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Amount of solar needed?


sharpness

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I've been following various threads on solar & have gained a lot of information. One thing I haven't got yet is how much solar capacity do I need. We used to be marina based with shoreline & charger so always came back to the boat with fully charged batteries. We used the boat lots of weekends & holidays & all worked well. We have a bank of 3 x 110ah which are now just about cream crackered so much reduced capacity but will be replaced soon. We are now retired & have moved the boat & no longer have a shoreline so how much capacity do I need to keep a new set of batteries topped up. There's nothing left on when we are not using the boat so no drain other than self drain but I don't really want to run the engine just to keep the batteries topped up.

Thanks

Steve

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Steve, the preceding replies don't really cut it. You need to replace the electricity you use. To establish this, you need to do an energy audit. See Tony's site for full details then post the results for further advice. 

Www.tbtraining.co.uk. I think...

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1 hour ago, sharpness said:

 There's nothing left on when we are not using the boat so no drain other than self drain.............

You need to clarify the usage, for example I am betting that "using the boat" doesn't coincide with "running the engine", eg there will be periods of using the boat when electrical items are on (for example, the fridge) when the engine is not running (for example, overnight). Also things like lighting, TV, etc etc into the evening once the engine is off. And the morning usage before the engine is started.

True, a typical leisure use (with the boat being used as a boat, ie travelling daily) will obviously not tax batteries as much as not moving/not running the engine but using electrical devices, but there's obviously still a time period in which electrical devices are drawing load, while the motor isn't running. And unfortunately, these don't coincide with the time period solar is best at (for example, an overnight stay with evening use of lighting and telly, will draw down on the batteries and the solar won't have had much chance to recharge before setting off for cruising the next day).

Then we must consider how much time it takes for the batteries to properly recharge. Its not so much the power of the alternator, but the degree of discharge which dictates a time requirement. It may well be that lightly-discharged batteries can be adequately recharged in 6 hours, from the engine alone, thus negating any need at all for solar; or that the usage is actually quite high in the evening and a late start to the day means that they are more deeply discharged and there simply isn't enough available cruising time to recharge batteries.

Without actual numbers to flesh out the above, all anyone can do is guess - so how about, between 0W and 400W of solar power will do it?

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As others have said it all depends upon your use of electricity and how you run/use your boat. If you run your engine several hours each day you are on the boat then as said 40 watts or so just to keep the batteries charged when you are at home. If you want to stay in place for several days without running the engine I have seen 200 watts often mentioned but that depends upon your electricity use, e.g. gas or electric fridge. Maybe rather less with a gas fridge. The problem comes when you want to stay in one place in the winter because of short days and cloudy sky so for that we are probably looking at cost effectiveness/value. I rather suspect on a narrowboat it may not be possible to install enough panels unless some go on the sides and maybe on masts. I   think most people just accept they will have to provide alternative charging during the winter months and fix as much solar as they can afford.

Remember solar is very good at the long low current charging needed to get batteries to 100% charged while the engine alternator is good at stuffing lots of amps in the batteries for the first hour or so of charging but then becomes very inefficient cost and wear wise. So when you need to do engine charging do it earlier in the day rather than later and let the solar complete the charge.

To maximise battery life you really need some form of battery monitoring so you know when the optimum time to start charging has arrived and also when to stop engine charging so you do not waste fuel etc. How do you do this?

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personal experience

140w will (May - September) keep up with 3 110Ah batteries connected to a 12v waeco fridge (compressor type), plenty of lighting (LED), a few hours of 12v tv use, normal water pump usage (freshwater and shower drain) and various small items (phones etc) being charged from 12v leads.

with similar usage in March - April and October we need an hour of engine every day or two to top the batteries off to the same level as above, the solar is still be doing the bulk of the work

in November - February the solar panels look pretty but are next to useless for anything more than trickle charging batteries that have no load on them, the batteries are kept pretty well charged but the panel will take a week or more to fully charge them after a full days use

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^^^ this ^^^

A most helpful practical illustration of what solar can be expected to do, and aligns with my own personal experience. 

