I'm considering running two 250w/24v poly panels; tracked/pointed during spring/fall, and flat during the summer; thru the Tracer 3215BN 30a mppt controller. I can also get 200w/24v mono panels, locally, but would much prefer poly, seeing as I need the panels to perform well in low light conditions, as is common during early spring/late fall. I have noticed when running two 140w/12v panels, tracked/pointed, thru the 30a Tracer, it is not uncommon to see 300w in and out, this time of year. That's an increase of 7% over their rating. So, 500w (2x 250w) + 7% = 535w.But I've also seen a 25% drop in power, at noon, going flat vs. tracked. So, 535w - 25% = 401w flat.On a 60% soc bank, charging voltage would begin in the low 13's range, say 13.2v. 401w / 13.2v = ~ 30a... the controllers max current rating. But I would prefer not to max out the controller, especially in hot summer conditions, so I would prefer to lower wattage, and thus max/peak output amps, during the summer months.Again, it's only during the peak summer period, say a month or so either side of the solstice, that concerns me. The rest of the year maxing out the controller will likely not be a problem, even when pointed.So I'm wondering if de-rating the panels would be the solution, and how I should go about doing so? Would it depend on series vs. parallel wiring, for example? Or would it depend on the number of diodes and/or cells, for instance?Your input is much appreciated!

The panels should derate themselves in the summer heat. If two 250s is too much at times, switch one of the 250s off?Do you even need the extra AH in the summer? (not so much furnace and lights on less time with longer days)Just get one 250 and another 140 12? IMO avoid 100w monos--low amps.I derated my 130w panel for a time so I could run it temporarily with a 105w limit controller. Just tape over one cell in a corner and it knocks the panel down to about half. That was enough for the job at hand (maintain batts while away from rig for a few days)

BFL,I slept on it, and while sitting down by the beach this morning, it became obvious I need to keep things as simple as possible... but without eliminating my tilt 'n' twirl plans. So I'm looking to go with the two 24v panels, only (which is what I would have done originally, if planning a rooftop install, from the get go). But I cannot find 24v/200w poly panels to meet the Tracers 390w/12v bank rating.Yes, I could simply go with a single panel, pointed, at times, but that would reduce peak amps to about 20a... less than what I'm seeing with the two 12v panels in series/mppt/pointed.Usually I won't need every ounce of energy I can squeeze out of a single 24v/250w panel, in summer, but there may be times when I need to replenish Ah's quickly, or during/after a/c use. At these times I'd like to get the most out of the system, without taxing the controller too hard.The panels I'm looking at are 60 cell... 6 rows of 10. My understanding is those 6 rows will be separated into 3 double rows of 10 (20 cells/section), by two diodes. So covering one cell would close down 20 cells, or 1/3 of the panel. Two 250w panels would therefore be de-rated to ~ 333w, for about 27a peak; perfect.What I don't know is how (external) wiring effects things, if at all? Do the diodes operate differently if the panels are wired in parallel vs. series?(I'll either keep the 12v system as a portable, or sell it.)

Regarding Heat derating:››› Electrical Characteristics at STC (Standard Test Conditions: Irradiance 1000w/m², Module Temperature 25°C, Air Mass 1.5):• Module Type: LDK250W Poly• Maximum Power at STC(Pmax): 250Wp• Maximum Power Voltage(Vmp): 30.30V• Maximum Power Current(Imp): 8.27A• Open-circuit Voltage (Voc): 37.7V• Short-circuit Current (Isc): 8.39A• Power Tolerance (%): 0-3• Maximum System Voltage: IEC EN:1000V/ UL: 600V• Module Efficiency (%): 15.50• Temperature coefficients of Pmax: -0.45%/?• Temperature coefficients of Voc: -0.32%/?• Temperature coefficients of Isc: +0.06%/?• Operating Temperature(?): -40?~+85?• Nominal operating cell temperature (NOCT): 45±2?Is it correct to say the above co-efficients are to be used at above 45C panel temp, not 25C? If so, that may not allow for very much heat DE-ration. And how hot do panels get in the summer, anyway?

• Nominal operating cell temperature (NOCT): 45±2?looks like that panel gets 25C warmer with 80% sun.The coefficients are used with temps different than 25C STChttp://pvcdrom.pveducation.org/MODULE/NOCT.htm

My Can Solar 60 cell 255w has three diodes. Check the owner's manual for which ones you are looking at. Anyway, you can play with how many cells to cover to get what.------------------EDIT Quick Test- 60 cell 255w panel- all in sun Isc 8.79, three cells one end covered (all on one side for half of 6) Isc still 8.77.As soon as the fourth along (next to middle of panel) covered Isc dropped to 4.4, then when fifth along (leaving only the corner one open, Isc was 0.2 and with all six of the end covered, still 0.2.You can get intermediate amounts above 4.4 by covering the first three and then part of the fourth.I haven't tried other combos, but obviously you can tinker till you get what you are looking for.--------------------------- Your requirements are changing and you should have the requirements clear before going to design. :)You haven't even done part of this summer yet on solar to learn what it can do or not, so maybe you will have more ideas by Fall. Meanwhile do some "masterful inactivity" ? What if you get a job at a shipyard in BC or NS? Anything can happen.

