Battery Bank Size?

[Dockhead]

Originally Posted by noelex 77

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.

This is all very good theory, but . . .

sizing of the battery bank is not actually a function of the capacity of charging sources. They are really kind of separate problems, and if anything, I think one would be more likely to make changes to charging sources to suit possible battery bank size, and not the other way around.

In my opinion, the battery bank should be as big as reasonably possible and practical. Other than cost and weight, a larger bank is all good. I don't think there is an "optimum" size, other than bigger.

If your charging sources can't put in an efficient amount of power (at least 10%, better 15% of C), then they should be increased. It's relatively cheap and simple to install a larger alternator, or better, a second alternator. A large-frame school bus alternator is a tremendously valuable charging source, and they come in very large sizes, large enough to be 10% of any imaginable battery bank (actually, since main engine alternators often work at speeds where they produce less than all of their output, you generally want to size them bigger than 10% of C). Of course you have to take care with the belt drives (double belts are required on the bigger ones) and might have to change some pulleys on the engine.

If you don't have an alternator that can put out enough power to feed a battery bank which you could otherwise afford in terms of weight and cost, then you should change it. That means it's a bottleneck in your system. 5 hours a day of running your main engine is ridiculous, in my opinion. That would not be pleasant cruising. That's just a too small alternator.

To give you an example from my usage -- I have a 110 amps (* 24v) school bus alternator, which is theoretically more than 25% of C for me. If I run the engine at a fast idle, I reckon I'm getting 80 amps (has a very steep charge curve -- made for school busses ), which is a bit less than 20%. And indeed, as the theory would suggest, I can get my batts from 50% to 80% in less than two hours of fast idling, whereupon I have about 100 amp/hours of juice to play with until the next chargin session. I stop at about 80% when I'm off-grid, because the batteries start accepting less and less power, so run time versus stored power falls off.

Remember it takes a combination of charging power and bank capacity to store a given amount power in a given amount of time -- you are limited by the lesser of acceptance rate of your bank or the charging capacity of your charging source. So to reduce your charging sessions to tolerable periods of time -- and I suggest that 5 hours is way too much, especially if you're using the main engine -- you need to consider both of these factors.

If you're using a generator, there is another factor to consider -- charging batteries can be combined with other activities which require the use of the generator. Hence a lot of cruisers will try to group together their AC power tasks and do it all at once, and the batteries are getting charged incidentally at the same time. This improves the efficiency of the whole process, and also, if you have a lengthy job requiring AC power, like washing and drying clothes, you can take the chance to get your batts above 80% for once, which they need from time to time for proper health. Obviously you also need a large enough battery charger to use your generator runs effectively -- better between 15% and 20% of C (I have 17%). And a large enough generator to run that charger. My charger at 70 amps and 24 volts nominal, needs about 2.5kW of input power when it's charging at maximum rate.

My charger takes about two hours to get the batts to 80% from 50%; then another hour to get them from 80% to absorption phase, 85% or 90%.

[Stu Jackson]

It doesn't matter how big your house bank is (excepting an unusually tiny one) because the amount of time required to recharge is exactly the same based on how much you take out!

Yes, yes, let's not quibble here about battery acceptance making the last 15% the hardest to recharge and all that...

If you take out 125 ah every day, you simply have to put it back, whether your bank is 800 ah or 200 ah.

I see these discussions, over and over, and keep wondering what makes this seem so difficult for some to seem to understand. Dockhead explained it very well, and I'll bet it's not the first time he's had to type this material.

Take it out, put it back. Ways to put it back: alternator, generator, solar, wind, trailing spinning log, crew pedaling on bicycles...

[noelex 77]

Originally Posted by Dockhead

In my opinion, the battery bank should be as big as reasonably possible and practical. Other than cost and weight, a larger bank is all good. I don't think there is an "optimum" size, other than bigger.

If we leave out cost weight and size sure the optimum battery size is infinite, but these factors cannot be ignored.

It is possible to calculate the relative benefits of a larger battery bank. Generator run time, maximum days between runs, battery life,cost per year etc can be predicted with a reasonably degree of accuracy.
This is the best way to make an informed decision about bank size.

With boats that get most of their power from a generator the maths falls in favour of a very large bank. Larger than most boats can physically fit.

