solar panel recomendations please

[noelex]

My maths does suggest that anything you can do to lengthen the life of your batteries or make them charge faster will be beneficial, no matter what your system is.

You are ignoring other important factors such as cost (capital cost and cost per year), weight, size,( loss of storage area) and complexity.
These factors need to balanced otherwise using your logic the battery bank would sink the boat.

Boats running generators need much larger battery banks, but for boats running primarily on solar a lot of people are fitting battery banks that are much too large.

People frequently focus just on battery life. Sometimes carrying around the weight of an extra 200-400Ahrs for little extra gain other than another 6 -12 months of battery life.
I think if they purchased those extra batteries separately and only got 6-12months life out of them ( which is effectivly what they are doing) they would be very disappointed.

 

[sailinglegend420]

You are ignoring other important factors such as cost (capital cost and cost per year), weight, size,( loss of storage area) and complexity....

Capital cost per year is less if you amortize over the life of the batteries as each year their cost goes up. Weight and size depends on your boat - we still don't know what yours is or how much solar you have, so your comments aren't very helpful. Very very few people have enough solar power to be able to put back the daily Ah usage and put 100% charge back into the batteries. If you do then you must have a large boat, so you shouldn't be worried about the weight of one extra person for an extra 250 Ah of battery.

....Sometimes carrying around the weight of an extra 200-400Ahrs for little extra gain other than another 6 -12 months of battery life....

You could double your battery life - that could go up from 5 years to 10 years. If you only get 1-2 years out of your batteries then you have a problem.

 

[noelex]

You could double your battery life - that could be up from 5 years to ten years. If you only get 1-2 years out of your batteries then you have a problem.

To put some hyperthtical numbers on it as an example.

Lets take a typical cruising sailor getting most / all of his power from solar.

Overnight he uses 50Ahrs beore the solar panels start charging. This would suggest he would use about 120AHrs a day. The solar panels will need to produce a bit more than this in the worst season. So over much of the year the average output will be much higher
Say he has a 400AHr battery bank.
50 AHr discharge takes him down to 87.5 % if the batteries are full. Lets call it 85%. This sort of discharge will apply for much of the year, because of the higher production over the better seasons will comfortably exceed the consumption.

During winter the average discharge will be higher, because there will be days where consumption exceeds output (although on average it must meet it).

For much of the year the discharge will be around 85%, but an average over the year might be 75%.
If we increase the battery capacity to 600AHrs the average discharge changes from 75% to 83%.

These sort of changes in the depth of discharge only make a small differance to deep cycle battery life. Batteries will die of old age even if the depth of discharge is kept very low.

The above is only a hypothetical example, but the numbers are realistic of what goes on in the real world. Despite this many cruisers will it much larger battery capacity 800 or 1000 AHrs is not unusual.

As a general rule the cheapest battery cost for deep cycle lead acid batteries is when the discharge cycles are around )50-60%. There are some other practical advantages in a larger battery capacity so aiming a bit higher is sensible, but beware of overkill.

 

[sailinglegend420]

To put some hyperthtical numbers on it as an example......

Figures are good - very good - and you can prove anything you want to with them.

Take your daily figures for summer alone - not averaged over the winter as well. 120Ah/day with a 400AH bank is a 30% DoD - 120Ah/d with a 600Ah bank is a 20% DoD. On my line battery curves that's a difference between 1800 life cycles and life 2800 cycles. I call this a large difference.

But to size the bank and the solar panels you need to take something closer to an average situation. For most people the average situation is their batteries NEVER get to 100% every day - probably only 85%, that's certainly all they will get if there is no sun and they run an engine or a genny.

For a 120Ah/day usage you would need 120 x 4 = 480 watts just to replace the lost Ah. To get above the 85% state of charge you need another say 40Ah = an extra 160 watts. So the solar panels would need to be a massive 640 watts!

 

[noelex]

Figures are good - very good - and you can prove anything you want to with them.

Take your daily figures for summer alone - not averaged over the winter as well. 120Ah/day with a 400AH bank is a 30% DoD - 120Ah/d with a 600Ah bank is a 20% DoD. On my line battery curves that's a difference between 1800 life cycles and life 2800 cycles. I call this a large difference.

No this is a common mistake.

Lets assume you are using 120AHrs a day . Overnight (before the solar output exceeds your draw) you might use 50Ahrs.
The cycle is 50AHrs
Not 120Ahrs
The cycle ends when the batteries are charging again.
So the cycle is 50Ahrs not 120AHrs. 30% down is incorrect 12.5% is the correct calculation. If you add a small amount for the batteries not being fully charged (although often they will be) and some diminished battery capacity with age you still come up with a very low discharge.

For a 120Ah/day usage you would need 120 x 4 = 480 watts just to replace the lost Ah. To get above the 85% state of charge you need another say 40Ah = an extra 160 watts. So the solar panels would need to be a massive 640 watts!

