Solar panels - sizing wiring for voltage drop

[T1 Terry]

With a lead acid battery 12.2v isn't going to be doing any charging, 13.8v for a trickle charge, 14.4v to 14.8v to get any serious charging happening. That's what needs to be available, not what the terminal voltage will be until the battery reaches 80% SOC. Now start to bring in the Vmp losses for every degree over 25 deg C, in realistic world cell operating temps the Vmp will be down around 15v, doesn't leave much room for voltage drop any where else does it, so the charging rate drops because the available solar voltage to battery terminal voltage differential has dropped.
Try the same experiment with a power supply acting as the solar panel and dial the voltage down and watch what happens to the current, open circuit voltage, not battery terminal voltage.
Remember, the solar panel doesn't have to charge the battery, there will be no current build up if there is resistance any where between the panel and the battery, the current simply doesn't flow with no ill effects to the panel, voltage differential is the only thing causing the battery to charge from the solar panel.

T1 Terry

[galacticair]

Thanks everyone for the detailed responses and healthy debate! Very informative discussion as far as I'm concerned.

In mid-day fairly strong sun today, I saw about 5-5.5A max go into the charge controller (according to the Steca PWM charge controller display), and the charge controller was outputting the same into the batteries at 13.9V (float mode setting, with 1x 220Ah AGM battery) until mid-afternoon, at which point it started regulating down the charge a lot (started the day in mid 90% charge range, day was intermittently cloudy and sunny). No shade on panel during that reading.

I can't read the actual voltage coming from the panel without unwiring, but so far the amps results seem consistent with the general losses observed with PWM as the battery pulls the panel down below Vmp: 125W / 13.9V = 5.7A (vs. nominal panel rating: 125W = 17.5V VMP x 7.2A IMP). So a 20% loss that comes mainly from PWM regulation, not the wires... This is consistent with Foggysail and Noelex's comments, if I understand those correctly.

Reading the responses, it seems the big uncertainty is what will happen with clouds. I'm definitely concerned that the high voltage drop will let the panels fall under battery voltage in cloudy weather, just when you want maximum efficiency... I figure in clouds the voltage drop will fall significantly below the theoretical maximum (lower amps), so hopefully <10% (vs 16-20% with IMP of 7amps based on Noelex's calculations).

I guess I'll have to look at the Steca regulator in cloudy weather to see if it cycles from high Amps to zero Amps when the clouds roll by (instead of falling from high Amps to lower Amps in the more usual case where the effective voltage from panel to controller remains higher than battery voltage). Sounds hard to quantify just how high the losses will be in the cloud case...

Thanks again everyone for your thoughtful contributions!

[noelex 77]

Originally Posted by foggysail

OK--- what did I overlook?

You are making some calculations which is good, but there are a few flaws. The devil is In the detail.

Vmp is 17.5 (not 17.6), but this is measured with a cell temperature of 25c
Typicall cell temperature when producing 7A will be 40-45c. This means the actual Vmp will be around 16v.
You allowed 0.3v for the controller so we are at 15.7v, but we also have some voltage drop at all of the connections.
We also have voltage drop from localised shading.

However the bottom line is if the battery voltage under charge is only 12.2v undersized wiring will have very little impact with no shading. The loss will only be a few percent. However, these sort of voltages will only be seen if the battery is under a reasonably heavy discharge.

Typical battery voltage when charging are muh higher Average battery voltages under charge are over 14v Once the "Vmp" voltage at the battery after accounting for these losses is below the battery voltage the power output is reduced considerably.
Above this level the power loss is much less than the % loss in the wire would suggest, below this level it much higher.

If we use 14 v we only have 1.7v to allow for the voltage loss in the wiring and connections assuming the panel is performing at peak output, all cells are in full sun, up to specs, with no shading. Approaching absorption voltage the leeway drops further.

All these examples are for non MPPT regulators the losses for MPPT will be higher.

[noelex 77]

Glad to see we have not lost you

5.5A @13.9v is a bit low and suggests your system is under performing.
With a PWM regulator you should be approaching 7.2A under ideal conditions with perhaps high 6s being a normal good result.
However the results of undersized wiring with PWM regulators is very erratic depending battery voltage so even these sort of numbers do not mean there are not significant losses at other times. Undersized wiring will produce large, or small, losses with not much in between.

