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Battery bank size verses charging capacity

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(Note I don't know much about this topic so excuse me if I have the terminology and details slightly out)


We currently have a 660ah battery bank made up of 3 x 220ah lifeline agm batteries on our beneteau oceanis 50.


Alternator is I think the standard 80amp alternator that comes with the yanmar but the most I have seen it inputting is 58amps. The surveyor reckoned it was only a 35amp but clearly it's more than that.

In addition to that we have 450watt of solar and a 300watt hydro generator that will only get used on longer passages(gets in the way for general boating).

240v charger is 40a.


I believe my batteries are buggered now and have a total capacity around 100amp max. We have a victron battery monitor which tells me SOC(although I have only set this to the default of 200a), draw/charge, total consumed energy, and voltage.

We draw around 5-6amp with the fridge freezer depending on the setting and everything else can make it up to 10-12 with nav lights and instruments + bit more for autopilot but I haven't really been able to see What it draws as we have only done day time sailing so far the the solar takes over. I'm assuming this is around 1-3amp depending on the sea conditions.

Solar is generally putting in 10-15a from 9am until 7pm obviously with the biggest input being the middle of the day.

Haven't tested the hydro yet.


Currently the batteries will go flat overnight after around 70-80amp of total consumed energy with the DOC saying arpund 70%(set for a bank of 200ah)

If I turn the fridge and freezer off at 8pm when the solar finishes then bavk on for an hour before bed to get the temp down again we can do the night without the batteries getting too low. If I leave it on then I'm below 12v by about 3am and get the battery alarm so have to shut it down. Voltage jumps up to about 12.2v when everything is shut off.

The other day I turned everything on in the evening to see how long it would last and got to 11v and under 12v with no load by 10pm with a total consumed energy of 70amp.

Batteries seem to back to full charge by 3pm allways which also is probably not good as it means that the batteries are only using a small amount of their copacity.


So I think I need to order new batteries. Got quoted 1800euro including instilation and testing for the same 660ah bank by some trustworthy local guys. These guys have already been to the boat and checked out the system and the batteries which tested pretty average. One dropped 3 volt with the loss thing the the other 2 dropped close to 2 volt. I don't know exactly what this means and the language barrier made it hard to fully understand.


Do I stick with the same size or go slightly smaller? I have been told that i dont have enough charging capacity to have a bank that big and if it gets low I might never be able to get it back to full charge.

Any recommendations or ideas here?


What size bank to other people run for typical offshore cruising?

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Hi Booboo,for a bank that size you need a min of 160 amp alternator and a smart regulator, Belmar or Ample Power are the well known models

Problem with your alternator apart from being too small, is that it is machine sensed so it will cut back very early.

Have a look at www.yachtwork.com noting he is now off skippering a big cat, for some ideas.

I would still maintain your bank size. This is a typical problem for production boats not having enough charging capacity

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I second adding a second ALT and use the smaller for Engine bank charge. But doing so is not always easy to do and is why these engines installs often had far too small ALT's. Plus, the Boat Builder thinks most will be day sailing and will be connected to shore power within a few days.
As Marinheiro says, you really do need a much bigger ALT. If it were at all possible, you could actually go as high as a 220A ALT, but physical size and Hp required to turn it could be impracticable.
In saying that, the Shore power charger is far too small as well, but as that charges 24/7 while on shore power, you can get away with it.

Before turfing the Bank out and replacing, you need to do a capacity test.
One important point, get the Data entered and set correctly. Having the capacity entered incorrectly will upset the calculations and give you an incorrect capacity reading. If your monitor can have a value for Purkets Law that can be entered, it is essential that is done. This will then give you an accurate set of information on what the bank is doing.
You then need to fully charge the bank and then carry out a load test and see if the Sum calculated by the monitor equals what the bank is capable of.
Or, another way is to test Voltage with an accurate Digital meter. AGM's should be resting after this time of full charge and rest, at around 12.8V. It is possible to get 13V resting from new AGM;s. 12.8V is what I would expect to see from old AGM's. Resting voltage will give you an OK answer of how good the bank is. Capacity testing is obviously the more accurate though. There is another way of capacity testing, which is carrying out manually what the Monitor is actually doing.

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I had some guys come and take a look and test the batteries. They took one out and charged it on their charger and then load tested it. Then they tested the other 2 while in the boat. Sick of the bills in Euro for all the mucking around and figured it's time to just buy the new ones as one more session on the boat and I will have already spent a similar amount to the new set of batteries anyway....

