Derating 24v Solar Panels?

[jrnymn7]

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!

[BFL13]

I don't know all the ins and outs. I am just repeating what came out of a few threads this past winter on all this from the experts who actually know this.

There was mention of "impedance matching" on that demand thing. ISTR the same circuit that does the MPPT gets involved in choosing the voltage while in PWM too. All too complicated for me! Just use it as a "black box" with mysterious innards you don't need to know about 🙂

[jrnymn7]

BFL,

o.k., so no tracking, but still bucking, when in pwm mode, hmmm.

But what about this?

"The input amps to the controller with an MPPT controller (in MPPT or PWM mode) is whatever it comes to when you divide panel watts by "panel voltage" The "panel voltage" is whatever the controller chooses at the moment. In MPPT mode it will choose Vmp. In PWM mode it will choose a voltage (which will be higher than the Vmp for MPPT) that provides the amps to match the "demand" on the output of the controller."

...if the controller is not tracking in pwm mode, how does it "choose" a voltage? Or do you mean to say, it will simply allow panel voltage to match Vbatt + Rcirc, and then divide watts by that voltage for amps?

I've noticed that when there is a large drop in power (due to clouds), the controller will quite often continue/resume at a lower voltage than before the power drop, once the clouds have passed.

[jrnymn7]

MrWizard wrote:
i ran MPPT untill, i got a burned up terminal in Jan
then i remove the blue sky 2000e, and installed the Xantrex C40 PWM spare
which is what i am currently using
i have been shopping for another controller to be my spare

I did buy a JoyQ ? 20 amp controller $15.99 on amazon so i could sell the three 42w unisolar panels as a system, ready to install

those are the panels i removed to install the three New windy nation 100w poly panels
which increased my solar from 505 to near 700 watts
sometime this week i'm going to double up on the 8ga wire that is the feed from the panels to the controller

Ah, yes, you were running 12v panels thru mppt, as I've been doing recently. How did you have things wired back then? All the panels in series, I'm guessing? And now, in parallel?

[MrWizard]

i ran MPPT untill, i got a burned up terminal in Jan
then i remove the blue sky 2000e, and installed the Xantrex C40 PWM spare
which is what i am currently using
i have been shopping for another controller to be my spare

I did buy a JoyQ ? 20 amp controller $15.99 on amazon so i could sell the three 42w unisolar panels as a system, ready to install

those are the panels i removed to install the three New windy nation 100w poly panels
which increased my solar from 505 to near 700 watts
sometime this week i'm going to double up on the 8ga wire that is the feed from the panels to the controller

[BFL13]

jrnymn7 wrote:

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)

The buck converter in the MPPT controller is still in effect when the MPPT tracking is not on (only on in "Bulk")

The input amps to the controller with an MPPT controller (in MPPT or PWM mode) is whatever it comes to when you divide panel watts by "panel voltage" The "panel voltage" is whatever the controller chooses at the moment. In MPPT mode it will choose Vmp. In PWM mode it will choose a voltage (which will be higher than the Vmp for MPPT) that provides the amps to match the "demand" on the output of the controller.

The "demand" can be just battery charging or that plus loads, so you can still get lots more amps even though you are in Float (no MPPT) if you increase the demand above what the battery wants by adding load.

(You can run loads more than what the battery wants with any ordinary PWM controller too up to the panel's current output at the time. You can see this when the controller's "solar amps" readout is higher than the Trimetric's amps (the Tri shows only the amps the battery is taking)

The actual voltage the MPPT controller can pick in PWM mode is something less than the Voc you would measure if the panel were disconnected. That Voc is affected by panel heating. EG, I would see about 34v Voc with a panel rated 37 Voc on a typical day. This is a limit on "overhead"

[jrnymn7]

MrWizard wrote:
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

Understood... 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. And yes, watts are watts, but why not work with what the panels have to offer? What the hot panel loses in voltage, can be partially recovered in the way of increased amps, by the tracker. This further reduces the amount of bucking required, for the same given (albeit reduced) power in and out... i.e; a much higher % of the power going into the cc is in the form of amps.

[JiminDenver]

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.

[MrWizard]

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

[jrnymn7]

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)

[jrnymn7]

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.

[BFL13]

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.

[MrWizard]

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

[2oldman]

jrnymn7 wrote:
Would I need to keep a 24-12v bucker on hand?

yes

[jrnymn7]

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?

[BFL13]

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.