Help with panel testing (Update with results)

[Bear in NM]

I finally picked up a second, foldable 100 watt panel to go with my fixed 135 glass panel. I want to do some testing this weekend with parallel and series connections with the mppt controller in my new trailer.

Does anyone have a tip on how to max out the amps/watts of the setup, short of draining a perfectly topped off battery?

Thanks,

Craig

 

[john61ct]

Put a dedicated controller on the unlike panel.

Suggest Victron 75/15, under $100, room for two more when yo want to expand.

 

[Bear in NM]

John,

Sorry, I was not clear. I want to just max out the amps/watts for testing between parallel and series in combining the panels for my given controller. I have meters to place in line to measure amps/watts, but all of my batteries are fully charged, and I do know that a full battery will not "tax" the components for a good evaluation of whether I would see more watts with parallel or series for the panels.

Craig

 

[dwh]

Put a load on it. Got an inverter? Make a pot of coffee.

 

[Bear in NM]

Ok guys, thanks for the giggles. I figured the battery drawn down might be the only reasonable solution. Maybe I'll throw one of my inverters with the coffee pot into the load terminal
of my solar controller :^)

I am figuring out that for a guy like me who likes to test, and who likes to control variables, all this solar stuff is loaded with so many variables that I need to be less anal.

Thanks,

Craig

 

[dwh]

Just put the inverter on the battery and plug something in. It'll draw all the solar can produce plus a little more from the battery. Run the battery down a few percent, no biggie.

 

[Bear in NM]

Yes, I was kidding about the load terminal of my controller. As a matter of fact, it was probably one of your posts last year that taught me that little tid bit. I saw your posts in the recent thread helping the member with his solar, and discussing mc4 connectors. I have all of the goodies needed to jump headlong into mc4, arriving tomorrow. Will be a fun weekend. Although my UPS driver will probably not find 50 feet of #10 fun.

Craig

 

[Bear in NM]

well, you could probably use this shielded #10 for a tow strap, so no telling how it will arrive. Maybe coiled around the trucks tow hooks.....And if I make a couple of 25' runs, that may be how I transport it on my truck

Craig

 

[vtsoundman]

I finally picked up a second, foldable 100 watt panel to go with my fixed 135 glass panel. I want to do some testing this weekend with parallel and series connections with the mppt controller in my new trailer.

Does anyone have a tip on how to max out the amps/watts of the setup, short of draining a perfectly topped off battery?

Thanks,

Craig

Sounds​ like part of your test is to evaluate the MPPT ability /max power under various conditions.

You'll need a load that will follow the source and allow the MPPT to function properly. A resistive bank like lightbulbs or toaster/space heater will not yeild the results...but your solar battery charger will.

Discharge the battery a good amount...use a known qty like an inverter with fixed load. It will draw constant power as the battery voltage changes. Then do your power/current observation​s either with the inverter plus batt charger... or keep the number of variables low (batt charge function only).

For understanding energy/total harvest...the real figure of merit is recharge time of the battery.

sent via the cone of silence

 

[Bear in NM]

Soundman,

Thanks. I should probably clarify further. I found a "Hi I am Amy from the Alt-E Store" youtubes where she was demonstrating that for her particular pair of mismatched panels (both 12v nominal) that in parallel, she was seeing more amps/watts than when connected in series. That just got me to thinking, might be good to test. She did not detail her load, was just switching wires into some kind of metal box. Probably a resistive load bank of some sort, like noted above.

When you say solar battery charger, do you mean solar panel? I understand that the solar panels are only chargers, but have not heard them referred to as a solar batter charger, per se.

Per the advice above, my plan was to discharge my battery to a point where I know that the discharge state is low enough that I can plug in both configurations while the battery "tells" my controller to fix the rate at bulk. At least control that variable. I figured that the only true test would be a total recharge over time, but was hoping to get some kind of idea with a point in time measurement, as I saw in the video. In order to do a timed recharge, I would probably have to draw down my bank for each configuration, let them rest to get an accurate SOC, charge or discharge and wait, the battery for the second test to equal the same SOC. Trying not to turn this into a whole weekend affair. We are having our monsoon season here now, trying to run two long term tests would be pretty tricky.

Perhaps the point in time measurement will be meaningless, but it has me curious. I was also going to make a couple of #10 runs of wire, to test against my in-use #12 landscape wires. My panels are all remotely deployed, away from either my trailer battery bank, or my solar suitcase. The suitcase will require parallel, as it has 2 pwm's. My trailer has the mppt.