A small panel is great in summer but each time I double my solar panel capacity I only seem to extend the useful 'solar season' by a week or two at each end, so 200w might be a good 'sweet spot' to go for. 

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2 hours ago, Mike the Boilerman said:

^^^ this ^^^

A most helpful practical illustration of what solar can be expected to do, and aligns with my own personal experience. 

A small panel is great in summer but each time I double my solar panel capacity I only seem to extend the useful 'solar season' by a week or two at each end, so 200w might be a good 'sweet spot' to go for. 

But this is not what the op asked for. He simply wants to keep a bank topped up, not provide for, and in the circumstances described by the op, 200 w is way over the requirement.

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12 hours ago, sharpness said:

We used the boat lots of weekends & holidays & all worked well. We have a bank of 3 x 110ah which are now just about cream crackered so much reduced capacity but will be replaced soon

This may be an indication as to how much solar you need - or - it may not be.

 

The question is why are your batteries knackered ?

The boat lived in a marina, with the batteries on charge all week and 'maxed out', you couldn't get another amp into them if you wanted to.

You go away for the weekend / week and gradually flatten the batteries because you are taking out more than you are putting in by just running the engine a couple of hours a day.

Back to the marina and the batteries get back to full-charge but with a reduced capacity because of your mistreatment.

 

You now no longer have this 'fall-back' lifeline and will have to generate all your own electricity, so, the question is how will you be using the boat, will you expect the solar to replace (daily) the amount you have used, are you prepared to replace the batteries again in a couple of weeks, 6 months, 12 months ?

What is your daily consumption ? It is obviously greater than your historical daily 'replacement method', will you be changing your engine running hours (longer cruises) or do you expect the Solar to replace everything you use (ie when you moor up for a couple of days without running the engine)

 

Without knowing how you use the boat, what your electrical audit would show, what you plan to do in the future, and what you want to achieve, it is impossible to suggest suitably sized solar panels.

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54 minutes ago, Phil. said:

But this is not what the op asked for. He simply wants to keep a bank topped up, not provide for, and in the circumstances described by the op, 200 w is way over the requirement.

 

I must have misunderstood then. Not that easy browsing and posting on an ifone 5!

Edited by Mike the Boilerman
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1 hour ago, Phil. said:

But this is not what the op asked for. He simply wants to keep a bank topped up, not provide for, and in the circumstances described by the op, 200 w is way over the requirement.

However as is so often the case we have not been given the facts we need and the OP may not even know what they are.

I very much doubt that when they return to their mooring they run their engine until they leave the boat so the batteries start with being discharged to an unknown degree. While I expect a 12 watt panel will keep up with self discharge I doubt it would do much to recharge a partially discharged 330 Ah bank over a day or so. It all depends upon how long the OP will be away from the boat for.

The answer to the question as asked may well be exactly as you say but resolving the problem may be vary different.

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1 hour ago, Phil. said:

But this is not what the op asked for. He simply wants to keep a bank topped up, not provide for, and in the circumstances described by the op, 200 w is way over the requirement.

However, many of us don't believe that the OP actually wants what he's asking for. If his batteries are fully charged when he leaves the boat then he does indeed only need to counter self-discharge. However, if the batts are at anything less than 100% SoC then he needs more than that.

What we're attempting to define is the answer to the above. If 100% how does he know? If not 100% then what?

Tony

I see Tony B posted much the same while I was typing. 

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20 hours ago, Mike the Boilerman said:

Steve, the preceding replies don't really cut it. You need to replace the electricity you use. To establish this, you need to do an energy audit. See Tony's site for full details then post the results for further advice. 

Www.tbtraining.co.uk. I think...

Thanks Mike, I did all that when I started fitting out the sailaway 15 years ago and ended up with the 3 x110ah bank which has done us well. We don't have big power needs & I like to keep things simple. Nothing has changed other than we have moved out of the marina & off the shore line. I've replaced the batteries 3 times I think, cheapo lead acid. & I try to look after them but there's better things to do than play with my batteries all day.

