solar for about 120AH/day you would be at 125 AH/day lost and you could get by with a saller bank.

And when at anchor that 400W of solar will probably meet your loads (does on my boat).
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Old 04-09-2013, 07:19   #4
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Re: Battery Bank Size?

The first question is how are you going to replace the energy.
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Old 04-09-2013, 07:49   #5
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Re: Battery Bank Size?

I'll second Noelex's comment. Until you tell us what your charging capability is underway, no one can give you an intelligent answer.

David
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Old 04-09-2013, 08:01   #6
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Re: Battery Bank Size?

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I'll second Noelex's comment. Until you tell us what your charging capability is underway, no one can give you an intelligent answer.

David
I disagree, but ain't no expert.

To me you go about it in the following order:
1 - daily AH demand
2 - battery capacity needed for #1 based on how often you want to have to charge
3- charging to support #2
4 -what is needed to support #3

If you don't like the answers then you look at ways to change #2 and #3 (because who really wants to reduce #1). So if you add solar or wind you start over knowing this.

One could cruise around on a 200AH bank with a load of 200AH/day and just a main engine if you are willing to start the motor 4 times a day.
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Old 04-09-2013, 08:06   #7
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Re: Battery Bank Size?

Battery bank size is linked to your ability/capacity to make energy. If you have a genset onboard, some solar panels and a windgen, 300-400 bank of deep cycles might be enough. But if you cannot charge daily, you may want a much bigger bank.

You must add up all the daily loads and then decide on how often you can realistically charge the bank. And that's that.

Remember to add some margins for charge efficiency and quick discharge. Etc.

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Old 04-09-2013, 08:16   #8
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Re: Battery Bank Size?

When working out your charging capacity remember your alt specs are often quoted max charge cold. Actual charge cold is MUCH lower in all real life situations. Use quality, external regulators to get the max out of your alternator capacity.

Sun - you can count on 4-5 sun-panel-hours on a sunny day. (on the easy route) Maybe 30-40% days will be sunny.

Wind - you must correct for wind loss when running, many windgens make nothing below 5kts apparent, very little below 10kts apparent. I think you can count on at least 50% of days with less than 10kts apparent. (again, on the easy route)

I think it is smart to carry a spare alternator. Fixing your while at sea can be a hell of a job. Swapping units is much easier.

Using energy efficient devices (LED lights, efficient fridges, well sized and tuned APs, etc.) goes heaps towards saving bucks on batteries and charging equipment.

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Old 04-09-2013, 22:10   #9
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Re: Battery Bank Size?

At present, I have a 115 amp alternator and are setting up combined 290 watt Solar which I have worked out should give me around 120amps a day.
Allowing on the high side, worked out I will be using 226 ah per 24 hr period. Going to run the water maker off the engine a couple of hours a day so not sure how many amps will come off the Alternator for a couple of hours of running but hoping it will top up what the solar is not replacing. Take your point about having enough for 4 days in case of repairs. If thats the case, I will be looking at 800ah Battery Bank. Thats a lot of Batteries or is that the norm for long distant cruising?
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Old 04-09-2013, 22:15   #10
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Re: Battery Bank Size?

PS - not looking ideally at a wind generator as the majority of the trip will be running with or reaching so very little wind horsepower for the Wind generator. Have looked at the Duogen tow generator, but hoping with the solar and alternator a couple of hours a day, I wont need it?
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Old 04-09-2013, 23:58   #11
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Re: Battery Bank Size?

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Originally Posted by Falshator View Post
At present, I have a 115 amp alternator and are setting up combined 290 watt Solar which I have worked out should give me around 120amps a day.
Allowing on the high side, worked out I will be using 226 ah per 24 hr period. Going to run the water maker off the engine a couple of hours a day so not sure how many amps will come off the Alternator for a couple of hours of running but hoping it will top up what the solar is not replacing. Take your point about having enough for 4 days in case of repairs. If thats the case, I will be looking at 800ah Battery Bank. Thats a lot of Batteries or is that the norm for long distant cruising?

800 ah is a nice round figure for you to aim at. Ideally the less % use of your bank...you are talking about 25% the more life cycles you can expect to get than sucking the bank down 50 or more %.

Thats the size bank I have now with a 200amp alt and 600W solar. The result is my batteries are topped up most days by lunchtime by solar, never having to run the motor. It may seem overkill to some here but I never have to worry about power use at all.

I struggled all across the Pacific with battery issues...my old bank + 200amp alt. I've learned now how important cable sizing is!!
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Old 05-09-2013, 01:48   #12
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Re: Battery Bank Size?

Lots and lots of batts and solar or a small gen set and lots of batts and solar ! Both of these together will power most any boat !!
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Old 05-09-2013, 02:11   #13
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Re: Battery Bank Size?

We've done just fine the last three years cruising in mixed weather conditions from the Caribbean to Alaska to the South Pacific. Our typical consumption is around 100Ah day at anchor and 200Ah underway.

