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Old 10-05-2020, 08:37   #1
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How big of a battery bank do I need?

How big of battery bank do I need?

I'm trying to determine how big a bank of batteries I would need to run 24 hours per day, say to cross large bodies of water. Note I also have 350 Watts of solar on a MPPT controller (might even add more).

Here are my power consumptions estimates. What am I missing or how far are these off? Based on these I will consume around 150 amps per day. Does this seem about right?

Equipment 24 hours Draw Comments Alder-Barbour Cold Machine 60 amp ,cabin lights LED 8 amps, Auto Pilot 6 amps (note have monitor wind vane), Bilge Pump 1, doesn't run much, running lights LED 8, 1 Anchor Light 10 amps , Laptop/Camara charting 6 amp, Chart Ploter 18 amps, AIS 8 amps, charging phone 6 1amps . Let's say my draw daily would be 150 amps.
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Old 10-05-2020, 09:31   #2
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Re: How big of a battery bank do I need?

Hi,

It is a budget question. I will assume you have lead batteries, which donít like being discharged more than 50%. If you are using 150a/day, you will need around 300-400 ahrs. assuming you will fully recharge daily.

Now you need a way to fill the battery bank. You have some solar, you probably have an alternator, maybe a little generator, etc. Do the math, and see what you come up with.

Keep in mind it is difficult to charge lead batteries as they begin to fill up because the resistance increases as they are charged. Ex on our boat with a 400 ahr lead bank, our gas generator would put in 65a or so for the first 20 min. After an hour we were down to maybe 20a, even though we had a ways to go to fill the bank. The generator (or engine alt) is good at the beginning, solar is good towards the end to do the final charge over hours.

In MX where the sun is bright, our 600w of solar recharged our batteries to full daily by noon or 1pm, and Iím guessing we pulled about 120a daily. Often half of our panels where in shadows from the mast.

Final advice, donít worry too much and just go for a bit. You have some solar and presumably an engine alt. Try that, and see how it works. If you need more solar, add it later.
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Old 10-05-2020, 09:48   #3
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Re: How big of a battery bank do I need?

TC, that's 150 amp hours per day. It is a not unusual load for a mid sized sailboat.


Based on the correctly mentioned 50%, a 400 ah house bank should be fine for a one night overnight.


Four golf cart batteries or three or four Grp 31 12V would work. Has for me for the past 23 years.


Good luck.
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Old 10-05-2020, 10:17   #4
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Re: How big of a battery bank do I need?

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Originally Posted by Stu Jackson View Post
TC, that's 150 amp hours per day. It is a not unusual load for a mid sized sailboat.


Based on the correctly mentioned 50%, a 400 ah house bank should be fine for a one night overnight.


Four golf cart batteries or three or four Grp 31 12V would work. Has for me for the past 23 years.


Good luck.



Just remember that on a multi-day passage, or off grid at anchor for days at a time, you will not get your batteries back to 100% every day. Lead batteries can be cycled efficiently from 50% to maybe 85%, so if your daily budget is equal to 35% of the nominal capacity of the bank, then you MIGHT get 24 hours average between charging.


Most people don't size their banks by just putting in whatever will give 24 hours or however many hours of budgeted consumption. Mostly we just put in whatever we can fit, and deal with it. My batteries don't give me 24 hours of my normal consumption but I have a heavy duty generator on board so it's more or less tolerable. I'd love to have 36 hours. But is wishes were horses . . .



If you can fit about 400 amp hours of true deep cycle batteries (golf cart batts are the best value), then you MIGHT get roughly a day between charging, with a budget of 150. Budgets of course are budgets -- an essential first planning step, but reality will inevitably vary, and usually not in the favorable direction.


I have 450 amp hours * 24v, so like 900 amp hours @ 12v. It doesn't last me a day, but ours is a pretty electrical-intensive vessel.


I was hoping this year to go to lithium batteries which lets you use practically about 80% of nominal capacity instead of 35% to in the best case 50% like lead. I was going to go with about 350 amp/hours * 24v. Alas the pandemic crisis made me decide to conserve time and money and go with golf cart batts after all.
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Old 10-05-2020, 10:52   #5
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Re: How big of a battery bank do I need?

Most cruisers spend 3 or more days at anchor for every day offshore. If you size the system to be able to spend an indefinite time at anchor, you can always run the motor a bit when on passage.

