Solar - more cells = better?

[CatNewBee]

Originally Posted by CarinaPDX

...
For smaller systems - say less than 200W (YMMV) - MPPT controllers don't usually make sense as the extra cost could be better used to simply buy another, or larger, panel. With larger installations the panels can be arranged serially to provide a higher voltage, and thus enable the use of smaller cables and make good use of an MPPT controller.

Greg

Hi Greg,

All OK so far, except the serial connection of the panels. This is sub-optimal, because in series the current is defined by the weakest part, so your weakest panel (and the one with a small shadow) will drop your string current, you will simply loose a lot of power. Always connect panels in parallel on a non-stationary install where shading of some cells can occur.

Parallel connected panels are more resilient towards partial shading. If you want higher voltages, chose panels with more cells. Also consider to add a string-diode (usually Schottky-diodes) per string to prevent reverse-voltage / reverse current if modules are shaded or have a short cell / hotspot / damage... This applies for installations of more then 3 modules / strings in parallel. Otherwise the current of the other strings can fry a panel - especially if you use flex-panels the heat can damage the structure below.

[john61ct]

Victron's full-featured 75/15 MPPT is often available for well under $100.

That is fine for a panel up to 65V, maybe 70 if it will never see icebergs. No idea how many cells that would be, and it's completely irrelevant.

Is anyone calling that expensive? Very robust, 5-year warranty good support.

The BT dongle functionality being built-in adds maybe $20, fully customizable charge profiles for any bank including future LFP, logging real-time performance data to the app on your smartphone.

A large single 220-250W panel would get the most from that, up to 300W if you found a real bargain.

And one SC per panel optimizes partial shade handling, each gets its own MPP optimization independent of the others.

Putting a nominal 12V panel on it would lose most of the MPPT efficiency gains, much lower value proposition.

The bigger SCs handle higher voltages than that and of course lots more amps.

[CatNewBee]

Well 15A is kind of too small. If you use a 60V 330Wp panel, you have 24A+ output at 12V. The 15A MPPT is ok for smaller panels and also nice for 24V systems, bud could be "underpowered" with a single high performance panel.

More smaller controller add up installation costs and also losses for the transformation. They are "cheep", but compared to what?

Usually the larger more expensive ones are also more sophisticated and better than the cheap ones.

So as always, it depends. You can find 150V / 100A MPPT's for 600..700$ It looks expensive, but you would also need 7-8 of the 15A to reach the same power.

[john61ct]

Originally Posted by CatNewBee

Well 15A is kind of too small. If you use a 60V 330Wp panel, you have 24A+ output at 12V.

No. The max volts is Voc rating, which by definition will never be seen IRL. Whether it's 40V or 60V is irrelevant to the wattage rating.
The 15A is not an upper limit for panel input, Victron cares about the max volts, going well over on amps is fine wrt the SC, and the watts specs are for final **charging output** maximum.

Yes in rare peak conditions some of the potential watts of overpanelling will be wasted, (why I said "if you got a great deal") but in normal use the average output is much greater than sticking to the 220-250w number.

My point was optimizing the $100 SC investment, spending as much on a couple anemic 100W 18V panels would be a relative waste of its potential.

And going to a much more expensive SC just to capture a very occasional few extra wH even more wasteful, put that money where it will do more good!

My biggest point, which is rarely seen, is with such inexpensive top-quality SCs you can optimize partial shade handling by going to a 1:1 ratio, rather than worrying about serial vs parallel.

And take full advantage of the great deals on big grid-tie panels rather than messing with lots of little ones.

[john61ct]

Originally Posted by CatNewBee

More smaller controller add up installation costs and also losses for the transformation.

I have no idea where you get the latter, and I have not found the former to be a significant issues, if at all

Originally Posted by CatNewBee

Usually the larger more expensive ones are also more sophisticated and better than the cheap ones.