Yes I agree.
This illustrates my point further.
Even for a boat with a very large solar array 640w in my hypothetical, but realistic, example 400AHrs still gave a much cheaper battery cost per year and reasonable battery life.
As we scale this system down to a smaller solar panel array and smaller daily consumption the battery size would of course become smaller.

The recommendation to go for very large battery banks was, and still is, good advice for boats getting most of there power from a generator, or the main engine, but I see a lot of boats that use solar power predominatly fitting battery banks that are much larger than they need to be. Increasing weight and adding to the running costs of their boat.

 

[sailinglegend420]

Lets assume you are using 120AHrs a day . Overnight (before the solar output exceeds your draw) you might use 50Ahrs.
The cycle is 50AHrs

There's a danger of our exchanges getting boring! If I do take your figures the DoDs are 12.5% for a 400Ah bank and 8.3% for a 600Ah bank. That's 4500 Cycles and 5500 cycles. That's a 22% improvement in life cycles.

What I omitted to add was you should calculate for the worst case scenario. No sun, and then the cycle would be 120 Ah, not 50 Ah, so in that situation my figures are valid. They would show a 55% improvement in life cycles with a larger Lifeline battery bank. (45% using Trojan's curves)

....If you add a small amount for the batteries not being fully charged (although often they will be) .......Yes I agree.....

You agree that to get a battery 100% charged you need over 600 watts. Very few boats have that in the real world, so batteries are often only ever charged to 85%. So if the sun is shining and I take your cycle as 50Ah then that represents DoDs of 10% and 15% between banks of 400Ah and 600Ah. On my battery curves, and probably everyone else's, that shows 5000 cycles or 3500 cycles - that's a 43% improvement.

So whichever way you look at it there is a significant improvement if life cycles with a larger bank.

Does anyone else please have any other figures?

 

[noelex]

There's a danger of our exchanges getting boring! If I do take your figures the DoDs are 12.5% for a 400Ah bank and 8.3% for a 600Ah bank. That's 4500 Cycles and 5500 cycles. That's a 22% improvement in life cycles.

You are misunderstanding how cycle life is measured, and its practical application for boat owners.
Normally it it calculated with one cycle directly after the next. So there is little account of the battery ageing.

Batteries die from both cycles, and well, age. (A bit like most of us ) If you keep a lead acid battery under optimum conditions with no discharge at all ( other than to stir up the electrolyte) it still has a reasonably short life. When you get to predicting deep cycle battery battery life with these low DoD aging of the battery plays a significant role.

For example 5500 cycles for someone sailing a couple of months a year would equate to battery life of close to 100 years!.
This is of course nonsense. The batteries will die a long time before this number of cycles.

For real life, practical information, an owner who changed is DoD from 12.5% to 8.3% would see very little improvement in their battery life, nothing like 22%. In both cases battery aging will kill the batteries first.

No sun, and then the cycle would be 120 Ah, not 50 Ah, so in that situation my figures are valid. They would show a 55% improvement in life cycles with a larger Lifeline battery bank. (45% using Trojan's curves)

This is another common mistake
People assume 0AHr output for a bad solar day. A very bad day for 640w of solar would be 40 AHrs output (so the batteries 120-40=80 AHrs, or 20% DoD)Of course even this will occur infrequently, So using this figure to calculate the battery life as if this depth of discharge was occurring every cycle would not give the correct result.

No sun, and then the cycle would be 120 Ah, not 50 Ah, so in that situation my figures are valid. They would show a 55% improvement in life cycles with a larger Lifeline battery bank. (45% using Trojan's curves)


You agree that to get a battery 100% charged you need over 600 watts. Very few boats have that in the real world, so batteries are often only ever charged to 85%.

If you achieving all your power from solar, over much of the year the batteries will reach 100% (or very close to it) very frequently.
If you reduce the size of the panels you need to reduce your daily demand. The results are the same.
If your batteries are only ever charged to 85% you are not going to be self sufficient on solar. You need to reduce your demand, or increase your solar watts.

85 % is a sensible maximum to aim for when charging with the main engine or generator. When the charge acceptance has dropped to, say 20A its not worth running the engine much longer, but with solar this restriction does not apply.

 

[Seren]

Noelx and Sunlegend420.

Fascinating,well-tempered and informative debate.Although the physics is getting out of my range one thing you both seem to agree on - that there is little gain in fitting an MPPT controller.
This agrees with the view of the marine electrician ( RN trained and owner of a company serving the electrical needs of vessels in a large oil/gas port ) who fitted my BP 2 x 60w rigids. I gave him the 'chapter and verse' that I had downloaded on the benefits of MPPT but he and his colleagues could see little gain for my Med cruising situation.