Note if you are regulating the results should be ignored.

The times when undersized wiring will have the greatest impact is with high battery voltages, high temperatures and localised spots of shade (not generalised shade from a cloud)

If you can lead a second temporary wire in addition to the wiring that is already in place it would be worthwhile to demonstrate the improvement. You can then you can decide if its worth making a permanent change.

[hellosailor]

Being non-metric I just have to ask, does "25mm2" really mean what it says? 25mm squared, as opposed to 25 square millimeters" ?

25mm2 would be 625 square millimeters, in conventional YnGlitch. An one-inch square busbar, damned heavy cable.

[foggysail]

Originally Posted by noelex 77

You are making some calculations which is good, but there are a few flaws. The devil is In the detail.

Vmp is 17.5 (not 17.6), but this is measured with a cell temperature of 25c
Typicall cell temperature when producing 7A will be 40-45c. This means the actual Vmp will be around 16v.
You allowed 0.3v for the controller so we are at 15.7v, but we also have some voltage drop at all of the connections.
We also have voltage drop from localised shading.

However the bottom line is if the battery voltage under charge is only 12.2v undersized wiring will have very little impact with no shading. The loss will only be a few percent. However, these sort of voltages will only be seen if the battery is under a reasonably heavy discharge.

Typical battery voltage when charging are muh higher Average battery voltages under charge are over 14v Once the "Vmp" voltage at the battery after accounting for these losses is below the battery voltage the power output is reduced considerably.
Above this level the power loss is much less than the % loss in the wire would suggest, below this level it much higher.

If we use 14 v we only have 1.7v to allow for the voltage loss in the wiring and connections assuming the panel is performing at peak output, all cells are in full sun, up to specs, with no shading. Approaching absorption voltage the leeway drops further.

All these examples are for non MPPT regulators the losses for MPPT will be higher.

Yes, panel temperature along with the type of regulator do make a difference. My analysis restricted things to 25C and of course a PWM as I mentioned.

Yes, my 12.2v example is below 14 but at 14 the battery is getting close to full charge and will begin to minimize charge acceptance. Now the main reason for going into all the details even assuming the conditions that I outlined in the beginning was to show the minor affect that wiring sizes have.

The affect that NOCT of 47C changes the Immp from 7.4 to 5.94 or 1.46 amperes, almost a 20% reduction that will further reduce the wiring voltage drop. Next Vmpp changes from 17.6..or if you wish 17.5 to 15.5 Vmpp under the same 47C conditions.

For a battery of 14v plus 0.3 PWM switch drop gives a delta between the panel Vmpp and the battery/switch combo of 1.2v. The wire resistance for this condition is 1.2/5.94 = 0.2 ohms. Now this is important! Wire length of 110' of #12 wire.......which most think too small.....has a resistance of only 0.0175 ohms which is below that which would impede a 14v battery from accepting the Kyocra KC139 panel's Impp output current of 5.94 amperes at NOCT of 45C.

Too much attention is paid to wire sizes. Sure, if your running parallel panels and I am, that needs to be considered. I have a total wire run (to and from) of about 40-50' and I use #10. Actually, I started running installing 2 #10 home runs. Made the first one, the second one I ran out of wire half way there. The result now is I have 1 1/2 home runs from the panels to the PWM.

Some time ago I read in one of the forums, I don't think it was Crusiers, where a guy recommended to someone that he should use 00 wire from his panels to the batteries!

[goboatingnow]

Originally Posted by hellosailor

Being non-metric I just have to ask, does "25mm2" really mean what it says? 25mm squared, as opposed to 25 square millimeters" ?

25mm2 would be 625 square millimeters, in conventional YnGlitch. An one-inch square busbar, damned heavy cable.

mm2 is a wire gauge For example 25mm2 is approx 10mm in diameter , by the way , 5mm x 5mm is 25mm2 !!

