MPPSolar PCM-3012 Solar Reg. & Panel Config

Submitted: Saturday, Apr 13, 2013 at 12:51
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Ok so i've been learing as i go here and I'm getting the (click on link) MPPSolar PCM-3012

It has a maximum AMP input of 18A and 300w.

Ideally my understanding would be to get 300W of panels which is pretty obvious. The difficulty is whether to get 12v or 24v panels. My understanding is that the best option would be 2 x 24v panels in parallel because i would get approx 10amps plus about 35-6 volts, and the excess volts can be converted into amps.

Is this the best option to ensure in good light and bad that the voltage output is in the optimum range for the mppt?

I have been looking for 150w 24v panels but can't find reasonable price ones so thinking of doing 2x75 in series and then parallel to another 2x75 in series. I would prefer just 2x150w panels if possible.

Next question - if i just got 2 x 150w 12v panels like these here, then i would have 18v @ 16.5A but i'm worried about the overall voltage if they end up in shade and below the minimum (15v) required for the MPPT controller... how many extra amps could i expect the mppt to produce with 2 x 150w @ 18v?

Which scenario would result in better overall charging capacity?


experts descend... (I do realise i have another thread going but thought this was a little more concise)
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Reply By: Member - Frank P (NSW) - Saturday, Apr 13, 2013 at 13:32

Saturday, Apr 13, 2013 at 13:32
Sleeprequired,

You have to find out if that regulator can take nominal 24V panels to charge a 12V system.

Because they list two models, one for 12V and one for 24V, I suspect it can't, but the suspicion needs to be verified. There are many, many solar regulators that have that restriction.

I have also found out through my own search for the same type of setup that you're looking at, that if a reg has that restriction, it also means you may not be able to series up 12V panels. I don't understand the reasons why, but that has been the answer from a number of regulator manufacturers or distributors.

Some of them will do both 12 and 24 volt systems and auto-detect the voltage, but still have the restriction of nominal 12V panels for a 12V system and nominal 24V panels for a 24V system.

Ask the manufacturer or distributor. If you're lucky and get a "yes" then you can either use 24V panels in parallel or 12V panels in series.

If you use panels in parallel, ideally you should put blocking diodes in the output of each panel to stop backfeed from a lit panel to a shaded one. This stops wasteage of solar output from the lit panel and protects cells in the shaded panel against heating from the back current.

If you connect panels in series, you will need bypass diodes on each panel. Without them, if one panel gets shaded then the output of all panels in the series string will be zero. Your panels may already have them in the junction box.

As per one of your possibilities, I have 2 x 60watt in series and parallelled that with another 2 x 60 watt in series. I can place the pairs wide apart to chase the sun, but inevitably one pair will get shade while the other is lit. All four panels have internal bypass diodes built in. I added the blocking diodes in the output to each after noticing discolouration of some cells due to overheating from backfeed.

The best way to do it, if you can, is to use the 24V panels or the 12s in series. Have the reg close to the battery and long leads, if required, between the panels and the reg. The higher voltage of the 24s in parallel or the 12s in series reduces the current in the panel-to-reg cables, and this keeps losses to a minimum.

Cheers

Frank
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Follow Up By: Sleeprequired - Saturday, Apr 13, 2013 at 13:53

Saturday, Apr 13, 2013 at 13:53
Thanks Frank,

I have been talking to the supplier and showed him the panel i have, and the panel i was looking at getting, and he suggested wiring in series to combine the voltage so that side is ok for sure.

given I have a 130w 12v folding setup already, and was looking at a 200w 12V panel, he said hooking those in parallel might end up reducing the voltage below the operating range of the mppt controller, and that while hooking them up in series would only produce approximately 260 watts and 8amps, it would allow the mppt to convert the excess voltage to amps.

the panel i WAS going to add is a 200w here, and then the one i currently own is here. I know it has an MPPT controller but i don't really trust it. Is there a way i can find out for sure? If it was i'd just hook a 250w panel up to the new controller and be done with it.

There were a few other questions in there now that we've sorted out the MPPT i'm looking at in terms of capabilities



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Follow Up By: Sleeprequired - Saturday, Apr 13, 2013 at 13:56

Saturday, Apr 13, 2013 at 13:56
specifically about the voltage from 2 x parallel 150w 12v in shade dipping below the minimum for the mppt controller
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Follow Up By: Sleeprequired - Saturday, Apr 13, 2013 at 13:57

Saturday, Apr 13, 2013 at 13:57
also do those panels have the bypass diodes built in?
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Follow Up By: Member - Frank P (NSW) - Saturday, Apr 13, 2013 at 22:06

Saturday, Apr 13, 2013 at 22:06
"specifically about the voltage from 2 x parallel 150w 12v in shade dipping below the minimum for the mppt controller"

The Vmp (max power voltage) of 12V panels is typically around 18V, plus or minus. They vary a bit. Vmp is the max power voltage of the panels and the MPPT reg will try to hold the panels at that voltage and take the resulting current. That is Maximum Power Point Tracking. Your proposed MPPT reg will track the Vmp if the panel voltage it sees is in the range 15V to 37V (in a 12V system). More sun, more amps. Less sun, fewer amps - but all at 18V. That's simplistic, but about how it works.

