New small system

cptdondo · August 2019

I'm looking to build a small system. 300w of solar and a charge controller. I'd like to be able to add a second 300w panel later.

I am having a hard time finding a controller that doesn't cost an arm and a leg that can handle that range.

I don't mind using PWM as for a system that small I'm not sure that it would make a huge difference.

Any suggestions for a controller?

BB. · August 2019

As always, details matter... What Vmp/Imp/Pmp are the panels rated?

PWM will work find with Vmp~18 volt panels and array (typically around 140 Watts max, although there are some ~200 Watt Vmp~18 volt panels occasionally available).

Almost anything else, really needs a MPPT type charge controller. Also, if your panels are a longer distance from the charge controller, a "higher voltage" array and MPPT controller makes these distances less expensive to support (much smaller AWG cable needed).

140 Watt panels are easy for a single person to move around/install/etc.

200+ Watt panels, you really need two people to move them around safely (especially if on roof/windy conditions).

Do you want anything else (remote communications, etc.)?

An alternative is to install a second 300 Watt array and PWM controller (two or more controllers are fine if wired to one battery bank).

-Bill

Photowhit · August 2019

Yet another alternative is to use 72 cell 300+ watt panels (most are) which are in the correct range, 35-40 volts) to use a PWM charge controller to charge a 24 volt system. This only 'sucks' if you need 12 volts or adverse to shopping on line for a 24 volt inverter.

https://www.solar-electric.com/samlex-pure-sine-wave-inverter-pst-600-24.html

Also I hope the 300 watt panel is local. A single panel may cost more to ship to you than the cost of the panel it's self!

There are some pretty good cheap PWM charge controllers made by Y-Solar, or for $33 you can buy a 30 amp Renogy PWM CC through Amazon (though it might well be made by Y-solar as well);

https://www.amazon.com/dp/B07G1PL1B9/ref=twister_B07G4DGMPJ

cptdondo · August 2019

I was looking at the 300W panels as they they the best bang for the buck. But I looked up the freight - EEEK!

I’m going to be in Arizona in November so I may just pick those up in person.

So right now the thought is that I get one of these:

https://www.solar-electric.com/rec310tp2m-twinpeak2-310-watt-monocrystalline-solar-panel.html

And this controller:

https://www.solar-electric.com/victron-energy-smartsolar-mppt-100-20-charge-controller.html

Unless I need to step up to a 30 amp controller. But the way I understand it I can put two of those panels in series or in parallel and still be within the parameters of the controller.

Photowhit · August 2019

cptdondo said:
Unless I need to step up to a 30 amp controller. But the way I understand it I can put two of those panels in series or in parallel and still be within the parameters of the controller.

Yes-No and dang, this is a pain to explain...

Yes this will work for 1 panel, The panel you choose is not a 60 cell or a 72 cell panel, but actually 2 - 60 cell panels on one piece of glass. So it works, for that controller. BUT that controller will likely not work for other 300+ watt panels which are mostly 72 cell panels and may exceed the limits on voltage, in strings of 2, on cold days due to higher VOC.

As to using 2 panels with MPT type charge controllers. MPPT (Maximum Power Point Tracking)uses the total wattage of the panels and down converts to incoming power to a voltage best suited to charge your battery bank. Charge controllers are rated on output. so a 310 watt panel charging a 12 volt battery bank, through a MPPT charge controller, can see up to about 310 ÷ 14.5 (charging voltage for a 12 volt battery bank) = 21.3 amps. So a 20 amp wouldn't work for 2 but might for 2. They do make a 30 amp and a 50amp version though.

In the real world solar panels are rated at STC (Standard Test Conditions) but are more likely to produce closer to Normal Operating Cell Temperature values (NOCT). REC publishes the NOCT on their spec sheet as 232 watts for the 310 watt panel(NOCT runs about 75% for most panels)

https://www.solarelectricsupply.com/fileuploader/download/download/?d=0&file=custom/upload/REC-TwinPeak-2-Mono-solar-panel-datasheet.pdf

So you might just squeeze 2 panels on a 30 amp MPPT charge controller and still be cost effective.
232 x 2 = 464 watts
464watts ÷ 14.5volts = 32 amps so a bit more than it's rated, I think Victron can be over paneled a bit, but worth checking.

