Best configuration for wiring panels

Rkg76 · February 2016

So the system components I have purchased over the last couple of years consist of the following:

Solar panel specs - Canadian Solar 200 Watt Panels (10 of them)

Vmp 28.9V
Imp 6.93A
Voc 36.2V
Isc 7.68A

Inverter MS 4448 PAE Magnum

Magnum 175 E-panel

Combiner box and breakers

ME Remote

Midnight classic 150

8 S-605 (468 amp hr) Rolls batteries

My question is around wiring the panels, is it best to use 9 of the 10 I have and wire them 3 in series or use all 10 and wire 2 in series? Or should I pick up a couple more and have 12 of them (so 4 sets of 3)??

I had quotes from 2 installers, one of them was going to use 9 panels and the other was going to use all 10...so it had me confused, so I have turned to you guys!

Thanks

mike95490 · February 2016

I'd go 3 panels in series, it will keep the heat in the controller. Add a 4th string if you want more power !

Vic · February 2016

Hi Rkg..,

Agree that strings of three PVs is really the only way to assure that you will have enough input voltage for that Classic to find a good Maximum Power Point.

Strings of two can mean that there will be insufficient input voltage, especially for Flooded batteries, or perhaps when the battery is cool, and the PVs are warm/hot.

Your PVs appear to be 60-Cell types, but with a Vmp, a bit on the low side, which can make strings of two even more marginal, IMO.

Good Luck with the new system. Vic

zoneblue · March 2016

Canadian Solar 200 Watt Panels (10 of them) Vmp 28.9V
Inverter MS 4448 PAE Magnum, Midnight classic 150, 8 S-605 (468 amp hr) Rolls batteries

My question is around wiring the panels, is it best to use 9 of the 10 I have and wire them 3 in series or use all 10 and wire 2 in series? Or should I pick up a couple more and have 12 of them (so 4 sets of 3)??

pairs of 60 cell panels will not charge any 48V bank correctly, so scotch that idea.

9 * 200W/58V*0.77= 24A = 0.05C... low C rate; CC just crawling along.
12 * 200W/58V*0.77= 32A = 0.07C... getting closer to the 0.1C that you want
15 * 200W/58V*0.77= 40A = 0.085C.. keep going
18 * 200W/58V*0.77= 48A = 0.1C... bingo.
24 * 200W/58V*0.77= 64A = 0.13C... thats probably your particular sweet spot as CC better utlised.

Odd size those batteries.

Rkg76 · March 2016

Ok, looks like I will need to add a couple of panels and at least make 4 strings of 3 in series. Is it ok to add 2 different panels that are roughly the same specs? or do I need the exact same panels in the series?

Also, this is primarily a weekend use only place so the batteries will generally have all week to charge up, the only thing running while not there during the week will be an energy star rated electric fridge.

Thanks

BB. · March 2016

For series panels--You want to match Imp to within 10% or better (i.e., 5.0 amps + 5.5 amps per panel).

For parallel connections, you want to match Vmp within 10% or better (i.e., 17.5 volt + 18.6 volt Vmp in series, not 18 volt and 35 volt panels in series).

It does does depend on charge controller type too (PWM vs MPPT) your final "acceptable" configuration.

-Bill

Rkg76 · March 2016

Bill, what will happen if the Imp is different by 18%...anything catastrophic??

BB. · March 2016

Yes--In theory, the bigger Imp panel puts 18% more current through the smaller rated panel and overheats wiring/cells to destruction. A worst case scenario. Practically speaking, that would only happen if there was a dead short out in the array somewhere--Something that we always want to be careful about is system/home safety.

In day to day operation, the panels will probably balance to a degree (the higher Imp panel will have lower output voltage/current because the smaller panel simply cannot supply that higher level of current).

It is just not good to put "mixed ratings" together on solar arrays--Outside of standard variation (5-10%)--Thinks can go wrong. Also, the farther out the ratings are, usually the worse the overall performance will be (easy 10% variation can reduce output by 10% of "name plate rating" very easily).

-Bill

vtmaps · March 2016

BB. said:

For parallel connections, you want to match Vmp within 10% or better (i.e., 17.5 volt + 18.6 volt Vmp in series, not 18 volt and 35 volt panels in series).

a little bit mixed up

BB. said:

Yes--In theory, the bigger Imp panel puts 18% more current through the smaller rated panel and overheats wiring/cells to destruction. A worst case scenario.

The series fuse rating of the panel with the lowest Imp in the string should determine the protection for that string. But hopefully a string with an 18% mismatch is just a thought experiment.

--vtMaps

Rkg76 · March 2016

Well...I have 10 panels that I purchased quite awhile back and I don't seem to be able to find the same panel anymore as it was discontinued by the manufacturer, so as I now need to go to 12 panels so I can group into series of 3 panels I need to find 2 more that I could match with the 1 that will be remaining...or I just go with 9 panels and have a slower charging rate...

BB. · March 2016

Yea... I probably need to think out the Series/Parallel connection descriptions a bit more clearly.

If 60 cell Vmp~30 volt panels... You can connect them in three in series x three strings in parallel for 9 total panels. That will work find for 48 volt battery bank and an MPPT type charge controller (with correct specifications). The one extra, just save it as a spare or sell on Craig's List.