With boats that get most of there power from solar/wind the maths produces very different results. In many cases a larger battery banks makes the battery costs per year significantly higher while adding weight and reducing storage volume with little gain apart from a slightly longer period before changing batteries. Given the nuisance of changing more batteries this is of dubious value.

Boats that rely on engine charging fall somewhere in between and of course most boats use a multitude of changing sources.

As a general rule of thumb most generator equipped boats fit battery banks that are too small and most boats with large solar arrays fit battery banks that are too big.

[Fuss]

Originally Posted by Falshator

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?

As a very rough guide as my boat is only slightly bigger.

I have 24v...625a batteries

I use sometimes between 150 and 180a in 24hrs maybe less

I put it back with .....
405w solar, in the med its around 90a daily in summer.
110a alternator and 55a alternator about 30 mins a day or 2nd day (motoring into or out of anchorage).
propshaft alternator when sailing faster than maybe 4.5kts

sometimes I run my 6.5kw generator with 2 x 75a battery chargers, but normally this is not in use. Feel like removing it sometimes, but a good backup I suppose.

Would be much better if fridge and freezer were more efficient
no gas on board only electric cooking.

Planning 400a of Lifepo4 batteries next time round, maybe in more than a few years, when I do this I will replace the 110a alternator with a 150a model which is onboard as a spare at the moment.

[Dockhead]

Originally Posted by noelex 77

If we leave out cost weight and size sure the optimum battery size is infinite, but these factors cannot be ignored.

It is possible to calculate the relative benefits of a larger battery bank. Generator run time, maximum days between runs, battery life,cost per year etc can be predicted with a reasonably degree of accuracy.
This is the best way to make an informed decision about bank size.

With boats that get most of their power from a generator the maths falls in favour of a very large bank. Larger than most boats can physically fit.

With boats that get most of there power from solar/wind the maths produces very different results. In many cases a larger battery banks makes the battery costs per year significantly higher while adding weight and reducing storage volume with little gain apart from a slightly longer period before changing batteries. Given the nuisance of changing more batteries this is of dubious value.

Boats that rely on engine charging fall somewhere in between and of course most boats use a multitude of changing sources.

As a general rule of thumb most generator equipped boats fit battery banks that are too small and most boats with large solar arrays fit battery banks that are too big.

I doubt that we are really disagreeing on anything significant.

Naturally you are right that a battery bank can be too big at some point, and my previous post probably muddled that point. I think a too big bank would be quite rare on a cruising sailboat, but it is theoretically possible.

If you have so much solar that you reliably produce more per day than you consume, then of course you're right -- you only need enough battery capacity to get you through the night, although, although -- a bit more, like double or triple, will help you on cloudy days . . .

For main engine charging, a battery bank bigger than what is necessary to have enough charge acceptance to take all current the alternator can give, and bigger than what is needed to store what you need for the period you want to run off your batts, will be wasted. Although, although -- alternators can (and often should) be replaced with bigger ones, and even if an increase in bank size doesn't give you more usable charge acceptance, it still gives you more storage capacity with a shallower discharge . . . and will run heavy loads more efficiently with less Peukert . . .

As to battery cost per year -- a bigger bank will always last longer than a smaller one, all other things being equal, because for the same amount of power consumption, you don't need to discharge them as deeply, and the same rate of charging will be a gentler % of C, both of which extends the life of the batts, in some cases dramatically. Also, you can keep using a bigger battery bank later in the batteries' life cycle, because 50% of a battery bank which is bigger than you theoretically need may still be enough to run your boat, whereas you might need to replace a smaller battery bank, sized to theoretical perfection, when it has lost only 20% or 30% of its capacity (or whatever). So you will get more years out of a bigger bank, all other things being equal, and the difference in battery cost per year will in practice be far less than in theory, and in many cases may not be different at all, and it's even conceivable that a bigger battery bank can be actually cheaper, for some people, than a smaller one, per year.


So in theory, everything you say is exactly right; in practice, however, a bank which is a somewhat bigger than what you theoretically need is almost always better, provided of course you can afford the weight, bulk, and cost.

[Dockhead]

Originally Posted by Fuss

As a very rough guide as my boat is only slightly bigger.

I have 24v...625a batteries

I use sometimes between 150 and 180a in 24hrs maybe less

I put it back with .....
405w solar, in the med its around 90a daily in summer.
110a alternator and 55a alternator about 30 mins a day or 2nd day (motoring into or out of anchorage).
propshaft alternator when sailing faster than maybe 4.5kts

sometimes I run my 6.5kw generator with 2 x 75a battery chargers, but normally this is not in use. Feel like removing it sometimes, but a good backup I suppose.