Thanks,

Craig

 

[vtsoundman]

Soundman,

Thanks. I should probably clarify further. I found a "Hi I am Amy from the Alt-E Store" youtubes where she was demonstrating that for her particular pair of mismatched panels (both 12v nominal) that in parallel, she was seeing more amps/watts than when connected in series. That just got me to thinking, might be good to test. She did not detail her load, was just switching wires into some kind of metal box. Probably a resistive load bank of some sort, like noted above.

When you say solar battery charger, do you mean solar panel? I understand that the solar panels are only chargers, but have not heard them referred to as a solar batter charger, per se.

Per the advice above, my plan was to discharge my battery to a point where I know that the discharge state is low enough that I can plug in both configurations while the battery "tells" my controller to fix the rate at bulk. At least control that variable. I figured that the only true test would be a total recharge over time, but was hoping to get some kind of idea with a point in time measurement, as I saw in the video. In order to do a timed recharge, I would probably have to draw down my bank for each configuration, let them rest to get an accurate SOC, charge or discharge and wait, the battery for the second test to equal the same SOC. Trying not to turn this into a whole weekend affair. We are having our monsoon season here now, trying to run two long term tests would be pretty tricky.

Perhaps the point in time measurement will be meaningless, but it has me curious. I was also going to make a couple of #10 runs of wire, to test against my in-use #12 landscape wires. My panels are all remotely deployed, away from either my trailer battery bank, or my solar suitcase. The suitcase will require parallel, as it has 2 pwm's. My trailer has the mppt.

Thanks,

Craig

A MPP is the maximum power point of a solar module. With just a little bit of sunlight, a module can produce its OCV or open circuit voltage. The more sunlight the higher the current.

However just because the module was producing the highest voltage in no way means it's producing the highest power... power is a product of voltage and current. This implies that one can lower voltage and increase current and have a higher product as a result.


There are many YouTube videos where people are ecstatic about the fact of module is producing its rated voltage. This has absolutely nothing to do with the amount of power the modules producing.

Typically the highest power point ...or the maximum power point ... of the module occurs when the module voltage is about 30-35% less than the ocv.

Since the maximum power point trackers or mppt operate by varying the voltage... And seek to find the highest product of voltage and current...the voltage point is much more critical than the current. Parallel connections allow dissimilar modules to have the same voltage operating point but produce different current.

Series connections must be done with modules whose current characteristics are very similar or identical.

It is this phenomenon that allows you to place two modules with different power ratings but the same or very similar OCVs and MPP voltage ratings in parallel.

These ratings will be on the back of the module. Decent modules will have these ratings at a particular temperature in a particular sunlight intensity called STC or standard test conditions.

So for your system, I'm assuming that the 100 and the 135 watt module have the same ocv and same mppt voltage reading.

Modules voltage is greatly impacted by the temperature of the module. This is called a temperature coefficient. Simply put if the modules are hot the module will produce less voltage and will produce less power as a result.

Since many people use modules whose ocv is around 18 volts

sent via the cone of silence

 

[Bear in NM]

Soundman,

Thanks, and I am aware of most of that. I have in line meters that track volts, amps, watt hours, "Power" etc. The OCV of my foldable Goal zero nomad 100 is indicated as 18-22, while my glass panel 135 is labeled as 22. Figured that was close enough.

I use a white folding camp table to help some with the thermal side. But, it is curious that goal zero uses a black fabric to encase all of their panels. I did see 66 watts out of it last weekend, but did not record any of the other measurements, as I was simply testing for function before a trip. This was solid sun, with the panel pointed reasonably well. But I was on a flat black trailer lid. Both panels are mono, but given the black enclosed fabric of the GZ, I figured that alone might reduce the raw percentage, assuming all other variables are constant. Yes, I know assuming....

The plan was to also test and record max watts at this time, and retest this winter when it gets butt ugly cold.

And for the record, I dig this minutia of this new found solar hobby, but on my last two trips I just plugged in the glass panel, my fridge, my lights, and enjoyed the camping. No fretting over panel placement, occasional shade or the monsoons, other than making sure no rain was getting at the electrical. My 135 is on it's second season with a 70ah portable battery bank, and it has worked for my needs. I need to test my new 100 on a trip or two, to see if I can do a set-up and forget type deployment.

Craig

 

[vtsoundman]

Sorry I got interrupted and accidentally posted this before I was finished with it. If your 100 and 135 watt module are the same voltages, their current ratings will be different. Parallel operation should be better.

In a series circuit, all series elements must have the same current but may have different voltages. In a parallel circuit all parallel elements must have the same voltage but can have different currents.

