Simple Solar For A Simpleton

[VirtualVagabond]

Originally Posted by Jimbo485

Parallel, Vic. With an individual breaker (not a switch) for each. It makes it simpler to use medium diam cable from each panel to an intersection point with the other panel. From here to the batteries you need heavy cable. We use 4 AWG, but you have to size yours for your situation.

I can't do a diagram, but the positive from each panel needs to have the breaker. The two positive cables join to each other after their respective breakers. We use a battery post in the cockpit and the breakers are immediately before this point. You might install it on the ceiling of your doghouse? The two negatives go straight to the negative post. From these two posts, you run the heavy cable directly to your batteries.

Most importantly, you need to have some way of measuring the voltage FROM THE COCKPIT, so you can turn off one or both panels when necessary. If your nav instruments cannot provide this info in the cockpit, you will need to install a voltmeter across the two posts. Or simply buy a cheap multimeter and "store" it in between these two posts where you can see the voltage.

If you leave the boat, turn the panels off and let the MPPT and your other panels handle everything. You will get in the habit, when in the cockpit, of naturally looking around at the wind, waves, boats dragging down on you, sail trim etc and part of that will be a glance at the voltage during the day. It is not hard, but needs to be part of your routine so that you don't overcharge your batteries.

Thanks Jimbo, but I'm going to need a bit of help here to make sure I've got it.

Your discription of how to connect it all is clear and makes sense, but...

What exactly is a breaker?
Ditto for battery post?

When you turn off the panels, do you use the breaker? It's sounding like a very close cousin to a 'switch' to me

Vic

[atoll]

Originally Posted by VirtualVagabond

Thanks Jimbo, but I'm going to need a bit of help here to make sure I've got it.

Your discription of how to connect it all is clear and makes sense, but...

What exactly is a breaker?
Ditto for battery post?

When you turn off the panels, do you use the breaker? It's sounding like a very close cousin to a 'switch' to me

Vic

a breaker as opposed to a switch has an amp rating and will trip when this rating is reached,ie in the case of a short ,if your panels are rated at say 30 amps you need a "breaker" that is 40-50 amps so as only to trip if there is a catastaupic failure like a dead short.

if using a simple switch,a fuse needs to be fitted.

again cant reccomend more that you read this website for info in laymans terms.
Coleman Air Breakers, Switches and More

the battery posts are the pos and neg terminals on a battery,be very careful on wire sizing and switches as you are dealing with high amperage and fire is a very real risk with undersized components

[Dsanduril]

Vic,

As Atoll says, circuit breaker and switch have the same function with your hand, but the circuit breaker will also open automatically when the current exceeds its rating. You probably have a bunch in your DC panel. West Marine Breakers (not an endorsement, just a picture)

As to the "post" mentioned by Jimbo, I think he is just indicating to run a single heavy cable (+ and -) from the batteries to the panel location, then connect the panels to that cable.

Having said all that, my recommendation is that you don't really need a breaker, a switch will do, but if you can get a marine breaker near the same price then go ahead and use one. Again IMO, you really do need either a breaker or a fuse very close to wherever your cables connect to the battery(ies).

In the solar situation the panels become essentially inactive if they are disconnected. On the other hand, a short in the wiring will drain your batteries. And, as you know from starter motors, batteries are quite capable of delivering a giant current when they can (as in a short). If your breakers are near the panels, and there is short in the wire between the breaker and battery, the breaker will do nothing to interrupt the current. Thus the requirement that you install something near the battery. For your two 140W panels this could be something along the lines of a 30A breaker or fuse, if you wire them separately then 15A on each one.

Ideal situation, put in a breaker near the panels and near the battery(ies) that way you protect things from both ends, since either device can be a source. But, if it's one or the other, again IMO, far more critical to have over current protection near the battery.

[Dsanduril]

On further reflection, there really is no point in putting a circuit breaker in-line with single solar panel. The operating current and short circuit current of a single panel are essentially the same, for the 255W panel posted above the operating current at MPP is 8.15A, the short circuit current is 8.66A. You'll never find a circuit breaker with that kind of discrimination (at least not that you can afford). Far more critical to have something at the battery end, where the short circuit current is in the 100s or 1000s of Amps.

Thoughts?

[foggysail]

Originally Posted by Dsanduril

If you connect a 2A panel in series with a 6A panel the whole string will operate at 2A.

Not sure about that. As I understand solar panel construction, they use bypass diodes across the cells. If there are bypass diodes, the higher current panel will prevail but the voltage sum will be less than the two panels in series. The topology model as I see it is more complicated than just two current sources connected in series.

[Dsanduril]

Not sure about that. As I understand solar panel construction, they use bypass diodes across the cells. If there are bypass diodes, the higher current panel will prevail but the voltage sum will be less than the two panels in series. The topology model as I see it is more complicated than just two current sources connected in series.

Panels are constructed of individual cells, which are connected in series or parallel (or both). Bypass diodes are usually placed around one or more cells (commonly more) in a panel so that if one or more of a group of cells are shaded the current can bypass those cells and continue to flow. Basically you are taking the group out of the series string, which means that voltage drops.

If your panels have internal bypass diodes then it is possible for some of the current from the higher current panel to flow through the bypass diodes in the lower current panel. This essentially connects the panels in parallel, with the additional problem that you get voltage drop across the diode(s). You can also install whole panel bypass diodes, which have the same overall effect, but a smaller voltage drop.

Is it better than the simple model I proposed, yes, but not by much. If you think that shading of one panel as compared to the other will occur on anything more than an occasional basis you are better off in parallel, where you can easily get all of the possible power from the higher performing panel and still get all of the available power from the lower performing one.

