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
JiminDenver wrote:
jrnymn7 wrote:
yep... that's one of my concerns.

On hot days, when Vmp is down and Isc is up, it makes sense to run parallel, thus taking advantage of the higher amps, and decreasing bucking from say 50v to 14+v, to 25v to 14+v, in my case.

Jim, you may want to consider power venting the cabinet with one or two small fans.

The only reason I bought the TS-MPPT-60 over the Mid nite was because the MS didn't have a fan to listen too.

I am going to rerun a test in parallel to see what kind of heat it produces. Should it still produce that kind of heat I will have no choice but to mount the inverter and controller in a box on the tongue and use AGM batteries inside the trailer.

Ah, so you have the controller inside the rig?

I went with the Tracer and mt50 remote, in order to avoid having to bring cabling into the camper... although I do like BFL's idea of setting it up to look like a 50's style music box, next to the dinette :)
BFL13
Explorer II
Jun 23, 2015
I kept my PWM Solar30 after going MPPT based on Mr Wiz's example to always have a spare controller on hand. Smk reminded us that the PWM controller can indeed do 24v panels IF the batts are also 24v.

I posted my test results using the PWM 24-24-- worked great.

So in an emergency where the MPPT controller has fried/died, you can put the batts in 24v mode, run the panels with the PWM controller and get 12v to the rig somehow.

On that, I have used a spare 12v battery during the day while the bank is in 24v mode getting recharged by solar while disconnected from the rig. Then around supper, I put the bank to 12v for the big draws and recharge the 12v spare with a trickle charger overnight off the inverter. Be easier to tap for 12 out of the 24 during the day and not carry the spare 12, but as PT says, this is not good for the batts in the long run.

For intentional 24v then you need a 24v inverter and a converter for the 12v.
jrnymn7
Explorer
Jun 23, 2015
BFL,

How would I tap 12v's off the 24v bank, in that case? The four 6's would be all in series, correct? I'm thinking, just wire all 4 in series, then take a pos and neg off two adjacent batterires? (EDIT, never mind, that would not work with 6v's.)

Or would I be best to keep a 24-12v bucker on hand?
2oldman
Explorer II
Jun 23, 2015
jrnymn7 wrote:
Would I need to keep a 24-12v bucker on hand?
yes
MrWizard
Moderator
Jun 23, 2015
jrnymn7 wrote:
red,

Does it not go without saying, as Isc goes up, so does Imp? If using pwm, available panel current would go up as the panel warms. If using mppt, the power curve shifts in favour of higher current, lower voltage. The mpp goes up and to the left on the knee, correct?

As solar radiance goes up voltage and current rise
As the panel heats up voltage and current drop
The best solar is on top of a cold mountain on a clear day
Second best is a cool clear spring day with panels pointed directly 90° with Sun
Warming does Not increase output, cooling does
Try this test take a panel and controller connect to a battery on a 90° day, point into sun read the panel voltage come back an hour later, load or no load the panel voltage will be less, now without removing the meter take a cool wet rag and wash the the panel face
Watch the voltage climb back up
Remember MPPT works with difference between battery charge level and panels , the lower the difference the less MPPT gain
BFL13
Explorer II
Jun 23, 2015
The panel's Isc goes up a bit with more panel heat but voltage goes down more so power goes down too.

With PWM you are getting current, so more is better. As long as the panel heating leaves you enough voltage overhead you are good.

With MPPT it doesn't matter what Isc does, since you are not using that. You are using panel output power (reduced by heating), bucking that, then using controller output divided by battery voltage to get whatever amps that comes to for charging the battery.
jrnymn7
Explorer
Jun 23, 2015
MrWizard wrote:
jrnymn7 wrote:
red,

Does it not go without saying, as Isc goes up, so does Imp? If using pwm, available panel current would go up as the panel warms. If using mppt, the power curve shifts in favour of higher current, lower voltage. The mpp goes up and to the left on the knee, correct?

As solar radiance goes up voltage and current rise
As the panel heats up voltage and current drop
The best solar is on top of a cold mountain on a clear day
Second best is a cool clear spring day with panels pointed directly 90° with Sun
Warming does Not increase output, cooling does
Try this test take a panel and controller connect to a battery on a 90° day, point into sun read the panel voltage come back an hour later, load or no load the panel voltage will be less, now without removing the meter take a cool wet rag and wash the the panel face
Watch the voltage climb back up
Remember MPPT works with difference between battery charge level and panels , the lower the difference the less MPPT gain

Please see page 5 of 5, Temp Comp

For the 24v panels I'm considering...Temperature coefficients of Isc: +0.06%/

Although it's a much smaller coefficient than that of Voc, it is positive, not negative/inverse.

I have seen well over 9a Isc from my 12v/140w poly's, with an Isc rating of 8.37, on a mild day, pointed.
jrnymn7
Explorer
Jun 23, 2015
BFL13 wrote:
The panel's Isc goes up a bit with more panel heat but voltage goes down more so power goes down too.

With PWM you are getting current, so more is better. As long as the panel heating leaves you enough voltage overhead you are good.

With MPPT it doesn't matter what Isc does, since you are not using that. You are using panel output power (reduced by heating), bucking that, then using controller output divided by battery voltage to get whatever amps that comes to for charging the battery.

But how about when the mppt controller is in pwm/abs? Is it still bucking, or simply passing thru something very close to Isc? (up to what the bank can accept)
MrWizard
Moderator
Jun 23, 2015
That is not because of increased power due to heat
Increased power over rated power is not uncommon with New quality panels
a year from now they will probably be putting out rated power Not increase above rated
JiminDenver
Explorer II
Jun 23, 2015
I would call Tracer and ask. The Eco-w doesn't say it will or wont clip amps but I have seen someone running a pair of 150ws in series and pushed it past its 20a limit. It stopped at 20.1a. It will also clip amps if it gets hot too.

I believe someone here uses a Rogue 30a MPPT controller with 490w, maybe they could chime in on how often they see it clipping amps.

I would over panel the MS in a heartbeat if I had the room. It would be great for lower light situations and let me do more earlier. It would also let me use higher watt items during the peak hours. It would be nice to see the full 60a the controller is capable of.

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