Now we're off the shore line I thought I should have something to keep thins topped up.

12 hours ago, Tony Brooks said:

As others have said it all depends upon your use of electricity and how you run/use your boat.

To maximise battery life you really need some form of battery monitoring so you know when the optimum time to start charging has arrived and also when to stop engine charging so you do not waste fuel etc. How do you do this?

Thanks Tony. We generally go out for a few days & having left the marina with a full bank & some moving its enough to keep the batteries above 50% on the Smartguage. I think in 15 years I've only run the engine to charge batteries less than 10 times.

If we go away for a longer trip we generally move each day & with low battery needs we don't have a problem. With a newish bank we've been able to stop 2 or 3 days  sometimes. When the bank is getting on a bit & reduced capacity, it gets a bit more marginal but again after 3 or 4 years use I've replaced the batteries.

 

Thanks for your help

Stev

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10 hours ago, Alan de Enfield said:

The question is why are your batteries knackered ?

The boat lived in a marina, with the batteries on charge all week and 'maxed out', you couldn't get another amp into them if you wanted to.

You go away for the weekend / week and gradually flatten the batteries because you are taking out more than you are putting in by just running the engine a couple of hours a day.

Back to the marina and the batteries get back to full-charge but with a reduced capacity because of your mistreatment.

 

You now no longer have this 'fall-back' lifeline and will have to generate all your own electricity, so, the question is how will you be using the boat, will you expect the solar to replace (daily) the amount you have used, are you prepared to replace the batteries again in a couple of weeks, 6 months, 12 months ?

What is your daily consumption ? It is obviously greater than your historical daily 'replacement method', will you be changing your engine running hours (longer cruises) or do you expect the Solar to replace everything you use (ie when you moor up for a couple of days without running the engine)

 

Without knowing how you use the boat, what your electrical audit would show, what you plan to do in the future, and what you want to achieve, it is impossible to suggest suitably sized solar panels.

That's exactly what has been the pattern over the last 15 years, we've certainly used 3, possibly 4 sets of batteries in 15 years so my "mistreatment" cant be that bad.

Now we are retired, we've moved the boat to a mile away. We can & do decide at breakfast time to go boating for the day, we never sit on the mooring, we sometimes just go to Gloucester shopping (2hrs) or for a run so wouldn't use hardly any lectric and would probably arrive back at 100% on the SG. If out for a few days, not a lot of running, with the fridge & TV on we'd get back  closer to 50%. I'd like the solar to start topping the bank back up & then keep it topped up till we go again which could be a few days or a fortnight.

9 hours ago, Tony Brooks said:

I very much doubt that when they return to their mooring they run their engine until they leave the boat so the batteries start with being discharged to an unknown degree. While I expect a 12 watt panel will keep up with self discharge I doubt it would do much to recharge a partially discharged 330 Ah bank over a day or so. It all depends upon how long the OP will be away from the boat for.

The answer to the question as asked may well be exactly as you say but resolving the problem may be vary different.

Thanks Tony, you're right, see my comment to Alan above & my earlier reply about very rarely running the engine to charge batteries & being away from the boat.

 

9 hours ago, WotEver said:

However, many of us don't believe that the OP actually wants what he's asking for. If his batteries are fully charged when he leaves the boat then he does indeed only need to counter self-discharge. However, if the batts are at anything less than 100% SoC then he needs more than that.

What we're attempting to define is the answer to the above. If 100% how does he know? If not 100% then what?

Yes Tony, that's what the dilemma is, as earlier in this answer, sometimes they are 100% on the SG, sometimes not, so how long is the piece of string, there are so many variables, so should I just go for say 100w which is far more than needed to counter self discharge. With a proper controller would any "surplus", assuming the sun does shine occasionally, do any harm?

 

Thanks for all your help

Steve

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3 minutes ago, sharpness said:

should I just go for say 100w which is far more than needed to counter self discharge. With a proper controller would any "surplus", assuming the sun does shine occasionally, do any harm?

