Variable Current Outputs - How to estimate current output?

[zcapozzi]

Hi all,

When a cloud goes over my panel, I'm able to measure the difference in voltage that is being produced. I have a 100W 20V panel and I've got a sensor that is measuring the voltage as it goes into the charge controller. I've noticed that full sun is about 20-22 volts (which is consistent with the optimal operational range). When it's cloudy, but enough for a shadow, I'll be around 14-15. Full clouds or rain puts me around 12 - 12.5. During the morning and evening, obviously there is a gradual ramp up and ramp down to 0.

What I'm having trouble figuring out is what the real output is at these times. Basically, I'm trying to figure out a way to estimate watts that are being produced by just using the voltage as a guide. I know that P=IV, so I'm not looking for a mathematical way to do this. My main questions:

1. Does the wattage of the panel get reduced with cloud cover? Is there a way to estimate the loss?
2. Is there a common high level understanding that the panel produces .01 amps during cloudy days, 1 amps during partly cloudy times, and 5A in full sun? Any anecdotal evidence would be helpful here.
3. How does Ohm's law get affected by cloud cover?

Thanks in advance.

[inetdog]

Hi all,

When a cloud goes over my panel, I'm able to measure the difference in voltage that is being produced. I have a 100W 20V panel and I've got a sensor that is measuring the voltage as it goes into the charge controller. I've noticed that full sun is about 20-22 volts (which is consistent with the optimal operational range). When it's cloudy, but enough for a shadow, I'll be around 14-15. Full clouds or rain puts me around 12 - 12.5. During the morning and evening, obviously there is a gradual ramp up and ramp down to 0.

What I'm having trouble figuring out is what the real output is at these times. Basically, I'm trying to figure out a way to estimate watts that are being produced by just using the voltage as a guide. I know that P=IV, so I'm not looking for a mathematical way to do this. My main questions:

1. Does the wattage of the panel get reduced with cloud cover? Is there a way to estimate the loss?
2. Is there a common high level understanding that the panel produces .01 amps during cloudy days, 1 amps during partly cloudy times, and 5A in full sun? Any anecdotal evidence would be helpful here.
3. How does Ohm's law get affected by cloud cover?

Thanks in advance.

1.The wattage of a panel is reduced by a reduction in the amount of current produced (both Imp and Isc) as the light level goes down. For a panel which does not have a load connected you should not see Voc change much if at all. If you use an MPPT CC or inverter, you may not see much change in Vmp either.
If you put a constant resistance load on the panel, then you will lose a disproportionate amount of power since you will be operating far below Vmp. Unless you measure the current as well as the voltage, you cannot accurately measure the power, particularly into a non-resistive load.

2. One person's definition of cloudy is very subjective. What you can say is that if you use an incident light meter for photographic purposes or a radiometer the panel output should be directly proportional to the light reaching the panels. But mottled (partial) shade will have a disproportionate effect compared to uniform light reduction.

3. No change in Ohm's law, but you have to realize that the panel is a current source which varies with light rather than a voltage source which varies with light, and so a fixed value resistor will not make a good load for the panel.

[zcapozzi]

1.The wattage of a panel is reduced by a reduction in the amount of current produced (both Imp and Isc) as the light level goes down. For a panel which does not have a load connected you should not see Voc change much if at all. If you use an MPPT CC or inverter, you may not see much change in Vmp either.
If you put a constant resistance load on the panel, then you will lose a disproportionate amount of power since you will be operating far below Vmp. Unless you measure the current as well as the voltage, you cannot accurately measure the power, particularly into a non-resistive load.

Thanks for the response. To paraphrase...your central suggestion is that the wattage is changing because the current is being reduced not because of anything having to do with the voltage drop. I talked to the manufacturer, and they tried to tell me that if the voltage was 12.25 (50%), then current would be half (2.65A), which seemed crazy to me. But I think I get your point. Voltage itself isn't going to be a good indicator (probably why it never really goes below 12V). If I'm going to figure out the power created, I would need to have a way to calculate amps as well as voltage.

I was hoping there was a way around having to put another current sensor into the system, but if that's what it will take, then so be it. Thanks. I figured out the ACS sensor for measuring the current from the battery to the inverter, I supposed I can do the same for the panel to the charge controller. Any tips or words or warning for doing this?