Derating 24v Solar Panels? | Good Sam Community

jrnymn7
Explorer
Jun 23, 2015
The specs say the Tracer has current limiting protection... but that could mean a few things, so I've fired off an email.
jrnymn7
Explorer
Jun 23, 2015
smk, I hope the Tracer qualifies as a 'good' controller :) If so, a great deal at $150 (incl remote monitor)
JiminDenver
Explorer II
Jun 23, 2015
jrnymn7 wrote:

Good point regarding using the solar to power loads. I don't usually think in terms of powering loads, just charging. Of course, I do realize solar is running loads all the time, in essence... just thru the bank first.

Yes, I'll try contacting ep solar, and see what they have to say. Maybe as long as I add a fan or two, it'll handle being maxed out.

The loads don't run through the bank first. The power out of the controller is there with no resistance unlike there is when the bank discharges. The only reason any power comes out of the bank is because the demand is higher than the output of the controller. Also I have started a system up, started a inverter running a light and then removed the battery. as long as the sun shown the light stayed lit.

That's why we have a A/C that pulls 450w, one of the burners on the cook top pulls 475w, a heater that pulls 400w and so on. The batteries are there for the short term big loads, over night and bad weather. Most days they are rarely out of float.
jrnymn7
Explorer
Jun 23, 2015
JiminDenver wrote:
jrnymn7 wrote:

Good point regarding using the solar to power loads. I don't usually think in terms of powering loads, just charging. Of course, I do realize solar is running loads all the time, in essence... just thru the bank first.

Yes, I'll try contacting ep solar, and see what they have to say. Maybe as long as I add a fan or two, it'll handle being maxed out.

The loads don't run through the bank first. The power out of the controller is there with no resistance unlike there is when the bank discharges. The only reason any power comes out of the bank is because the demand is higher than the output of the controller. Also I have started a system up, started a inverter running a light and then removed the battery. as long as the sun shown the light stayed lit.

That's why we have a A/C that pulls 450w, one of the burners on the cook top pulls 475w, a heater that pulls 400w and so on. The batteries are there for the short term big loads, over night and bad weather. Most days they are rarely out of float.

Are you saying you have some loads tied directly to the controller's 'load' terminals?
JiminDenver
Explorer II
Jun 23, 2015
jrnymn7 wrote:
smk, I hope the Tracer qualifies as a 'good' controller :) If so, a great deal at $150 (incl remote monitor)

Like the Eco-W, once you have proven it's functionality and reliability it will no longer be just a el cheapo controller. It will be a functional lower cost option and the more options the better.
JiminDenver
Explorer II
Jun 23, 2015
jrnymn7 wrote:

Are you saying you have some loads tied directly to the controller's 'load' terminals?

No never have used the load terminals. The power just skates across the battery terminals to the inverter or any load. There are fan, pump and other systems that use no battery at all. When the sun shines they work.
jrnymn7
Explorer
Jun 23, 2015
o.k., gotcha... I hadn't thought of it that way.
red31
Explorer
Jun 23, 2015
jrnymn7 wrote:
I'm just saying it would be better to run parallel/mppt, if and when the panels and controller are hot. Less bucking means higher efficiency, lower heat generation, and thus better overall performance.

Same as when not hot. 1% efficiency difference between 33v and 66v (12v system), same power. I'm sure the owner's manual graph ignores voltage drop ;)
JiminDenver
Explorer II
Jun 24, 2015
Actually I tested one 250w panel tracking and got 18 amps. That would have been 54 amps for the three panels in parallel. When I connected them all in series I only got 48 amps. The heavier wiring is worth it for the extra amps and cooler controller in my opinion.
BFL13
Explorer II
Jun 24, 2015
red31 wrote:
jrnymn7 wrote:
I'm just saying it would be better to run parallel/mppt, if and when the panels and controller are hot. Less bucking means higher efficiency, lower heat generation, and thus better overall performance.

Same as when not hot. 1% efficiency difference between 33v and 66v (12v system), same power. I'm sure the owner's manual graph ignores voltage drop ;)

24-24 is more efficient than 24-12 but then you have more "overhead" with 24-12 in low light that might get you more AH in a day? It is all scenario dependent it seems.

Just stick the darn thing out in the sun and you get what you get!
( Not that I would accept such advice :( )

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