That sounds like a good compromise. You can't harm the batteries no matter how big the array is and 100W should raise them up to 100% quite quickly during sunny weather. 

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20 minutes ago, sharpness said:

Yes Tony, that's what the dilemma is, as earlier in this answer, sometimes they are 100% on the SG, sometimes not, so how long is the piece of string, there are so many variables, so should I just go for say 100w which is far more than needed to counter self discharge. With a proper controller would any "surplus", assuming the sun does shine occasionally, do any harm?

Unfortunately, when Smartgauge says 100% it doesnt mean that your batteries are "full". It's dealt with in more detail elsewhere, but my Smartgauge shows 100% long before the amps being drawn are low enough to suggest that the batteries might be close to "full".

Smartgauge seems to be good at telling you when to start charging your batteries, but not when you can stop.

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On 29/05/2017 at 23:14, Richard10002 said:

Unfortunately, when Smartgauge says 100% it doesnt mean that your batteries are "full". It's dealt with in more detail elsewhere, but my Smartgauge shows 100% long before the amps being drawn are low enough to suggest that the batteries might be close to "full".

Smartgauge seems to be good at telling you when to start charging your batteries, but not when you can stop.

 

Except that it isn't. 

I used to charge my batteries when the SG got down to 50% which used to be the benchmark, but this lead to my new set getting wrecked over one short winter.

Apparently one is supposed to recharge the batteries immediately they have been discharged, even a little bit. A good compromise seems to be to recharge them every day when in use, whatever the smartgauge says.

Edited by Mike the Boilerman
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question from a newbie.

 

Is there not a meter that can be fitted to tell exactly how much power has been drained from your battery bank each day?

Would that be a difficulty thing to wire in to an existing set up, and if not how much would it cost roughly?

Surely if you were able to keep a record of how much power was taken from your battery bank each day or week then  deciding how much solar you need or how often you are going to need to run your engine (or separate generator) would be simple.

Finally,  would there be a problem (apart from the initial expense) from having too big a solar array?  Could too much power damage you battery banks?

This charging of battery banks,  how big your banks should be, etc etc, seem to be a regular topic on the forum. 

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Ok just reading your updated post re your requirements. You have a battery bank of a theoretical 330 ah. Assuming that this is the reality, which with a knackered bank it isn't, then when the SG is at 50 you need to replace a minimum 165 ah in order to fully charge the bank, plus a bit over that due to inefficiencies. To give a rough estimate, a 100 w panel will produce approx 70 ah in summer and as low as 5 ah in the winter or a cloudy day. So to recharge your bank from SG 50 will take at least 3 days in good sunshine during the summer and far far longer in the winter. 

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12 minutes ago, efanton said:

Is there not a meter that can be fitted to tell exactly how much power has been drained from your battery bank each day?

Would that be a difficulty thing to wire in to an existing set up, and if not how much would it cost roughly?

Surely if you were able to keep a record of how much power was taken from your battery bank each day or week then  deciding how much solar you need or how often you are going to need to run your engine (or separate generator) would be simple.

Yes, that's exactly what most battery monitors other than the Smartgauge do. They use a shunt to count amp hours going into and out of the battery.

They're not very good for telling you what % charge you're at, because that relies on them knowing the true capacity of the batteries, which is hard to measure and changes over time. But for just counting amp hours used over some period, they're perfect.

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On 29/05/2017 at 23:33, efanton said:

question from a newbie.

 

Is there not a meter that can be fitted to tell exactly how much power has been drained from your battery bank each day?

Would that be a difficulty thing to wire in to an existing set up, and if not how much would it cost roughly?

 

Yes there is. An 'amphour counter'. But they are misleading as they drift out of calibration.

Cost is couple of hundred quid-ish.

On 29/05/2017 at 23:33, efanton said:

 

Surely if you were able to keep a record of how much power was taken from your battery bank each day or week then  deciding how much solar you need or how often you are going to need to run your engine (or separate generator) would be simple.

 

Would it? How will you know how much the solar will deliver? Or put another way, sun will shine in the coming week, say?

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