Would be much better if fridge and freezer were more efficient
no gas on board only electric cooking.

Planning 400a of Lifepo4 batteries next time round, maybe in more than a few years, when I do this I will replace the 110a alternator with a 150a model which is onboard as a spare at the moment.

WOW, that's some setup

In theory, everything you have is grossly oversized. Did you design that system, or did you inherit it from a PO?

On the other hand, everything is sized with enough reserve that you just don't have to worry about power, isn't that right? You don't need your generator much, but it's nice to have it there, isn't it? You don't need to worry about where the power will come from if you feel like firing up the electric kettle.

I don't actually think there's anything wrong with a system like that -- which will be very robust with big margins of error everywhere and lots of redundancy -- other than that it will have cost a good bit more than one designed to the theoretically "optimum" sizes.

[Fuss]

Further to what I just posted.

The main thing is to have everything in balance so that you are not constantly monitoring everything.

If I put some tape over my battery monitor and for the next 2 weeks cruised around, anchored here and there , motored out for 15 mins, motored in for 15 mins each day, maybe stayed 3 days in one place at anchor, then two weeks later I took the tape off, I would expect the batteries to be no worse than 80% full.

Thats what you need to aim for, a balanced system that fits your style of cruising. Without your motor being on just because you are needing your batteries to be fuller.

[Sandero]

The answer is there is no right answer... there are many depending on how much power you consume and how you replenish it to the batteries.

Obviously you need to set up a use budget for different conditions... and probably the most demanding condition will determine the bank's storage capacity.

You then have to add the charging sources into your budget again under different conditions. This should give you a matrix of possibilities... for bank size.

The changing options are as important as the actual bank size.. they include:

high output alternators driven by a smart regulator
generator
solar (regulated)
wind (regulated)

The later provide more or less free energy, the former require the use of fuel.

When I went through this exercise years ago for my 36' boat I decided that I would run the diesel at least one or more hours per day to charge batts, run the inverter at the same time to use some AC devices such as vacuum... generator hot water, and cool the refer which was engine drive. I often ran the engine during my SSB radio sched because it sucked a lot of juice and the alternator would meet that demand. Of course running the engine also added to (in most cases) boat speed.

It would be wonderful to NOT have to use the engine and rely on free energy. But for MY needs., hot water and refrigeration this was not possible. I could have used a 12v refer but that would increase the electric budget.. which would mean perhaps running the engine in any case. My cabin heater runs on electric and diesel.

For long passages the amount of stored fuel becomes an issue and aside from battery capacity fuel capacity enters into the battery charging puzzle.

My approach worked fine for the water I cruised in and the passages (back and forth between NE and Caribbean) I made. I don't stay is slips or overnight at docks and anchor (all chain w/ electric windlass). Anchoring is done under power so alternator is charging and windlass is not taxing the batts. Engine is 28 (4,600 hrs) yrs old and runs fine with regular maintenance and one major top end rebuild.

My engine is 36hp w/ 100 amp alt, maxcharge 614 reg
fuel capacity 40 gal (plus 20gal or more in jerry jugs on passage)
110 watts solar
house batts - 2 8D AGM
start batt - optima blue top
Link20 monitor

The OP's deal is likely completely different from mine and every other sailor's solution/ Take em with a grain of sea salt. YMMV

[Fuss]

Originally Posted by Dockhead

WOW, that's some setup

In theory, everything you have is grossly oversized. Did you design that system, or did you inherit it from a PO?

On the other hand, everything is sized with enough reserve that you just don't have to worry about power, isn't that right? You don't need your generator much, but it's nice to have it there, isn't it? You don't need to worry about where the power will come from if you feel like firing up the electric kettle.

I don't actually think there's anything wrong with a system like that -- which will be very robust with big margins of error everywhere and lots of redundancy -- other than that it will have cost a good bit more than one designed to the theoretically "optimum" sizes.

When I bought it, the setup was...
24v...625a batteries
no solar
70a alternator
no propshaft alternator

6.5kw generator with 1 x 80a battery charger

same fridge and freezer
halogen and normal bulbs everywhere
no gas on board only electric cooking.