I guess the question is should I be getting a slightly smaller bank? 3 x 180ah instead?

I imagine a bigger alternator could be a costly excersize with new pulleys and belts? Roughly what sort of figure would that be?

The solar will generally be keeping the batteries pretty charged and just a top up from the motor when needed. When at sea and needing the extra power from the nav lights, autopilot and instruments we have the hydro generator putting in 300w all day and night.

I also found it seems to be better to run the motor at night as the solar raises the voltage and tricks the alternator into thinking the batteries are charged, Is this correct?

So what's the most cost effective plan considering I can't do much myself and it's DAMN expensive getting anything done up here. Even staying in a marina to get work done is 60-100euro PER NIGHT! Burning money.....

Big battery bank, smaller battery bank or do I go the whole hog and do batteries and alternator upgrade. Is it actually nessisary.

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OK, agree with the above. The existing alt is rubbish for recharging a deep cycle bank, it's designed for recharging a start battery. Let it to that if possible, and add a new bracket and alternator.

Let's get the system right. Bank size first.

Proper bank sizing allows you the required bank power take each day without stressing the bank. Currently you are using say 80amp hours - but cannot run the fridge/freezer 24/7. A good fridge freezer will use around 50amp/hours per day, but some use over 100. If it's one with the thin plates inside, and no eutectic tanks, it's likely to run virtually continuously. So the question is, how much more to add to the 80amp hours already used. Remember that 80amps is in good sun, with pretty good solar charging. I would at least double that, and say at least 160amp hours per day.

So, batteries should never be discharged more than 50%. They charge fast to about 80%, then slows as it gets closer to full. The last part can easily take 5 hours or more. Many cruisers don't charge past about 80% for this reason.

So that gives us 30% of the bank for daily use. This means the bank size of 660a hours is about right. 25% of that for alt size, so 150-200 amp alt.

Get a good, hot rated alt. Use a serpentine belt. Make sure the bracket is heavy and up to the task - the loads are BIG.

For the existing batts, take them out, have them properly tested with a 20 hour capacity test. You can do this one battery at a time, so you can still live aboard. A simple capacity test with a meter is not sufficient - it only tests Max output over a short time, deep cycle is different, and you MUST do a proper 20 hour discharge capacity test to know if they are any good.

Once the capacity is known, put that and the proper perkets number into the battery computer, and then it will start to tell the truth. At the moment, the only thing it's telling you that's correct is the voltage!

Finally, a note about battery computers. Even when correctly programmed, solar systems can cause them to prematurely reset capacity to full. Normally, they have a function that when the charge rate drops to a set number (say 9amps on your bank size), they will reset to full. The problem is that solar might do this. Read the manual for your battery monitor!

Good luck, and welcome to the world of the cruiser :-)

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Just buy some new batteries and use less power with the solar and hydro when sailing and run the motor at night for an hour or so if you need to. Turn off the fridge at night. I survive with an 8 amp alternator and no hydro. Or could you run a portable genny to a battery charger as a booster when required

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Turning off a fridge/freezer is often false economy. It can take more power to bring it back down to temp than was saved by turning it off, depending on system. I don't know why that is, but it's my experience.

Also my experience was that it is hard to find anyone that really properly understands this stuff. The real, long term cruiser market is pretty small. Either dedicate some real time to research the particular alt and battery, regulator and monitoring system you have and want, or try to find someone locally you feel you can trust  after reading these two threads, then go with what they say.

Common mistakes are.

Unknown state of charge due to no, crappy, or poorly programmed battery monitors.

Batteries killed early due to above

constantly undercharged batts causing premature failure

Battery bank to small for power requirements

Automotive alternators that never charge the bank (Not low startup, not hot rated)

Lack of user knowledge - like that the batts capacity reduces over time, allowance should be made with batt monitor programming for that.


Many cruisers end up replacing their batts every 3-5 years. Properly managed, I got 14 years from my last set of VRLA batts. 

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My calcs indicate that your 450 watt solar should in a perfect world not counting for shading, age and transmission loss, produce 37 amps at full strength. Thats worth checking out.


Agree.  In the caribbean with roughly 300w of solar panels in our bank we saw 20 amps or even maybe a little more at times and an average of 10-15 amps while the sun was out and 3-5ish if it was cloudy.  Worth checking out whether your ones are perhaps not producing what they should or regulator over regulating or something.  Panels apparently degrade with age so if they're older they may not be delivering the full 450w.