I have not seen her videos, based on your comments, she likely has a Current mismatch and that's why parallel operation yielded a better result.

Earlier when I was referring a mppt solar charger, I was referring to controllers that have integrated battery chargers like a victron unit or a ctek. A quality battery charger will be able to output mid-to-high for 14s for a rapid recharge rate. Many of Diplostrats links and other posts reference this... And will help you understand what voltage you need for your particular battery chemistry.

However because panels run hot and modules in this class of application have a relatively high temperature coefficient, their mppt voltage in operation will be on the lower end of the spectrum. The temperature coefficient is a term that describes the impact of temperature on an element. In this case the hotter the module, the lower the output voltage.

As such, this doesn't leave the power electronics very much room to operate. In other words if the mppt of the solar array is 15.5V and you really need 14.7 for the battery, the Power Electronics may not be able to produce the necessary 14.7volts.

In my system I compared a ctek and victron solar mppt controller and integrated battery chargers... I found that the ctek underperformed when compared to the victron under the same test conditions.

I attributed this to the ctek having too frequent of an mppt search algorithm and /or entering pulse charge mode way too early in the process.

I also noticed that when I put my modules in series for a 36 volt solar panel array, I was able to get more power and improved energy harvest. When I did the math, I found out that the mppt voltage was too close to the absorption voltage of the battery/ battery charger if I connected the solar modules in parallel.

The Victron repeatedly produced higher peak power in both series and parallel configurations (than the ctek did in a parallel configuration). Further, the highest peak power occurred when the victron connected in series array.

Energy is the product of power and time. More power in the same amount of time will give you more energy. Batteries are energy storage devices...


Since we're not talking about a lot of voltage, voltage drop/resistive losses may consume relatively large amounts. A paltry 1 volt drop can easily translate into a 5 to 7% loss....

Depending on the length of wire and the amount of current flowing across it, the difference between #12 and #10 wire may not be all that much. The higher the current, the higher the loss. Therefore peak losses occur when the system is producing the most power. I suggest using any number of the voltage drop calculators available online for an estimate. For a quick swag, sum the MPPT current rantings for a parallel connected system. Use that current as the input to the calculator to understand the amount of voltage drop across a given circuit length.

It goes without saying however longer the cable... lower the gauge.
higher the current... lower the gauge.

sent via the cone of silence

 

[vtsoundman]

Soundman,

Thanks, and I am aware of most of that. I have in line meters that track volts, amps, watt hours, "Power" etc. The OCV of my foldable Goal zero nomad 100 is indicated as 18-22, while my glass panel 135 is labeled as 22. Figured that was close enough.

I use a white folding camp table to help some with the thermal side. But, it is curious that goal zero uses a black fabric to encase all of their panels. I did see 66 watts out of it last weekend, but did not record any of the other measurements, as I was simply testing for function before a trip. This was solid sun, with the panel pointed reasonably well. But I was on a flat black trailer lid. Both panels are mono, but given the black enclosed fabric of the GZ, I figured that alone might reduce the raw percentage, assuming all other variables are constant. Yes, I know assuming....

The plan was to also test and record max watts at this time, and retest this winter when it gets butt ugly cold.

And for the record, I dig this minutia of this new found solar hobby, but on my last two trips I just plugged in the glass panel, my fridge, my lights, and enjoyed the camping. No fretting over panel placement, occasional shade or the monsoons, other than making sure no rain was getting at the electrical. My 135 is on it's second season with a 70ah portable battery bank, and it has worked for my needs. I need to test my new 100 on a trip or two, to see if I can do a set-up and forget type deployment.

Craig

Sorry if I over-explained....also I think we may have cross posted. I do this stuff for a living and and like you, am naturally curious. I think it's awesome experimenting with your gear like this.

With respect to panel placement just point towards the South at about 30 degrees and that'll be fine. If you want to be super anal about it, adjust the modules a couple of times per day.

Many of the solar Goal Zero products I come across have not yielded impressive results. I love the fact that the portable easily packaged and ready to go however.

sent via the cone of silence

 

[dwh]

It's not the Voc rating that really matters and has to be close - it's the Vmp. You're starting with a 4v diff in Ocv, that's probably going to end up a pretty big diff in Vmp.

The MPPT algorithm will end up more or less averaging the array load and neither panel will be operating at its optimum Vmp.

I agree that you will almost certainly see more watts rigging in series.


As to a load test - again, just put a bigger load on the battery side than the solar can supply. There's no need to run the battery down first as long as something is sucking up all the solar can produce.

Later, when your battery is down anyway, you can do a charge test.