Again, not in thos case, the only one I can think of with Victron is dedicated sense wire for temp comp vs integral.

Originally Posted by CatNewBee

So as always, it depends. You can find 150V / 100A MPPT's for 600..700$ It looks expensive, but you would also need 7-8 of the 15A to reach the same power.

And completely lose out on the optimum shade handling, which is rarely a minor issue on sailboats. Total output gains from the *same* wattage inputs of going 1:1 from that *plus* more effective use of MPPT at higher voltage can easily be 30%+

Can also give better panel placement flexibility, reducing windage issues and keep thing looking nice.

My intention isn't convincing you, it's your boat your money.

Presenting information for consideration by others they may not have known.

And addressing the OP question on the non-importance of counting cells, once you go to more modern and flexible SC options.

[admiralslater]

I

Could not agree more regarding separate sc per panel. We are in the trade winds at the moment and the most southery pannel routinely puts about 300 more watts in the bank each day as it sees the morning sun first . If the pannels where linked I would lose that every day . Yesturday I put in 3.5 kilowatts

Originally Posted by john61ct

No. The max volts is Voc rating, which by definition will never be seen IRL. Whether it's 40V or 60V is irrelevant to the wattage rating.
The 15A is not an upper limit for panel input, Victron cares about the max volts, going well over on amps is fine wrt the SC, and the watts specs are for final **charging output** maximum.

Yes in rare peak conditions some of the potential watts of overpanelling will be wasted, (why I said "if you got a great deal") but in normal use the average output is much greater than sticking to the 220-250w number.

My point was optimizing the $100 SC investment, spending as much on a couple anemic 100W 18V panels would be a relative waste of its potential.

And going to a much more expensive SC just to capture a very occasional few extra wH even more wasteful, put that money where it will do more good!

My biggest point, which is rarely seen, is with such inexpensive top-quality SCs you can optimize partial shade handling by going to a 1:1 ratio, rather than worrying about serial vs parallel.

And take full advantage of the great deals on big grid-tie panels rather than messing with lots of little ones.

[hellosailor]

And in exactly which far flung outpost of empire is "U" considered an abbreviation for uoltage?

I've never seen in in CE, UL, EU, ISO, or Japanese literature or spec sheets. Where should I expect to find it in current use? (No pun intended, but quotes from the 1800's ain't gonna do it.)

[john61ct]

Looked at hundreds of panel spec sheets certainly never seen it there.

But I'm willing to accept it's used by some more academic/theoretical references.

Even if it's a typo, NBD.

Sometimes precise terminology is very important, other times the overall relative quality of information helps guide who you listen to and who you should take with a pound of salt.

[MartinR]

Originally Posted by hellosailor

And in exactly which far flung outpost of empire is "U" considered an abbreviation for uoltage?

I've never seen in in CE, UL, EU, ISO, or Japanese literature or spec sheets. Where should I expect to find it in current use? (No pun intended, but quotes from the 1800's ain't gonna do it.)

Just to complicate things.

In all scientific literature: U for voltage (electric potential), I for current. But unit is still Volt. For example: U=100V You can hardly write V=15,5V So this is not completely interchangeable. Unit for current (I) is A of course. Ump=Impulse Voltage, unit for this is Volt.

So U and Ump are variables, their unit being V. Using U and Ump is correct in the text. Y

[noelex 77]

Originally Posted by john61ct

A large single 220-250W panel would get the most from that, up to 300W if you found a real bargain.

On a 12v system a 15A controller is too small for the kind of wattages you are quoting. Most boat installations have limited room to install panels. Therefore it is important not to just look at performance/price ratio, but to also take into account the performance/unit area ratio. Even if just looking at the performance/price ratio you need to take into account the total cost. Installing the mounting system and even cables circuit breakers etc means the added cost of installing a reasonably sized controller that will handle the maximium power the system can produce is generally sensible. I believe it is also better not to rely on the controllers self protect mechanism to limit the current to maximium the controller can manage especially when the controller is installed with limited ventilation in a hot location. These conditions are not an uncommon for a marine installation and MPPT controllers are not the most reliable electronics even when their specifications are not exceeded.