What people often don't realise, is your batteries are "flat" once they get down to roughly 50% full, and they're difficult to get to 100% as the rate of charge they'll accept slows down as they get more full.  So actually of your supposed 660ah in your bank, you probably operate mainly in the range between 60-80% or between amp hours 400 (60%) and 530 (80%), meaning you have roughly 130ah to use.  If you're just measuring voltage (which is pretty much all you can do without a properly configured battery monitor), then the trap is that you can think you're full, because the voltage shows low when they're empty when you drop down to 50% as you go flat in the night, but bump them up to 60% with a quick charge from the solar and your voltage bounces back now the batteries are in their healthy range again.  But you're still only at 60% and still need to keep charging.  


In your case Booboo, if your batteries are flat in the morning, then even if you're putting in the full 15amps it sounds like you're seeing as a best case from your solar set up for a full 12 hours then you've generated 180ah.  in the real world if you're seeing 10-15ah in the good sunny part of the day you're probably only seeing 5ah for the first and last few hours and probably actually getting 100ah out of the solar over the day.  Then running the fridge during the day at 5ah subtracts 60ah over 12 hours, plus anything else you've run during the day - say 10ah for a few hours of stereo, waterpump running or whatever, and you've got a surplus of 30ah, which is 5 or 6 hours of fridge minus whatever you use in lights during the evening then you're back to where you started - flat with the alarm going sometime in the middle of the night.  


This wont be fixed by new batteries.  If this is the scenario then if you change the batteries it'll just keep happening.


It's possible they're no good, but as IT says above without proper testing it's very hard to tell.  You could try to get them FULLY charged by buying some bags of ice to put in the fridge/freezer and turning everything off for a couple of days, while you sit in a marina on shore power with a decent smart charger and give it heaps.  Then go back to your usual usage and see what happens - if you're flat again the first night, the batteries probably aren't holding their charge.  If you last a few days before running into these problems again, there's every chance the batteries are fine, you're just using a little more power than you're putting in each day, and after a few days you end up hovering in the low end of the healthy range of the batteries and never getting them properly charged.


As everyone above has said a bigger alternator gives you a pretty simple fix, if you have a big powerful alternator and can poke in 150amps then you'd get the same amount of charge you get from the solar setup over the day in less than an hour of engine running, which gives you the ability to easily get charged up again.

I have no idea what it'll cost there, but it's not a massive job really.  stab in the dark in NZ $1000 for a good alternator and smart regulator, and half a day for an engineer to weld you up a bracket to mount it and sort out a pulley and belt, and half a day for a marine sparky to wire it up?

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Prices are a bit out. the Balmar 94-12-165-IG, 165 amp, is around $1200 USD See http://www.defender.com/product.jsp?path=-1|328|2289948|2289951&id=3082924  Their smart regs are around 350 - 400 USD.


Plus you will need a bracket and pulleys, plus install. The bracket might have to be custom made depending on the engine. All up, this type of thing is usually 3-4 K in NZ, maybe $2.5-3K USD ??


The cost is why this type of thing is normally only found on long term cruising boats. 

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IT is spot on. (you have been studying IT) I won't add to what has been said, apart from be aware a large frame Alternator takes up some space and often those boats don't have enough of it.

However, a good thing to also check just for power saving, is the fridge freezer temp. The colder you make something, the more energy it takes to get it that cold. So ensure you have the temp set to enough and nothing more.


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Thanks for all the reply guys. It's certainly food for thought.


I have been in and out of marinas most of the time and had the batteries on charge a lot and often for over a week but then they still only last 1 night if I turn off the charger.

I just don't want to chuck more money away getting these batteries tested again. Although our charging capacity is low I'm pretty sure that right now it's not the immediate problem.

Interesting to note that this boat has done extensive cruising to the Caribbean and back with the current setup and the old owner said they vary rarely needed to charge with the motor, only after a long period of cloud cover. I would say the 8 year old solar panels have lost a bit of their output now but still do provide plenty of grunt.


I'm thinking of just doing the batteries for now as they will take the rest of the week to arrive and then seeing if I can find a friendly cruiser who can help me do the work upgrading the alternator (might even be able to trade skills...).