This disadvantage to working in parallel is that current goes up quickly, and most things (including many of the MPPT controllers) are sized based on current.

[Dsanduril]

I borrowed this image from elsewhere on the internet.

Series connected panels with bypass diodes:

Base Case - You get rated panel current and 2x rated panel voltage (2 panel string for simplicity).

Case 1 - Shading of one or more cells in group 1-18. In this case you will get full current from group 19-36, and full output from the other panel. In this case you will be better connected in series, you will get full current and 1.5x panel voltage - voltage drop of one diode (~0.5V).

Case 2 - Shading of cells in both group 1 and group 2. You could get full current from the unshaded panel, and 1x voltage - 2x diode losses (so ~1V). Better in parallel in this case.

Parallel connected panels:

Base Case - you get 2x panel current and 1x voltage.

Case 1 - In this case you will likely get nothing from the panel with shaded cells, it will not be able to get sufficient voltage to match the voltage output of the other panel. You essentially have an 18-cell panel with Voc about half the other panel. Better in series in this case.

Case 2 - The shaded panel is essentially out of service, the operating panel can provide full power to the system.

The cases are way simplified, they assume two cell groups/module (a common configuration in 36-cell panels), two panels per string, and hard shading that will prevent cell output. Real world cases are far more complicated.

[Jimbo485]

Vic,

Atoll answered your question about a breaker.

The term battery post was probably used incorrectly by me. What I meant to describe was a large bolt in the cockpit or your doghouse ceiling where the 2 two medium cables come in from the negative of each panel and from where one large cable goes to the battery negative. You can use battery terminals to attach the cables to each bolt (where I said battery post). Same for the positives.

[tartansail]

Originally Posted by noelex 77

This is a good discussion of MPPT technology.
I was pleased Dsanduril mentioned the voltage drop that solar panels experience when operating in the real world where cell temperatures are well above 25C. This is often forgotten when mathematically determining the gain from MPPT.

Factors that have not been mentioned are the inefficiencies MPPT has in converting the voltages and the inherent self consumption of the MPPT tracking circuitry.

It has been suggested in this thread the gain for MPPT on a boat is in the range of 10-20% on average. I think this is a reasonable figure although personally I think the average gain is a bit lower at 5-15%.

It is also important to note this is only with the very best MPPT controlers. Cheap controlers with poor tracking are much worse and they can loose out to simpler non MPPT controler.

Talk of a 50% gain is misleading. This sort of advantage will not occur unless the panel is incorrect for the instalation (Say using a 60 cell panel on a 12v system without MPPT for example.)

The information on this thread is too good to let it die!

So, I've decided to go with an MPPT and parallel panels. Given the need to adjust absorption voltages to the requirements of the batteries -- which seem to be 14.4 - 15volts for LA -- it makes sense to get a controller with the ability to adjust absorption and which has temperature compensation. Genasun, which is what is typically paired with the Solbian panels by the distributor, is not adjustable. You recommended Rogue earlier which is certainly better priced than Morningstar. Is there a reliable resource (other than the views of CF posters, which I pay attention to, but would like to complement) on the strengths and weaknesses of the various MPPT controllers?
Thanks!

[noelex 77]

Originally Posted by tartansail

. You recommended Rogue earlier which is certainly better priced than Morningstar. Is there a reliable resource (other than the views of CF posters, which I pay attention to, but would like to complement) on the strengths and weaknesses of the various MPPT controllers?
Thanks!

Unfortunatly there is not a good review site of controlers, which is a pity as he performance varies signifficantly. Contary to the popular view that all MPPT regulators have similar technology.

This discussion of the Rogue controler on a solar forum contains some independent reviews.

Rogue MPT-3024 - Page 24

[tartansail]

Originally Posted by noelex 77

Unfortunatly there is not a good review site of controlers, which is a pity as he performance varies signifficantly. Contary to the popular view that all MPPT regulators have similar technology.

This discussion of the Rogue controler on a solar forum contains some independent reviews.

Rogue MPT-3024 - Page 24

Thanks. Sounds like a task for Practical Sailor or some such.

[VirtualVagabond]

This thread has been fantastic... thanks all.

Breakers = trip switches Now I understand

We're making up the brackets and trying to work out how to bolt through the doghouse and run the wires so Sandy will still cook meals for me and keep the bed warm at night. She knows it all has to happen. Just doesn't want to see it...

Vic

[Jimbo485]

Cheap and simple with the brackets means some aluminium angle, rivets and screws. Screw one side of the angles to the top of the doghouse, with sikaflex on the screws, slot the panels in between the two pieces of aluminium and pop rivet the panels to the angle.

You can position the panels so they overhang the front of the doghouse a little to ensure decent ventilation beneath the panels. Alternatively, spacers to keep the panels a few cm above the doghouse.

Sikaflex is your mate when running the cables thru the doghouse roof.

[DeepFrz]

VB, don't screw anything into your coach roof. Through bolt and follow the instructions on MainSails site about sealing the core with strengthened epoxy.

[VirtualVagabond]

I know it's been a little while since I got the panels and controller. They are in place and I'm about to start wiring them up.

To get the ideal wire size, do I take the length of run from davits to controller... about 45 ft, or back again too, ie 90ft?

There are wires already in place by the forward thinking PO, but only #12 which seems hopelessly inadequate. Trying to run #4 would be an impossible task.

The controller is only about 10ft wire length to the batteries so running #4 for that bit will be easy.

Vic