Since we sail a performance cruising catamaran and have been cruising for a total of 8 years, both cost and weight matter. Thus, we find the 400Ah capacity stored in (4) led acid 6 volt golf cart style batteries a good compromise as they are modular and easy to replace anywhere in the world should a problem arise. ) 540 watts solar usually does the trick, but during extensive cloudy periods we run a Honda 2000, although a Honda 1000 can run a 50 amp charger just fine and weighs a lot less. As a back up we have twin Yanmar 3ym30's fitted with stock Hitachi 80 amp alternators managed by Balmar charge controllers.

Before adding expensive battery and charging solutions look to reduce energy consumption first.
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Old 05-09-2013, 03:11   #14
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Re: Battery Bank Size?

There's no scientific way to calculate how much battery capacity you need. How long is a piece of string? The bigger the better, since a bigger bank will accept more energy in a given amount of time, will lose less energy to Peukert effects at any given rate of discharge, and obviously will store more power. A bigger bank will last longer because you won't need to discharge it so deeply. But bigger is heavier and more expensive.

If your present bank is nominally 320 amp/hours at 12v, that sounds a bit small to me. My consumption is probably not that much greater than yours, but I have separate fridge and freezer, and I use a fair amount of inverted power for various things. At my latitude (50+) I also have to heat a lot, and the central heating system uses electrical power for fan coils and pumps.

My bank is 420 amp/hours * 24 volts, so like 840 at 12 volts, so more than 2 1/2 times bigger than yours. I would not want anything smaller. I can get away with one generator run a day in the summer with no heating going on, but other seasons I can't usually quite make it on one. Although I have a quiet heavy-duty diesel generator, you really don't want it running all the time -- kind of spoils the point of being in beautiful, remote places, doesn't it?

If you do start to count amps and daily consumption and so forth, keep in mind that you will get a lot less real power out of your battery bank, than its nominal rating.

First of all, if you are charging with a generator, you will mostly be using the range from 50% charged (you don't want to go below that) and 80% - 90% charged (takes inordinately long to get above 85% or so, so not worth while with a generator, usually), so that's only 30% to 40% of the total theoretical capacity of the bank which you actually get to use.

Second, rated battery capacity and realistic battery capacity are two different things. Batteries are consumables and gradually lose capacity over their lives. So you will not get anything like the full rated capacity of your batts for very long.

Solar is wonderful because it works silently whenever the sun is shining -- you never shut it down. It works great for that last 10% or 20% which are impractical to charge with a generator. Solar punches above its weight because of this -- has a lot more practical effect than you would think from the watts rating.
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Old 05-09-2013, 04:00   #15
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Re: Battery Bank Size?

The extra information means we can do some some electrical modelling on your system.
Its not clear if your watermaker is belt driven off the engine or electrically driven using the output of the alternator. I have assumed the latter, but this makes a difference to your required battery bank size as if it is belt driven much more of the alternator output is available and a larger bank is needed to absorb this power.

I have made a few assumptions
1. Your 115A alternator will put out about 90A continuous ( this assumes reasonable engine revs)
2. 290w of solar will give you 100AHrs. This assumes reasonably good conditions
3. The watermaker will consume about 45A

If we consider what would happen with various battery bank sizes

1. An infinitely large battery bank size:
So each day after solar you will need to make up 126AHrs from the engine alternator. During the 2 hour watermaker run you will be replacing 45A giving you 90AHrs. The remaining 36AHrs will be replaced at 90A, so will take just over 1/2 Hr.

So infinitely large battery bank you will require 2 1/2 hrs engine run time a day. You can go as many days as you want without an engine run, but every missed day will add 2 1/2 hrs run time. ( running the engine every second day would be 5 hour run time, every third day would be 7 1/2 hr run time etc)

2. Smallest possible batter bank (425Ahrs see below)
You don't want to run the batteries below 50%. 90% is likely to be your normal highest SOC. If you run the engine every day after the solar input your maximum deficit is likely to be 170AHrs on the worse solar days a minimum battery bank of 425 AHrs will be needed to keep the batteries above 50%.
A battery bank this small will probably just accept the 90A and 45A charge rate, so the engine run time will not change.

3. Compromise (850 AHrs)
If we size the battery bank so that the engine can be run every second if needed it allows use of the engine power during a long motor.
To go 1 day without running the engine requires a battery bank of 850Ahrs, but the run time is now 5 hours.


Conclusion.
I think the analysis shows what many would have guessed without the maths, that a battery bank of about 850 AHrs is optimum. There is little point going larger unless you anticipate times when when you will be motoring for more than 5 hours. The 850AHr battery bank (or even larger) has very little impact on your engine run time compared to the smallest acceptable battery bank of 425 AHrs.

The biggest problem I can see is the long engine run time you will have 2 1/2 hrs is a long time to put up with the main engine and this assumes a reasonably high output that will require above idle engine speed. Unfortunatly a larger battery bank has no impact on this.
I think you may find this long engine run time frustrating and would look at ways to reduce this, which really involves more/better sources of power generation combined with energy saving measures and a larger (or an energy efficient) watermaker.
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