The retail prices for getting stuff done here are just too much right now. The time delay of checking the 3 batteries on a 20hr capacity test then having to order the batteries will mean I'm likely stuck here for another 10 days which is another 600euro just in berthage alone. Right now 1800 euro for new 660ah batteries fitted and tested seems like a good option to just get out of here and go cruising. Remember im in the 'motor'terannean right now anyway so doing plenty of motoring!

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Sounds fair Booboo.


While it is likely the batts are tired, you dont know for certain.Your call, but personally even though I might make the call to replace, I'd still want to make absolutely certain that the new ones are being charged properly. A good multimeter  to check the batt voltage -AT THE BATTS - when charging will tell you whats up. Compare it to the spec sheet for the new batts.

Your call of course, good luck!

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Boo boo if the solar charge is high like 5 to 9 amps it's telling you the battery's have no resistance , Iif the battery's were holding charge the resistance would be low as they top up so the panels charge would be 2 to 3 amps constant my house battery has just done the same my panel charge is showing 4 to 6 amps and battery goes flat over night with out using it ( not holding charge ) your alternater will be small on a factory motor. I had to change up to 75 amps as 40 amps was supplied from New Deregulate your alternated and you have full charge on key , through the next step regulated it balances the input against resistance in less than an hour it would proberly switch off charged ,

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I would say the 8 year old solar panels have lost a bit of their output now but still do provide plenty of grunt.


Nope, rest easy there too. You will lose about 20% after about 20yrs. The most important thing is to make sure the panels are clean. Also remember that even a clear blue sky day in your view, may not be in the Panels view. Humidity can have a massive affect on output. Certainly dust in the air and smog will drop output. The best output is often seen when you have the real clear air after a good rain.


Boo boo if the solar charge is high like 5 to 9 amps it's telling you the battery's have no resistance

Depends totally on the load. If the load is being drawn all the time, like via a fridge running, then the Current is going to be consumed immediately and the panel reg will be poking in current. Hence why a Load test on the bank, while disconnected is the only accurate way to measure the Bank.

It is very simple to do. But if the monitor is set correctly, then that will do it for you. The tricky part is to get it set correctly.

That rest voltage test I said earlier tells you exactly this as a result. If the battery is reading a 12.7 -12.8V, after 24Hr, then it is in good condition. Little to no internal leakage. Showing a lower than 12.6V and the Battery is getting into it's Senior years. Less that 12.5V and you have issues and need to look at replacing the bank if you want the bet performance.

For Deep cycle banks, the ability to produce Capacity is related to time. They are slow and steady at producing Current. They do not produce high current amounts quickly. So a Load test over a 20hr period is best. This can also be done over a 10hr period and if you read the info on the battery itself, you will see an hr figure or either 10 or 20. That is the amount of time that the battery was originally calculated on by the manufacturer. 


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Wheels you saying the next step regulators are bad for charging up quickly ,what alternated takes 20 hours to fill a battery

Huh? Where did I say that? Or have I said something confusing? Next step are fine as are many others. In fact any smart charging reg is better than nothing.

The only thing I can think re the 20hr thing, is the test. Is that what you mean? This test is a draw down, not charge up. But you do start it with a fully charged battery.

On the battery somewhere, there should be a label with some specs. Among those specs should be either a 10hr or 20hr rating. That is the time taken that the manufacturer used to test the total AHrs of current the battery was able to deliver. 20hrs is the better rating, although it tends to be one that is much harder to come by these days. The Guys that make great batteries are happy to use that figure. 

So for ease of numbers, lets say the battery is 100Ahr @20hr. You divide the 100 by 20 which gives us 5A. So by applying a 5A load, the battery should be able to maintain that 5A for a period of 20Hrs.  Important: The voltage will be below 12V. In fact the Voltage is likely to be and should be no lower than 10.5V. When the Load is removed, that voltage will start to lift again and after a period of time, will be nearer the 12V area.

No Deep cycle will ever produce the 100A for 1hr. In fact there are two things that will happen if you tried. Firstly the Battery may produce current for around 30 to 40min. Secondly, you are likely to boil and damage the Battery. This is the difference between a start and deep cycle.

This relationship between battery capacity and rate of discharge is not linear and that is what Peurkets Law describes and the formulae allows us to calculate proper recharging of the battery. It is also why the recharge time not linear either and becomes longer as the charge nears full capacity. So for instance, you may get 80% of the capacity back into the Bank in 2hrs and still require maybe a further 3hrs to get the last 20% in to it.

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