With this setup, I needed to run the generator every day for 1 hour in the morning and 1 hour around evening time. This was then sortof filling the batteries back up with some engine motoring as well.

Daily life always had a focus around how full the batteries were, which is not want you want to be doing.
Thats why I changed it to the point where it all looks after its self.

The battery monitor is now mostly used to show if something is not working or if something has been left on that shouldnt be.

It needs around 150a of alternators, I originally tried 110a but its better with a total of 165a.

[Dockhead]

Originally Posted by Fuss

Further to what I just posted.

The main thing is to have everything in balance so that you are not constantly monitoring everything.

If I put some tape over my battery monitor and for the next 2 weeks cruised around, anchored here and there , motored out for 15 mins, motored in for 15 mins each day, maybe stayed 3 days in one place at anchor, then two weeks later I took the tape off, I would expect the batteries to be no worse than 80% full.

Thats what you need to aim for, a balanced system that fits your style of cruising. Without your motor being on just because you are needing your batteries to be fuller.

Yes, I think this is an extremely important point.

If you design a system which is balanced on a knife's edge, you have to manage it intensely and you end up worrying about power. And if something doesn't go to plan, or you lose some battery capacity, or whatever, it starts not working right.

Whereas if you have plenty of margins and redundancy, you just don't need to think about it. Which means you can do the stuff you went out to go cruising for, rather than being a full time maintenance engineer.

I mentioned on another thread that my Victron battery monitor died over the winter. I just used voltage this year to roughly judge battery state, and stopped counting amps and obsessing about the state of the batteries. I left the inverter going 24/7 so the five people on board could have their Ipads plugged in whenever they wanted, and didn't have to ask me for a charging session. I didn't worry about the parasitic load of the inverter.

I just ran the genset in the morning when I got up and charged the batteries when I was anyway making hot water so people could have showers. Then in the evening, with maximum loads at dinner time, I might put on the generator again, if the batts were below 70% or so. But in general I just didn't worry about power.

What all that cost me was having somewhat bigger everything than I theoretically need, especially battery bank (although it's considerably smaller than yours), and I probably ran a generator run or two which I might have avoided if I had obsessively run the system for maximum theoretical efficiency.

That's worth a lot to me.


P.S. I have all-LED lighting, which makes a dramatic difference in power consumption, plus I now have an LED anchor light. But when I'm at anchor, I usually run my electronics 24/7, so that's quite a bit of power. I do that because I make it a policy to be ready to bug out in an instant if the anchor drags (not that I've ever had an anchor drag in the last 10 years, but still seems to be good practice), and also, to run anchor alarms, and also because I like to keep an eye on the wind. Furthermore, I've found my new 4G radar -- which has practical range down to my toe rails -- to be terrific for keeping an eye on other boats in a crowded anchorage -- to be sure that I'm not swinging into anyone. In Ushant, where it was blowing 30 or 40 knots most of the time during our four nights at anchor there, I left the radar on 24 hours a day, and even set a guard zone in case someone started to swing or drag into us.

[Maine Sail]

One thing I see a lot of folks neglect what talking about bank sizing is the Peukert effect in relation to the Peukert corrected loads on a bank.

A larger bank will not be as dramatically affected by Peukert effect and you'll actually wind up with more usable amp hours out of a single larger bank, with the same daily load, than you do with two smaller banks with the same daily load. This means less cycling depth on a larger bank than on a smaller bank given the same daily loads..

For example a bank with a Peukert of 1.25 and an average load of 8A it looks something like this:

100Ah bank, Peukert 1.25, load 8A = 88Ah - "P" Corrected Load = 9A
200Ah bank, Peukert 1.25, load 8A = 211Ah - "P" Corrected Load = 7.6A
300Ah bank, Peukert 1.25, load 8A = 351 Ah - "P" Corrected Load = 6.8A
400Ah bank, Peukert 1.25, load 8A = 502 Ah - "P" Corrected Load = 6.4A
600Ah bank, Peukert 1.25, load 8A = 834Ah - "P" Corrected Load = 5.75A
800Ah bank, Peukert 1.25, load 8A = 1200 Ah - "P" Corrected Load = 5.4A

By using a larger bank, and considering the Peukert effect, it means that your bank will have shallower discharges, not just because it is a large bank, but if the average load stays the same, and you increase bank size, you will actually get more capacity out of the larger bank, at the same load, due to Peukert's.