You can always add 3x parallel strings of other Vmp~30 volt panels just fine up to the controller's limits (or add a second MPPT controller when it makes economic sense).

-Bill

Rkg76 · March 2016

BB. said:

You can always add 3x parallel strings of other Vmp~30 volt panels just fine up to the controller's limits (or add a second MPPT controller when it makes economic sense).

-Bill

Thanks Bill...so from what you say above it would be no problem to use 9 of the 10 identical panels I have wired 3 in series and 3 parallel groups and then just add 3 more panels of a different Vmp? I have a midnight Classic 150...

YehoshuaAgapao · March 2016

150V MPPT charge controllers on a 48V battery bank favor series sets of 3 60-cell panels. Each series string of 3 is hooked up to each other in parallel through a combiner box. 150V 60A MPPT charge controllers will easily handle 4 strings (12), and with your low wattage panels, you can probably fit 5 strings (15).

Ask the 10-panel installer if he is using a 600V MPPT charge controller, or possibly he wants to AC-couple the battery bank. Both these options are more expensive and AC-couple is not practical for off-grid. If he intends to wire 2 to a string, and don't know what you are talking about with 600V charge controllers, etc.. just go to the installer that wants to use 9.

YehoshuaAgapao · March 2016

If you will be mixing old/new panels with different specs, preferably have them on separate charge controllers. At least don't mix them in the same series string (current mismatch in a series worse than voltage mismatch in a parallel).

zoneblue · March 2016

Thanks Bill...so from what you say above it would be no problem to use 9 of the 10 identical panels I have wired 3 in series and 3 parallel groups and then just add 3 more panels of a different Vmp? I have a midnight Classic 150...

Repeat, your battery, and inverter need more solar. Parallel your existing 3S3P array with another 3S3P or 3s4P into the classic.

Rkg76 · March 2016

zoneblue said:

Repeat, your battery, and inverter need more solar. Parallel your existing 3S3P array with another 3S3P or 3s4P into the classic.

Would the fact that I will only use the place 2-3 days a week change your thinking that I need more panels...not in the budget to go and double the amount of panels I have right now anyways.

cow_rancher · March 2016

Rkg76 said:

zoneblue said:

Repeat, your battery, and inverter need more solar. Parallel your existing 3S3P array with another 3S3P or 3s4P into the classic.

Would the fact that I will only use the place 2-3 days a week change your thinking that I need more panels...not in the budget to go and double the amount of panels I have right now anyways.

I don't comment much here, but no... you need the same amount of panels to sustain your loads whether you are there one day a week or 7 days a week.

Rancher

Rkg76 · March 2016

Ok...thanks Zoneblue and Rancher...I guess for now I will maybe pick up 3 more panels and run 12 panels (3S4P) to start and see how that goes...

Rkg76 · March 2016

cow_rancher said:

I don't comment much here, but no... you need the same amount of panels to sustain your loads whether you are there one day a week or 7 days a week.

Rancher

The loads are much different when I am there vs not there though...i.e. I use a bunch of power over the weekend bringing the batteries down 50% DOD then doesn't the system have all week to bring the batteries back up, or are you guys saying I don't have enough with 1800 watts of panel to even bring the batteries back up on a 48v system??

vtmaps · March 2016

Weekend users can usually get by with less of a solar array. Rule of thumb is that batteries need between 5% and 13% charge rates. 1800 watts of panels can typically push 1350 watts into the batteries (if there are no loads). 1350 watts ÷ 58 volts (charging voltage) = 23.3 amps. That is just a 5% charge rate into a 468 ah battery, so you are within the limits of the rule of thumb.

Rules of thumb are general first approximations. The 5% end of the range is for situations like this, where the battery has several days to charge up without any day or night loads.

Most important is not to let the batteries get too far down on the weekends... batteries that are deeply discharged should be recharged quickly.

--vtMaps

YehoshuaAgapao · March 2016

I'd go with adding a string or two, even with weekend-only use - fill up the capacity of the existing inverter / charge control equipment - not too expensive

Rolls/Surrette handles deep discharge better than others (50-80% DoD vs Crown & Trojan), but they also have higher internal resistance and have higher capacity loss with heavy loads (rate of discharge). Shifting heavy loads (laundry, tools, etc..) to daytime along with the extra soloar will help alot if you are pushing the capacity limits on your batteries.

zoneblue · March 2016

Yeah bill always says that 0.05C is ok for weekenders. But you have to very careful with that. You get a week of nasty weather after a party weekend, and youve left a low bank low all week. 0.1C charge rate systems are always much happier.

westbranch · March 2016

The loads are much different when I am there vs not there though...i.e. I use a bunch of power over the weekend bringing the batteries down 50% DOD then doesn't the system have all week to bring the batteries back up, or are you guys saying I don't have enough with 1800 watts of panel to even bring the batteries back up on a 48v system??

that is "old thinking' from when lead was cheap and PV's VERY expensive.... trying to 'make do... now that PV is cheap and lead (or nay other comparable alternative) is expensive, treat the batteries with respect and keep them healthy and happy with a proper charge daily...!!

Best configuration for wiring panels

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