Originally Posted by john61ct

No. The max volts is Voc rating, which by definition will never be seen IRL.

It is important to realise that the Voc number quoted on the panel specs can, and very likely will, be seen and even exceeded in real life. The Voc is only quoted for STC conditions. Lower panel temperatures (< 25°C) and higher illumination (> 1000W/m2) than the STC conditions are likely at some stage. The better manufacturers also quote numbers for statistically poor panels (a tolerance of +5% or even +10% is usually quoted) . If you connect your panels in some conditions, a brief voltage at, or over the STC Voc will be seen.

Most controllers will be permanently damaged if their input voltage limit is exceeded even very briefly, so it is important to emphasise this point. It is a common and unfortunately an expensive mistake to make when choosing solar components.

Most manufacturers suggest assuming the Voc will be exceeded by 10 -15% at some stage, and this is good advice.

[noelex 77]

Originally Posted by admiralslater

I Could not agree more regarding separate sc per panel. We are in the trade winds at the moment and the most southery pannel routinely puts about 300 more watts in the bank each day as it sees the morning sun first . If the pannels where linked I would lose that every day . Yesturday I put in 3.5 kilowatts

There are some advantages installing a system with one solar controller per panel. In fact, that is what I just installed in our new system.

But in the situation you described (if I understand it correctly) the 300WHrs produced in the morning by the single panel that sees the morning sun would still be generated if the system had a single controller and the panels were wired in parallel.

3.5 KWHrs . On our first boat we thought we doing well when we upgraded our original solar system and we could average 12Ahs (about 150Whrs) .

[CatNewBee]

The only advantage is, you have more redundancy, if one fails.
Installation wise you must wire every single panel to the controller and then wire from the controller to your batteries with thicker wire.

With a parallel setup, you wire all panels in parallel to a distribution point (isolated with string diodes) and then you have only one cable down to the controller. Using high voltage panels in my example you have only 6Amps / panel at 60V, so 4mm² cable is sufficient to the distribution point. Having 5 Panels in parallel you need a cable for 30 Amps, so a 12mm² cable should be OK, the controller translates the power into 12V 100A, so you want some 35mm² to the battery, that should be nearby. With single controllers you have 5 times the holes for the cabling.

I do not want to convince you for a different design, with 2 panels you can use 2 controller, for a larger installation it's just the better design to keep it simple. Either way you would not lose any power. The shaded panels just deliver less Amps, while the exposed panel run full throttle. I would have chosen too 2 controller instead of one, but I don't found two suitable for 3 + 2 panels, it was cost wise more efficient to go for the big one (150/100) and it has BT communication built in for remote management.

[john61ct]

Originally Posted by noelex 77

Most manufacturers suggest assuming the Voc will be exceeded by 10 -15% at some stage, and this is good advice.

Which I very explicitly did in my post.

Pretty rare to come across single panels over 60Voc right? And voltage just needs to be say 40V, going higher doesn't actually buy greater output.

And again, the amp rating on Victron's model name has nothing to do with the max input limit from the panel.

From the POV of getting the most out of the 75/15, anything below 250w will be sub-optimal nearly all the time.

Overpanelling by another 50w does *not* pose any risk to the SC, does increase average output a lot in real-world condition, and again, I'm not suggesting spending more money doing so.

[john61ct]

Going to higher voltages reduces wire thickness required from the panels, which is where the distance is.

The SC should be quite close to the target bank.

[CatNewBee]

The victron controllers have two parameters for the current and the voltage:

Maximum input current / maximum input voltage
maximum charge current / output voltage

Input current is in general lower then output.

the 75/15 means maximum input Voltage 75V, maximum output current 15A.
The other parameters are in the data sheet.