Dave

[goboatingnow]

Just to restate my concerns

If the OP was to use 4mm2 ( #11) there would be in excess of 2 volts drop in that circuit , Vmp is likely to be in reality in full sun around 15-15.5 add the voltage drop plus losses in the PWM convertor( and dropout limits) and the likely hood is that that the battery will never get fully charged. ( or more correctly take a long time to charge ) ( charge doesnt fall off immediately at absorption cutoff )

Not to mention that in excess of 10 watts will be lost in I squared r losses which is a lot for a cable that may be in a conduit

Given a balance of cost , I would say the minimum is #5 , around 16mm2 to allow for acceptable voltage Loss of about 0.6v , #6-7 at a push .

Other then that you are going to need SEPIC converters at the charger end

A better idea would be higher voltage panels , mppt or get the distance down !

Dave

[foggysail]

Originally Posted by goboatingnow

Just to restate my concerns

If the OP was to use 4mm2 ( #11) there would be in excess of 2 volts drop in that circuit , Vmp is likely to be in reality in full sun around 15-15.5 add the voltage drop plus losses in the PWM convertor( and dropout limits) and the likely hood is that that the battery will never get fully charged. ( or more correctly take a long time to charge ) ( charge doesnt fall off immediately at absorption cutoff )

Not to mention that in excess of 10 watts will be lost in I squared r losses which is a lot for a cable that may be in a conduit

Given a balance of cost , I would say the minimum is #5 , around 16mm2 to allow for acceptable voltage Loss of about 0.6v , #6 at a push .

Other then that you are going to need SEPIC converters at the charger end

A better idea would be higher voltage panels , mppt or get the distance down !

Dave

Dave-- Are you sure of your figures? Imp = 7.2? Wire run 110 feet of #11 CU?

I calculate 110' of #11 wire to have a resistance of 0.139 ohms. Dissipation is (7.2^2)(0.139) = 7.2 watts or 0.13 watts/ft if two wires together in conduit. Voltage drop I get (0.139)(7.2) = 1. And yes, 110 feet is a long run!

Foggy

[goboatingnow]

Originally Posted by foggysail

Dave-- Are you sure of your figures? Imp = 7.2? Wire run 110 feet of #11 CU?

I calculate 110' of #11 wire to have a resistance of 0.139 ohms. Dissipation is (7.2^2)(0.139) = 7.2 watts or 0.13 watts/ft if two wires together in conduit. Voltage drop I get (0.139)(7.2) = 1. And yes, 110 feet is a long run!

Foggy

Well actually I used data for 4mm2 , which is in between 11and 12 and the tables I have are showing .23 ohms to .25 for 33metre giving 1.7 to 1.89 , And I derated for 40 degrees and run in a conduit. This is multi strand un-tinned thick wall PVC, this excluded any terminator resistance or derating for poor installation etc

Dave

[goboatingnow]

Originally Posted by goboatingnow

Well actually I used data for 4mm2 , which is in between 11and 12 and the tables I have are showing .23 ohms to .25 for 33metre giving 1.7 to 1.89 , And I derated for 40 degrees and run in a conduit. This is multi strand un-tinned thick wall PVC, this excluded any terminator resistance or derating for poor installation etc

Dave

Opps sorry , your right foggy , used data from one row away. , yes a total round trip resistance of 0.17 woud be correct giving 1.27 ( for multi stranded un tinned 4mm2 per IS 8135 class 5 ). And 9.5 watts loss. ( which is too much ) all at 7.5 A

So now I'd say 10mm2 would be a good compromise giving .5v drop and 3.5 W heat loss

These figures not derated for 40 degrees C and conduit run, but for free air , 20 degrees C. , so a little derating might be advisable
Dave

[noelex 77]

Originally Posted by foggysail

Yes, panel temperature along with the type of regulator do make a difference. My analysis restricted things to 25C and of course a PWM as I mentioned.

Yes, but you cannot use a cell temperature of 25C. Cell temperatures will be nothing like this in real life. There is no point doing calculations and reaching conclusions from unrealistic data.

