Adding Additional Panels

Ako · February 2023

Some months ago I had 4 x 460 Watt panels fitted with a total output of 1840 Watts controlled by a MPPT that shows it can handle 2000 watts however the most I have ever seen from the panels is 1640 Watts , this is ample for normal days , I go into absorption by mid morning and Float by mid afternoon .

During bad weather I am just a little short of what's needed to cover my basic loads so I need to squeeze as much as I can to avoid starting a generator so considering extra 360 Watts , not a lot but im hoping the additional panels would add to what we already get but not sure if it would be compatible with the existing panels as although they are Mono also they are a different make and size , I would al be exceeding what is shown for the 150/70 MPPT which is 2000 Watts , the additional panels would give me 2200 watts max , Voltage wise I have a 20% safety factor and also I would be within the current that the 70 Amps the controller is rated for .

The existing layout will only allow me to add 2 more panels physical up to a maximum of 200 Watts , there is no room for any other options .

I have posted the existing and proposed layout in hope that someone more knowledgeable than me could tell me if it would work or if I have overlooked something that would make it impracticable or damage my existing set up and if possible suggest an alternative method of wiring .

BB. · February 2023

Looking at current (Imp)... Estimate:

460 Watts / 41.9 Vmp = 10.98 Amps Imp (est)
180 Watts / 10.1 Vmp = 9.42 Amps Imp (est)

Usually suggest that Imp for series panels be within 5% to 10% of each other... This is getting a bit far apart (10.98/9.42=1.17) or ~17% different...

Just to compare... The Watts of your array with 4 panels and Vmp/Imp of those panels, vs 6 panels with "reduced" current of the 180 Watt panels (not exact, but lets see what happens):

4x 460 Watts * 1.0 Imp current rating = 1,800 Watts Parray-std
460 Watts * 9.42 Amps * 1/10.98 amps (reduced Imp of 180 watt series panels) = 1,579 Watt panels with reduced Imp current
1,579 Watt (of 460 Watt panels) + 2*180Watt (full Imp panels) = 1,939 Watt "hybrid" array STD output
1,939 W - 1,800 W = 139 Watt increase by adding 2x 180 Watt panels (adding 360 Watts of series panels)

The above is a quick guess... You probably will get some more power than the above math indicates because Vmp of the 460 Watt panels will rise a bit with reduced Imp current... But there is a good chance you will not gain near as much power vs adding a second MPPT charge controller for the 2x180 Watt panels (138W/360W=0.39)... Or only getting ~40% of the new 2x180 Watt panels worth of power if added to existing array and single MPPT controller.

Your Voc-array-cold looks OK (assuming Andalusia Spain and does not fall below 0C at your place--If top of mount/far north cold climates, Voc-array-cold could possibly be an issue with too high of Voc-cold for controller).

Your controller with "optimum" over paneling, would be:

70 amps * 28.8 volts charging * 1/0.77 panel+controller derating = 2,618 Watt array "optimal" over paneling...

My suggestions:

Probably too much difference in Imp for the two sets of panels--Not enough "new power" to make up for loss by low Imp with existing panels.
Find a pair ~180 Watt panel with Imp closer to 10.98 amps (ideally +/-5%, possibly +/-10% difference in Imp)
Get a pair of ~180 Watt panels (Vmp=18/24/30/36 volts etc.) and put them in series with a second MPPT controller connected to your battery bus in parallel with the first controller and get the "full ~360 Watt Pmp-std added power of the two new panels).

My guesses...

-Bill

Ako · February 2023

It does get cold here in the winter but in my 20 years i have never seen 0c , 2c is about as low as i have ever seen but that's rare , 8c is far more common .

Unfortunately the only option i have to increase input is with additional panels , adding Charge Controllers is not physically possible as it running another cable from the Array to the equipment .