If you have two such panels in parallel the Vmp will be 18V. The MPPT reg will try to holding the combined panel voltage at 18V. If the panels are equally lit, the amps into the reg will be double what one panel could provide.

If one of those panels is shaded, then there are two possibilities.

1 If you have no blocking diodes the MPPT reg will try to hold the panel voltage at Vmp, 18V. Some of the resulting current from the lit panel will backfeed into the shaded panel and be dispersed as heat in the shaded panel. THE REMAINDER will go to the reg and charge the battery.

2 If you DO have blocking diodes the MPPT reg will try to hold the panel voltage at Vmp, 18V. The lit panel cannot backfeed to the shaded panel, so ALL ITS OUTPUT will go to the reg and charge the battery. That is what you want.

Both of these scenarios assume that there is enough sun to allow the MPPT reg to keep the panels at their Vmp and take the resulting current.

If there is not enough sunlight on one or both panels to keep the voltage up to Vmp, then the system will be unable to track the maximum power point and will revert to a less efficient method. What that is depends on the design of the reg.

Cheers
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Follow Up By: Member - Frank P (NSW) - Saturday, Apr 13, 2013 at 22:45

Saturday, Apr 13, 2013 at 22:45
"also do those panels have the bypass diodes built in?"

According to the product description the 140 watt panels on eBay that you linked to here have bypass diodes.

So does the 200 watt panel here.

I cannot tell if the panels you already own have bypass diodes, but I suspect they do. You would need to open the junction boxes and have a look.

Cheers

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Follow Up By: Member - Frank P (NSW) - Sunday, Apr 14, 2013 at 09:56

Sunday, Apr 14, 2013 at 09:56
Sleeprequired,

Just a couple of other things ...

Most people don't bother with blocking diodes in the output of panels in parallel. For two years I didn't have them but then noticed the discolouoration in some cells which, after some research, I figured was due to backfeed and heating so I decided to install them.

Also, the 18V I was working with above is just an example. The fold-up pair you have has a Vmp of 16.88. The 200 watt panels you're looking at have a Vmp of 18.7. The 140 watt panels your looking at have a Vmp of 18.0. All those values are within your controller's specs.

Ideally all panels should have the same specs. You can mix them up a bit but you will lose a bit of efficiency as the controller will settle on something that is not quite right for one or more of them.

I am not sure what your losses will be with a lowest to highest spread of nearly 2 volts (16.88 to 18.7, = 1.82) You will need to get advice on that. My panels are all mono like yours but have different Vmp's that are within half a volt of each other and they work ok. Out of 400 watts total I get 28 amps out of a 30 amp reg in bulk charging phase. But I'm not sure if a nearly 2 volt spread in Vmp would work as well.

If you can afford the loss of efficiency, then it may not be worth chasing. But if you need all those 300 watts you're planning, then it's something to consider.

Cheers


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Follow Up By: Sleeprequired - Sunday, Apr 14, 2013 at 12:57

Sunday, Apr 14, 2013 at 12:57
Hey guys thanks heaps for all the information. When I was speaking with the dude who was selling the controller he said that if I hook up in series I can expect 260w and if I hook them in parallel about 302 wtt.

His point though was that at 16,88v he thought it was flying a little close to the minimum and with shade and cable osses it could drop below the 15v bottom limit.

So much smarter to wire in series IF I keep the one I've got and add it to a 200v one.

Given I've dumped all this cash though I think I'll end up getting all new matching panels. I just wish I could find 150w 24v panels to get amps and voltage spot on.

So frank do you have 12v or 24v panels?

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Follow Up By: Member - Frank P (NSW) - Sunday, Apr 14, 2013 at 19:23

Sunday, Apr 14, 2013 at 19:23
Mate, I've got all 12V panels. 4 are fixed to the roof of the van (the roof is an odd shape and I have to have 4 small panels) and 4 are portable. If the van is in shade I use the portables. If not, I leave the portables packed up. I can use them all together in poor sun.

They are arranged in pairs, the two in each pair in series. Then all the pairs are parallelled into the controller. The controller is close to the battery bank.

Each seriesed pair has a Vmp of a bit under 36V. The Vmp's are within about half a volt of each other. The controller needs a minimum of 18V for the MPPT to work, so it's all good.

If you want the higher Vmp and you're buying new panels, get 4 x 75 watt, 12V panels and series/parallel them. As long as the reg can take that you should be ok.

Cheers


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Follow Up By: Sleeprequired - Tuesday, Apr 16, 2013 at 13:44

Tuesday, Apr 16, 2013 at 13:44
ok what i might have to do is actually hook up 2 x 150w in series. Reason being is the cost of mounting 4 panels is going to be significantly more than mounting 2 panels which is a practicality that i forgot.

They'll both be on the roof of the campervan and will generally have the same sunlight conditions.

In summer that might mean they're both shaded simply because i think it's probably better to be in the shade than running an air-con and being in the sun :).

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