A good first estimate of a balanced system is 1 solar watt for every amp-hour of lead acid battery. More solar is always better, but 2 watts per amp-hour would be wasted most days.

Laptops can range from 1 to 4 amps of current draw.

I leave the VHF on 24/7, which can use from 6 to 20 amp-hrs per day, depending on model.

For a 45 foot boat with cold machine and hydraulic autopilot, my energy budget was about 80 amp hours at anchor and 160 on passage.
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Old 10-05-2020, 10:55   #6
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Re: How big of a battery bank do I need?

There's no point in a bank much bigger than you can charge in the time you want to spend charging or available charging source..
I'd say roughly 100-120 amps a day use for most cruisers in decent size boats.
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Old 10-05-2020, 11:23   #7
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Re: How big of a battery bank do I need?

Bank size doesn’t really change charge times as much as people think, small banks take 6 hours or so to fully charge, as do bigger ones and 6 hours of usable Solar is unusual and why unless it unusual conditions like being way North in Summer where you may have usable Solar at 6 AM, it’s very unusual that a lead acid bank is fully recharged by noon, most chargers left to their own programming prematurely drop to float, making you think your fully charged when you not quite there yet.

Bigger the bank the longer it will last because it’s being discharged shallower, but they are also heavier and take more room and cost more, but as was said most fit all the batteries they can and live with that as most boats don’t have large battery boxes from the factory.

It’s pretty common for most to have to motor some every day to keep the batteries fully charged, and or run a generator of some sort, so it’s best to accept that as opposed to not getting to full as over time that’s harmful to a bank.

A way to handle it is to motor sail until your Solar can finish off the charging, most people have alternators that will output more than their Solar array, so it’s best to motor in the morning when the bank can accept all the alternator can make, that way you get the most charging you can for the least motor running, I very frequently hear people running their engines in the afternoon because Solar didn’t get them full, and that’s not as efficient as doing so in the morning.
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Old 10-05-2020, 12:09   #8
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Re: How big of a battery bank do I need?

Well, sure. It's about how many amps you take out! But if you have a huge bank, drag it down, and a 40 amp alternator, it's gonna take a month to get it back in!
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Old 10-05-2020, 12:38   #9
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Re: How big of a battery bank do I need?

Good news bad news. Assuming your 350w of solar has an unobstructed view, it will generate around 1750w of energy on average. More in summer, less in winter. For purposes of sizing your batteries, your usage load of 150a is effectively reduced by about a third - more or less the daylight hours will run off solar, not battery. A 300ah lead acid battery is more than enough and you will rarely drop below 60% discharge.

The bad news is you're using more than you produce so you'll have to make up the balance somewhere. Although it looks like a 20ah deficit, it's probably closer to 50ah because there's a point in the day where your batteries are full and cannot accept the watts being generated. Every couple days you'll need to run your engine for a few hours. I would do it at the end of the day when solar has diminished. No less than about 60% dod or so.

Add another panel and another battery and you will be damn close to self sufficient underway if that is a goal (you're probably sustaining your power needs at anchor with electronics and running systems shut down)

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Old 10-05-2020, 13:14   #10
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Re: How big of a battery bank do I need?

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Originally Posted by Cheechako View Post
Well, sure. It's about how many amps you take out! But if you have a huge bank, drag it down, and a 40 amp alternator, it's gonna take a month to get it back in!

A 40 amp alternator is not good for anything.
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Old 10-05-2020, 13:39   #11
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Re: How big of a battery bank do I need?

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Originally Posted by Cheechako View Post
Well, sure. It's about how many amps you take out! But if you have a huge bank, drag it down, and a 40 amp alternator, it's gonna take a month to get it back in!

It doesn’t really matter much, if you take 100 amps out of a 200 AH bank or a 400 AH bank recharge times will be very similar, but the 400 AH bank will last several times as many cycles.
But will weigh twice as much and cost twice as much and take up twice as much room.

Now of course if you take twice as many amps out of twice as big a bank, and have a small charger it’s going to take longer to charge, but if you took the same number of amps out of the little bank it’s not going to last long.

Battery bank is a fuel tank, you don’t put a 10 gl tank On a 300 HP motor and you don’t put a 300 gl tank on a 10 HP motor, so of course you match charge sources with the bank, although other than price you can’t have too large a charge source, bank determines acceptance rate, so if you have a 100 amp alternator and it only charges at 50 amps, well it may run cooler and last longer.