If it were possible i would be happy to turn on extra panels only on the days when the weathers bad but i know that's not practical so i have to make do with a generator for the odd hour or so to make up for the shortfall even though we only require a little more than we get from the array on bad days

Your estimate of IMP on the 4 panels i have now is bang on 10.98 Amps BB . I have found the same company selling 200 watt panels with an imp closer to that at 11.06 .

Rated power (Pmax): 200W
Maximum voltage (Vmp): 18.10V
Maximum voltage Current (Imp): 11.06
Open circuit voltage (Voc): 20.25V
Current short-circuit current (Isc): 12.68A
Maximum system voltage: 1000VDC System voltage: 1000VDC

I have googled and find conflicting advice and information . Am i correct in thinking that the apart from the total voltage needing to be well under the rated 150 volts and the amps being no more than the 70 amps that the watts are not so much a problem as the output from an array is never the maximum it can produce and if it does and it exceeds the Charge Controllers limits that it only means it will be clipped and not do any harm .

BB. · February 2023

A pair of 200 Watt panels with Imp~11.06 would be perfect:

10.98/11.06= 0.993 or 0.7% difference

That is pretty much an exact match (panels are usually spec'ed to +/- 2% to 3% of ratings).

The Isc-array rating is 70 Amps max (Isc of the two parallel strings is roughly 2x 13 Isc = 26 amps for array... So that is fine:

https://www.solar-electric.com/lib/wind-sun/Victron_MPPT_Controller_spec_150-45_to_150-100.pdf

"Good Quality" (and I would suggest Victron are good quality controllers) MPPT controllers will "clip" (safely, part of normal operation) their output current to "rated output current". And they should also reduce their output current if the controller overheats (hot ambient temperatures, poor air circulation, etc.).

PWM controllers (pulse wave modulation) controllers are simply "on/off" switches and have no way to limit max/peak current safely. There the maximum Imp from array to battery bank (usually 1.25x safety factor). PWM cannot "safely/reliably" "clip" output current.

-Bill

Ako · February 2023

Thank you BB for the time you spent providing me with a comprehensive explanation but now im confused, last night i was pleased and relived that the panels could be added to my system so thin morning i want to order them of the dealer only to be told its a bad idea and would not work i would not gain anything but louse a lot of my existing input .

I have little confidence in the tech opinion of the dealership as they have given me misleading information in the past that errors on favouring them by selling me things sales , they show so much self interest in fact that so i would normally discount what they say but they were so adamant . They say it would only with with a second MPPT for the 2 additional smaller panels as they are a different Wattage and Voltage then the existing ones so connecting them in series would drag the others down to either the lower voltage of the smaller ones or the Amps of the smaller ones .

I ordered the 200 Watt panels and look forward to telling them that they were wrong AGAIN .

BB. · February 2023

Ako, I am certainly not perfect--But I do not see anything wrong here.

Note that every solar panel is a bunch of 0.5 volt cells (Vmp-cell) in series. As long as the current "matches" in series, and the overall string/array matches the input voltage/current of the MPPT controller, everything should be good.

The big thing for you now is to make sure you have the + and - terminals of the y panels correctly wired in series. Your panels are at "different ends" of the strings, so it would be easy to mix up the connections on the roof.

Also, double check the polarity of the MC4 connectors between existing and new panels (or what ever connectors are used on the new panels) are the same polarity as the existing panels. + & - sometimes are reversed (not all mfg. seem to follow the same pin and socket rules--And I would not be surprised if an error happened during mfg. either). And that (if you use the connectors) they are the same MC4/Whatever type--Again, there are a wide variation out there between brands.

And you may want to have a friend help, if you can. A 200 Watt panel is not small and a gust of wind or a slip on the roof could take it out of your hands during installation.

Good luck,
-Bill

Ako · February 2023

Thanks again for the advice and warnings Bill , i wil certianly follow them and double chech the conectors now for polarity rather than assume the manifacturer has then corect . Existing panels used only Black cable to both the positive and Negative but the new ones have Red on the positive but now i intent to check for myself just incase .

