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Old 25-11-2013, 05:11   #1
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How to choose MPPT size

I need some advice on buying a couple of MPPTs. We bought an older boat last year. It came equipped with a bunch of solar panels. It had two MPPT controllers, and one PWM controller. Previous owner explained that the PWM was for the smaller, old solar panels that came with the boat. He told me that if he had kept the boat instead of selling it, he had planned to re-do the hard top and get rid of the smaller panels as they didn't contribute much. What I have written so far is the sum total of my knowledge of this solar setup.

On the way home from the USA with our "new" old boat we were hit by lightning. Fried just about everything on the boat.

Once back in the Turks and Caicos, we had a local company rewire the batteries and engine rooms, with new heavy cables, new battery switches, etc. In the process, they just clipped off the leads to the MPPTs and threw them in a box with all the other junk they removed from the boat due to the lightning damage. Now the MPPTs are gone, and I don't really have much info about what they were.

I would like to add the solar panels back into the charging mix. Right now, our only charging source are the diesels with two new alternators.

Yesterday I put a meter on the three banks of solar panels. The wire leads are still there. In early afternoon sunlight, I was seeing outputs of 19, 21, and 27 volts from the three banks. Is there some way I can use this info to order a couple of MPPTs to hook my solar back up?

If that's not enough info to do it, can you tell me what I need to know to hook up these panels again? I do plan to junk the smaller panels.

This is a photo of the top of the boat when we bought it. I believe the six small panels in the center are the ones I was advised to dump. I have one more panel identical with the big one on the left, which has not been installed. I was told to just wire it in parallel with the other three of similar surface appearance. So I'll end up with four of the "blue" ones, and the four identical ones surrounding the six small dark ones.



Can someone please advise me on my next step? I'd like to order a couple MPPTs but don't know what size. I'm okay with basic wiring, just not real up to speed on solar just yet.
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Old 25-11-2013, 23:04   #2
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Re: How to choose MPPT size

Some posters with much greater knowledge than me advised me to read this guy. You can hire him for phone consultations as well. I did what he preached and it works.

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Old 26-11-2013, 01:02   #3
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Re: How to choose MPPT size

The first step is determine the sort of panels that you have and want to continue using.
The specifications are often on a sticker on the underside of the panel.
Failing this we need to do some detective work. Measuring the voltage of each panel in bright sunlight with no shadows will tell you the voltage of the panels.
The 27v you recorded from one suggests that they are high voltage panels that can only be used with a MPPT controller. Only panels of similar voltage can be connected together in parrallel so we need to know the results from each type of panel to know how to best wire them and if multiple controllers will be required.

It is also important to know the current output so you know what sized controller to buy. The easiest way to make an estimate of this is to measure the area the panel.

Look on the back first and see if the specs are there.
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Old 26-11-2013, 01:37   #4
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Re: How to choose MPPT size

What I did was to have separate charging for my start and house batteries.

I have two old 10W panels wired in parallel to an old controller to keep the start battery charged and 6 40 watt panels in three groups of two to a "MPPT" controller for the house batteries.

Each group of two 40W panels is wired together in series, then in parallel with the other groups of two 40W panels.

I brought a "MPPT" charger off ebay. I say "MPPT" as current advertisements no longer describe it as an MPPT controller and I would suspect it does not optimize the charge. However it does accept the higher voltage of the panels in series. It also has a voltage display of it's output that is useful in assessing how effectively it is working.

The two 10W panels are fused near the battery with an inline 3AG fuse and the 40W panels go to a circuit breaker before the battery.

If you measure the size of your existing solar panels and look at the pattern you should be able to find similar ones online (ebay?) and this could give you the approximate output of each of your panels.

Do make sure you use wire of a suitable size to connect it all up.
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Old 26-11-2013, 04:51   #5
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Re: How to choose MPPT size

They are already wired. The ends of the wires are within three feet of my battery bank, so that's okay. I don't need to run new wires.

Can I mix panels of different manufacturers and/or design? The outputs of any two panels are going to be different anyway, at any given time, correct? Shading, angle of the sun, differences in cells. I was wondering if it makes sense to wire these up to be port and starboard panel feeds, instead of grouping them by their manufacturer. The shade is always on one side or the other, meaning if I wired all the starboard batteries to one MPPT and the port side batteries to the other, one array would usually be in unobstructed sunlight.