You might move up to a 40 or 50 watt MPPT charge controller. I think Renogy makes an inexpensive 40 amp mppt.

cptdondo · August 2019

Thanks for the detailed explanation. That actually makes sense. I have a lot of roof real estate (I am working on an old ambulance and the roof is 12x8’, flat as a board, with 1/4” aluminum) so I can put whatever I want up there.

So on that Victron it has a rating of 100V for the panels. Would it be possible to run 2 panels in series on a 30 amp Victron?

I’m just trying to understand all of this.

BB. · August 2019

Depends on your low temperatures (and in RV, you can, in theory be anywhere). If you are in way below freezing weather, I would suggest Vmp-array to not be over ~69 Volts... If you are getting close or over, then "you have to do the math".

Higher end MPPT controllers are something like 145 to 150 Volts Vpanel max, and you can run a Vmp~100 volts (Standard Conditions/aka marketing numbers).

Midnite is nice because they have a shutdown mode (hypervolt TM) that allows them to shut down at Vpanel-max to Vpanel-max+battery bus voltage... Ie., a 24 volt battery bank, V-hypervolt range is 150-174 volts (controller is in "safe shutdown mode").

https://www.solar-electric.com/midnite-solar-kid-mppt-solar-charge-controller-black.html

Before you fall in love with any particular controller--Read the specifications and reviews. See what works best for you (I suggest staying away from fan cooled controllers and inverters--They can be very noisy in closed areas during day/operation).

To calculate Voc-cold and Vmp-hot for your array, some controller mfg. have online or downloaded programs to do the calculations. Here is Midnite's:

http://www.midnitesolar.com/sizingTool_kid/index.php (Kid, the 30 amp family)
http://www.midnitesolar.com/sizingTool/index.php (Classic, the larger MPPT controller family)

-Bill

Photowhit · August 2019

cptdondo said:
So on that Victron it has a rating of 100V for the panels. Would it be possible to run 2 panels in series on a 30 amp Victron?

I think so, the things you are looking for are if the VOC will exceed the limits on the coldest day of the year.

...and if you would be okay 'over paneling' the charge controller, as your array would be capable and expected to produce 32 amps or more in ideal conditions. I didn't look up or read through the manual, Victron makes quality products and I think you are okay. There is an axiom that 'loads grow' you might just go ahead and get a 50 amp charge controller and be prepared for 3 panels. You could run 2 in parallel with 'y' or branch connectors, They aren't going a long distance to the charge controller if on a van;
Image result for mc4 branch connector

If flat mounted on the roof, the panels won't be in ideal position very often. Not sure what you are taking when you visit Arizona, but I'd make 2 suggestions, check to see if they have inventory and let them know you are coming and check the size if driving something other than the ambulance. They will be similar in size to these 310-330 watt panels on my Scion XB;

cptdondo · August 2019

So the coldest weather I would normally expect is around 0*F - although I may be doing an Arctic trip and temps will drop to -50*C. But for planning purposes I’m going with 0*F.

What I’m hearing is a 50 amp controller, two of those panels in parallel, and I should be OK?

PNW_Steve · August 2019

I just put a 300 watt panel on my 5er. I used a Renogy Rover that cost a bit over $100 shipped. I picked this one because it will handle a second panel with ease.

I have been on the road for 10 days and it is working to my satisfaction. I have had shore power occasionally. Before solar I had to run my generator several hours a day when I am off grid. This is working out to be my first trip, in the four years I have owned this right, that I had zero generator run time.

Photowhit · August 2019

cptdondo said:

What I’m hearing is a 50 amp controller, two of those panels in parallel, and I should be OK?

Cake!

New small system

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