Ako · February 2023

Yesterday i was amazed to see my 1840 watts of solar panels produced 1910 watts and over 64 amps . It was a nice clear day and around 12c and i had far more appliences on than i would ever normaly use , mostly electric tools in the garage , the inverter was humming as it was right up to its limit but the panels produced well above what they should produce , Bill said i was doing well seeing 1640 watts from them so i hope i havnt done any harm .

Panels are 4 x 460 Watts / 41.9 Vmp = 10.98 Amps Imp each and i had a voltage of 29.5 so its stil far more than i was told i should expect from them .

BB. · February 2023

Happy to hear your system is performing well. In general, the current (Imp) is proportional to the amount of sunlight. The Vmp is proportional to the temperature. Cool panels (cold weather, wind) helps a lot and can increase Vmp to "standard" or even better Vmp voltage.

I would suspect that your panels (and MPPT controller) are doing fine. Increased voltage output of the panels should cause excessive stress on the panels. And since current does not increase--You get more power (P=V*I) because of the increase in Vmp.

Another issue to be aware of... Most controllers seem to give power numbers that are 0-10% over "actual laboratory grade" power meter readings. Some brands like Schneider seem to be pretty accurate (at least in times past). Others seem to lean more towards the 10% more "optimistic" values. One designer (semi-retired poster Solar Guppy) had reported that even his several brands of lab grade equipment did not agree "exactly" either.

If you ever get a good RMS (root mean square) reading AC+DC Current Clamp DMM (digital multi-meter) you can verify the readings of the MPPT controller (or not)...

https://www.fluke.com/en-us/learn/blog/electrical/what-is-true-rms

The only "damage" that I believe that you can cause (other than rocks through the glass, wind damage, time/heat, etc.) is having a shaded panel on a sunny day. My concern is that the bypass diodes that route current around "shaded cells" are not operating in the best of conditions. Bypassing diodes get hot, and these diodes are mounted under sun, glass, and plastic backing.... I would suggest trying to avoid all shading/partial shading of panels (branches, pipes, chimneys, etc.) during middle of day. This will help keep the diodes cool(er). If there is no shading on a panel, then the diode(s) are not conducting current.

Also, keep your electronics (MPPT solar controller, AC inverter, etc.) in a cool area (no direct sun) with lots of open air circulation. High temperatures and temperature cycling (cool at night, hot during day will stress components and can lead to early life failure. The old engineering rule of thumb... For every 10C over room temperature, "life" is cut by 1/2 (and every 10C drop, life is extended by 2x). And thermal cycling is a killer too (hot/cold/hot/cold).

Otherwise, I would not worry about your system. It is running fine.

-Bill

Ako · February 2023

Thanks Bill from what you say i think i have most of the points covered except for voltage i , was fortunate to have plenty of open space around the house so i mounted the panels where there is nothing around to ever shade them and my equipment is in our garage so cold for half the year and venelated the other half and i have used the Cerbo relay to turn on computer fans when equipment reaches 30c .

My only concern is what you said about high voltage " Increased voltage output of the panels should cause excessive stress on the panels " the voltage im seeing at the MPPT daily is closer to the VOC which the data sheet shows as 49.7 ( 99.4 in series ) and i have always been getting in the high 94 to 103 every day sinse the panels were fitted 2 months ago , until now i thought that was what i should be seeing but now i know its much to high i dont know how its possible to reduce it . The VMP is far lower at 41.9 ( 83.8 in series ) .

What would you advise .

BB. · February 2023

OPPS!

I would suspect that your panels (and MPPT controller) are doing fine. Increased voltage output of the panels should NOT (Corrected. -BB) cause excessive stress on the panels. And since current does not increase--You get more power (P=V*I) because of the increase in Vmp.

Better?

-Bill

B.

Ako · February 2023

Yes , so much better BB , Thank you , i should sleep better tonight than i did last night

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