Originally Posted by foggysail

The affect that NOCT of 47C changes the Immp from 7.4 to 5.94 or 1.46 amperes, almost a 20% reduction that will further reduce the wiring voltage drop. Next Vmpp changes from 17.6..or if you wish 17.5 to 15.5 Vmpp under the same 47C conditions.

Much better. Now we have realistic a Vmp of 15.5v

Originally Posted by foggysail

Now this is important! Wire length of 110' of #12 wire.......which most think too small.....has a resistance of only 0.0175 ohms which is below that which would impede a 14v battery from accepting the Kyocra KC139 panel's Impp output current of 5.94 amperes at NOCT of 45C.

So ignoring the voltage drop in all the connections, the voltage drop over the switches, fuses etc you are saying the Vmp will be exceeded, with this wire, when the battery voltage is above 14v.
When the battery voltages exceeds the Vmp the power output starts to be severely reduced. If Vsc was to be exceeded the output would be 0.

We still have not considered the significant effects of localised shading, plus the other factors mentioned above, but your calculations are already showing the solar panel output will be unacceptably reduced. Using a wire gauge where the the Vmp is exceeded at a battery voltage of 14v is very poor and will lead to inadequate performance as I, and others, have been saying.

Have a look at the almost vertical slope of the curve and the rapid drop in delivered current when Vmp is exceeded. (Vmp is the point on the graph at the peak of the knee, about where the kneecap would be)

[foggysail]

Originally Posted by noelex 77

Yes, but you cannot use a cell temperature of 25C. Cell temperatures will be nothing like this in real life. There is no point doing calculations and reaching conclusions from unrealistic data.


Much better. Now we have realistic a Vmp of 15.5v



So ignoring the voltage drop in all the connections, the voltage drop over the switches, fuses etc you are saying the Vmp will be exceeded, with this wire, when the battery voltage is above 14v.
When the battery voltages exceeds the Vmp the power output starts to be severely reduced. If Vsc was to be exceeded the output would be 0.

We still have not considered the significant effects of localised shading, plus the other factors mentioned above, but your calculations are already showing the solar panel output will be unacceptably reduced. Using a wire gauge where the the Vmp is exceeded at a battery voltage of 14v is very poor and will lead to inadequate performance as I, and others, have been saying.

Have a look at the almost vertical slope of the curve and the rapid drop in delivered current when Vmp is exceeded. (Vmp is the point on the graph at the peak of the knee, about where the kneecap would be)

Sure........... your quoted post above has high merit. But you overlook what it was I attempted to explain which specifically is "too much importance is placed on wire sizes!" I am not sitting at my keyboard recommending that solar users wire their panels with #18 or even #12 wire. My intent was to show that even with #12 wire with a total length of 110' could perform well in most cases. And I also want to point out that in most cases wire run lengths are much shorter. Even Dave (Goboatingnow) put emphasis on reducing the 110' wire run length for the OP's application.

So on shorter run lengths #12 could possibly meet all performance expectations in many, probably most applications. Personally, I recommend at least #10 minimum because I usually over kill stuff like this. I am also not in the camp of recommending 00 wire as I remember one once recommended. I also believe you are placing too much emphasis on connection voltage drops.

From my my earlier post #15 in this thread where my data came from Kyocera's KC130TM specifications:

"Now notice that this is for Mpp and for battery charging who cares what the wiring drop is if it is charged with a current source? This is true as long as the battery terminal voltage plus all the voltage drops do not EXCEED Vmpp at which point the panel's current avalanches downwards."

As your attached performance curves indicate.

[foggysail]

Originally Posted by goboatingnow

Opps sorry , your right foggy , used data from one row away. , yes a total round trip resistance of 0.17 woud be correct giving 1.27 ( for multi stranded un tinned 4mm2 per IS 8135 class 5 ). And 9.5 watts loss. ( which is too much ) all at 7.5 A

So now I'd say 10mm2 would be a good compromise giving .5v drop and 3.5 W heat loss

These figures not derated for 40 degrees C and conduit run, but for free air , 20 degrees C. , so a little derating might be advisable
Dave

Yeah Dave, I quoted just plain wire resistance in my simple example instead of multi-stranded untinned.

[sailorboy1]

you guys have some side bet on who can make this the most complex?