A larger bank will also support higher voltages at load and suffer from less voltage sag. Most electrical devices on your boat prefer good voltage, especially motors. Voltage sag is not a good thing and this is but one more benefit of a larger bank beyond life cycles, DOD and Peukert effect.

I am solidly in the larger is often better camp but larger needs to go hand in hand with charging sources. This is a systems approach not just a bank sizing exercise...

My starting point is always:

*Max out your energy conservation

*Know your average daily loads on the hook & under sail

*Now design a system

[Dockhead]

Originally Posted by Maine Sail

One thing I see a lot of folks neglect what talking about bank sizing is the Peukert effect in relation to the Peukert corrected loads on a bank.

A larger bank will not be as dramatically affected by Peukert effect and you'll actually wind up with more usable amp hours out of a single larger bank, with the same daily load, than you do with two smaller banks with the same daily load. This means less cycling depth on a larger bank than on a smaller bank given the same daily loads..

For example a bank with a Peukert of 1.25 and an average load of 8A it looks something like this:

100Ah bank, Peukert 1.25, load 8A = 88Ah - "P" Corrected Load = 9A
200Ah bank, Peukert 1.25, load 8A = 211Ah - "P" Corrected Load = 7.6A
300Ah bank, Peukert 1.25, load 8A = 351 Ah - "P" Corrected Load = 6.8A
400Ah bank, Peukert 1.25, load 8A = 502 Ah - "P" Corrected Load = 6.4A
600Ah bank, Peukert 1.25, load 8A = 834Ah - "P" Corrected Load = 5.75A
800Ah bank, Peukert 1.25, load 8A = 1200 Ah - "P" Corrected Load = 5.4A

By using a larger bank, and considering the Peukert effect, it means that your bank will have shallower discharges, not just because it is a large bank, but if the average load stays the same, and you increase bank size, you will actually get more capacity out of the larger bank, at the same load, due to Peukert's.

A larger bank will also support higher voltages at load and suffer from less voltage sag. Most electrical devices on your boat prefer good voltage, especially motors. Voltage sag is not a good thing and this is but one more benefit of a larger bank beyond life cycles, DOD and Peukert effect.

I am solidly in the larger is often better camp but larger needs to go hand in hand with charging sources. This is a systems approach not just a bank sizing exercise...

* Peukert
* Depth of discharge, and
* the necessity of considering charging sources together with bank size

have all been mentioned in this thread. But you, of course, expressed it much more eloquently

[noelex 77]

If we look at total battery cost per year an average cycle discharging to a SOC around 50-60 % is the most cost effective.

The depth of cyle will vary depending conditions but many solar equipped boats keep their batteries at a much higher SOC. They often forget when calculating battery size that the depth of discharge is only the amp hours that are used overnight until the panel output matches the consumption. They also fail to take into account that even a very bad solar day will produce some output.

Boats are compromises and larger battery banks do have other advantages, but many solar equipped boats think they are saving money in the long term with a large battery bank when the opposite is true. (This does not tend apply to generator equipped boats)

[Dockhead]

Originally Posted by noelex 77

If we look at total battery cost per year an average cycle discharging to a SOC around 50-60 % is the most cost effective.

The depth of cyle will vary depending conditions but many solar equipped boats keep their batteries at a much higher SOC. They often forget when calculating battery size that the depth of discharge is only the amp hours that are used overnight until the panel output matches the consumption. They also fail to take into account that even a vey bad solar day will produce some output.

Boats are compromises and larger battery banks do have other advantages, but many solar equipped boats think they are saving money in the long term with a large battery bank when the opposite is true. (This does not tend apply to generator equipped boats)

In support of Noelex's thesis is my father's experience. His boat is solar powered and has modest power needs (it's much more Spartan than mine). He has two big 8D AGM batteries which are ten years old and still going strong. He rarely discharges his bank below 70%. He could probably live with a smaller bank without affecting battery life. Solar is pretty cool that way.

[sailorboy1]

Originally Posted by noelex 77

They also fail to take into account that even a vey bad solar day will produce some output.

It is pretty amazing what solar can do when you would think it wouldn't do anything. I was getting 5 amps the other day during the rain!

It is cloudy today and my boat is out on the mooring with the refrigerator on. Even if the sun doesn't come out the panel will still replace most of the power from the refrigerator running overnight. The system wouldn't go into absorption and truly fully charge but my 460 AH banks can do this this for a long time.