I was just trying to point out the fallacy that I see often and that’s that you size a bank so that it’s 50% discharged by morning when you went to bed with it at 100%.
If you are racing, sure do that, but if your cruising maybe you don’t want to.
You can stress yourself over how much Solar do you need or you can put on as much as you can fit, or afford. Same for a battery bank, you can stress on trying to size it perfectly, or you just put in as much as you can.
In both cases your adding padding or fudge if you will and don’t worry so much what to do in Winter or cloudy days etc.
Then don’t forget seasons either, I have 660 AH and a kilowatt of Solar, In Summer it’s overkill, lots of extra capacity of both, I have way more than enough, but in Dec and Jan, I need to supplement the Solar most days or not quite make it to 100% daily. Huge difference between July and Dec in Solar. Depending on where you live of course, if you live on the Equator maybe not so much.

Then don’t forget that a battery bank is as strong as it’s going to be soon after installation, from then on it dies a slow death, losing capacity along the way. My 5 yr old 660 AH bank is now a 400 AH bank, I got by this year only cause it was a little oversized to begin with, but if I had sized by what many say and so that in the morning I woke up with a SOC in the 50’s, then after it degraded I would be waking up with it much more deeply discharged, and it would have likely died last year or maybe the year before.

So, maybe there is logic in sizing one say 25% to 40% oversized so that when it loses 25% or more capacity, it’s still OK.
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Old 10-05-2020, 13:56   #12
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Re: How big of a battery bank do I need?

A little correction.
A generally accepted rule is that a Solar array will on average make 1/3 of its rated watt capacity in AH.
Yes that’s mixing units, but what it’s saying is that on average I can get 333 AH out of my 1,000W array.
I use 150 AH overnight, so at first look it looks I will make it easily, but don’t forget that a battery is not 100% efficient, there is no free lunch, so that means that on average you will have it return 106 to 110% of the AH you took out to fully recharge a battery. Plus it takes 6 hours, if you don’t have 6 hours of good Solar cause it’s Winter, then it doesn’t matter how many Solar panels you have, you won’t quite make it to full charge.
Battery acceptance rates start out very high, meaning when a battery is at its lowest charge, it will accept its highest charge, my bank will take 185 amps for 20 min for example at the beginning but the last several hours it’s down to 15 or less slowly tapering down to only 3.3 amps, that’s when it’s fully charged.
Usually your biggest charge source is the alternator or a shorepower charge / generator, so don’t run them when the bank will only accept 10 amps or less, run them when it will accept all they can make, which is first thing in the morning. Cause you can get a lot more in the bank in a much shorter time that way.
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Old 10-05-2020, 14:05   #13
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Re: How big of a battery bank do I need?

150 amps (assuming 12 volts) is over estimation running all of that but anyway...

If you want 150 ah a day then 200 watts solar is enough on sunny days. In overcast conditions you will need 8-10x more solar to ensure you have the power, or about 1800 watts solar. In this way you won't need much of a battery bank; I have 40ah and 650 watts solar. This is the system I use: enough overcast solar to power everything. When the sun is out you can use a lot of power on stuff you don't really need every day like filling tanks from watermaker.

A larger battery bank will allow you to get by on fewer solar panels but eventually you will run out with enough cloudy days in a row. So I suggest 300ah battery and 600 watts of solar for you as a starting point.

If you use lead acid batteries, double the capacity to 600ah or realize you only actually will use 75ah a day and divide everything by 2.
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Old 10-05-2020, 14:49   #14
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Re: How big of a battery bank do I need?

when you guys are talking about 6v golf cart batteries.....what kind are you talking about? what type of lead acid batterys and amp hours are you using....because I need to upgrade

4 costco 125 amp hour cart batteries in series and in parallel will equal 250 aph at 12v

what can one expect from a honda e2000 for generating amp hours....
sorry for interupting
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Old 10-05-2020, 14:53   #15
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Re: How big of a battery bank do I need?

To the OP: frankly, I can't follow half of what's being said here. I've designed and installed several off grid solar systems and many of the statements here are new to me. Yes, you should not discharge your batteries below 50% but no one seems to be able to figure out how to factor in your solar charging.

Keel it simple. You use more than you replenish - adding batteries is kicking the can down the road, but it will be harder and take longer to charge a large battery bank with a small alternator. Get your solar running and take it from there. Get a decent state of charge meter and try to keep it above 60 % for your lead acid batteries.
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