If wiring in series, does it matter if the voltages of the panels in series are different? in that photo I posted, could I for example wire up the two blue panels and two of the square black panels on one side, and mirror that on the other side of the boat?

I don't have separate separate start and house batteries any longer. I have four big wet cell batteries, two in each hull, with switches that let me choose bank 1, 2, or both.

I'm still trying to get educated on the solar stuff here. You guys have been a lot of help so far. Thanks.
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Old 26-11-2013, 23:54   #6
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Re: How to choose MPPT size

If wired in parallel the voltages need to be the same (for MPPT the Vmp should be within 0.5v for best performance).
If wired in series the current output needs to be the same.

When these criterion are not met generally there is no damage, just reduced performance, but this can be dramatic if there is a considerable mismatch. We really need the specifications of the panels (from the sticker, or measured) to make meaningful suggestions concerning the best way to wire them up and if multiple controllers will be needed.
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Old 27-11-2013, 00:50   #7
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Re: How to choose MPPT size

There looks to be four different types of panels in your picture so if you are going to spend the not inconsiderable amount needed to buy a MPPT controller why not go the extra and buy new panels?

Or, rather than one expensive MPPT controller why not buy four cheaper ones that will take the higher voltage of each group of panels in series and then wire them all together at the battery?

It looks like there would be considerable shading so my opinion is that series connection may be better, provided the panels have suitable internal wiring. Other may have way better knowledge and experience than me.
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Old 27-11-2013, 01:59   #8
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Re: How to choose MPPT size

One other clue is the number of cells per panel
The panels with the blue corners look like 36 cell panels so these are likely to be nominal 12v panels (Vmp about 17v Voc about 21v) so will work with a PWM or MPPT controller

The silver sided panels are 48cell panels so will need an MPPT controller and it will need to be different to the panels with the blue corners (unless you are really lucky with the current output)
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Old 27-11-2013, 07:56   #9
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Re: How to choose MPPT size

As stated, I am dumping the six little panels in the middle of the photo. So I will have three different types to deal with. 2 of the big blue ones, 2 of the little blue ones, and the four identical dark gray ones. I'll be back working on the boat today and will remove a couple of them to see if I can find some kind of information on a label underneath. Thanks again for the help. I was considering just scrapping all of it and installing a small gen set.

I've also got a wind generator to sort out. It apparently can drain the batteries, although the manual says it has diodes in it. Probably another lightning bolt victim. But that's a whole different thread.

And thanks, again, for the help. It's taking a lot of time to sort this boat out. The solar is only one of the projects going on. Gotta love old boats. Lets you throw all your money into one hole. Simplifies things.
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Old 27-11-2013, 08:18   #10
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Re: How to choose MPPT size

There have been several informative threads here regarding solar power. I would first glean the knowledge already presented.
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Old 27-11-2013, 08:40   #11
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Re: How to choose MPPT size

I really don't see that it's any of your business.
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Old 27-11-2013, 08:45   #12
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Re: How to choose MPPT size

Not at all, hopefully saving you some time and leg work. I have benefited greatly from the different threads on solar power. Did not intend to offend.
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Old 27-11-2013, 08:49   #13
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Re: How to choose MPPT size

Sorry. That sounded pretty abrupt on my part. I've been reading threads on solar installations here for two days. None of them addressed this specific issue, a solar "newbie" with an inherited collection of various panels ending with bare wires, and no controllers.
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Old 27-11-2013, 08:54   #14
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Re: How to choose MPPT size

One of the ways I've found to learn about various subjects is to find the website of the equipment manufacturers, and download and read some of the manuals. Some of the earlier solar posts discuss the various different vendors, like, for example, Morningstar. You might give that a try. Good luck.
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Old 27-11-2013, 10:23   #15
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Re: How to choose MPPT size

A couple of observations:

1) The smaller panels in the middle look like the old harbor freight amorphous panels. Between the 4 of them you're probably getting 60 watts, so I concur that it makes sense to get rid of them.

2) For each type of panel you have we need to know both voltage and amperage. MPPT controllers are rated by both maximum voltage and maximum amperage. The easiest way to measure current is to short circuit a panel and use a clamp-on ammeter.

Once we know the voltage and current of each panel we should be able to come up with a wiring diagram for you.
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