[SunEagle]

Thanks for the response. To paraphrase...your central suggestion is that the wattage is changing because the current is being reduced not because of anything having to do with the voltage drop. I talked to the manufacturer, and they tried to tell me that if the voltage was 12.25 (50%), then current would be half (2.65A), which seemed crazy to me. But I think I get your point. Voltage itself isn't going to be a good indicator (probably why it never really goes below 12V). If I'm going to figure out the power created, I would need to have a way to calculate amps as well as voltage.

I was hoping there was a way around having to put another current sensor into the system, but if that's what it will take, then so be it. Thanks. I figured out the ACS sensor for measuring the current from the battery to the inverter, I supposed I can do the same for the panel to the charge controller. Any tips or words or warning for doing this?

There are devices that can be installed to measure the amperage running through a wire. I use a Uni-T model UT203 DC clamp on multimeter. It is not the best but works for me when I am checking my panel.

Attached is a picture during one of my tests where you can see it on the ground between the panel and battery.

[zcapozzi]

There are devices that can be installed to measure the amperage running through a wire. I use a Uni-T model UT203 DC clamp on multimeter. It is not the best but works for me when I am checking my panel.

Attached is a picture during one of my tests where you can see it on the ground between the panel and battery.

Interesting. I have an ACS712 sensor measuring the current from battery to inverter, so I went ahead and bought the same sensor to put between the panel and the charge controller. I got it to work once, so I figure it can't be too hard to figure out again.

[FreeBTU]

You could use a shunt to measure your output.
A shunt is a very low ohm resistor you connect put in your circuit.
You measure the actual resistance of the shunt after it has been installed (make sure the wire it is in is not connected!
Write down this one time measurement.
Now any time you want you measure the voltage drop of the shunt by simply putting the probes of your multimeter o each side of the shunt.

I=U-shunt/R-shunt
Now measure the output voltage. (U-system)
Watt=U-system x I

http://en.wikipedia.org/wiki/Shunt_(electrical)

[SunEagle]

Interesting. I have an ACS712 sensor measuring the current from battery to inverter, so I went ahead and bought the same sensor to put between the panel and the charge controller. I got it to work once, so I figure it can't be too hard to figure out again.

It should work between the panel and charge controller.

Most charge controllers require them to be connected to the battery before you connect the panels to it. Check the instructions of your CC to make sure you are performing the connections in the correct sequence otherwise the CC may not work properly.

I just looked at that ACS712 sensor. Be careful since it is only good for 5 amps. What are you using to get the readout?

[zcapozzi]

It should work between the panel and charge controller.

Most charge controllers require them to be connected to the battery before you connect the panels to it. Check the instructions of your CC to make sure you are performing the connections in the correct sequence otherwise the CC may not work properly.

I just looked at that ACS712 sensor. Be careful since it is only good for 5 amps. What are you using to get the readout?

Yup, I've been connecting the battery first, then the panel, so all's good there. Good point about the 5A. I won't ever be drawing that much off the battery, so that one works fine, but I forgot that the panel could be up to 5.29A. Will that fry the sensor or just result in accurate readings above 5A?

For the measurement, I'm using an Arduino to record the data points and send the data back to my website for display and analysis.

[SunEagle]

Yup, I've been connecting the battery first, then the panel, so all's good there. Good point about the 5A. I won't ever be drawing that much off the battery, so that one works fine, but I forgot that the panel could be up to 5.29A. Will that fry the sensor or just result in accurate readings above 5A?

For the measurement, I'm using an Arduino to record the data points and send the data back to my website for display and analysis.

I see that the ACS712 comes in more than just the 5 amp module. There is one for 20 and 30 amps too. I didn't see any mention about over loading that sensor but it may not like anything above 5 amp for even a short time. Just be careful.

Nice little device. My friend uses an Arduino control system to fly his RC plane via way points. Neat technology if you like to do your own programing.

[zcapozzi]

Neat technology if you like to do your own programing.

And I do. Honestly, part of the reason that I wanted to experiment with solar was so that I could have set up the data recording system and have a real-time website to track it. I can't seem to find any ACS712 20A breakout boards, so I might have to just try the 5A and hope for the best.

[RobW]

For the measurement, I'm using an Arduino to record the data points and send the data back to my website for display and analysis.

Hi, zcapozzi, would you mind sharing more about your Arduino setup? I'm interested in doing the same things with my system.

[zcapozzi]

Hi, zcapozzi, would you mind sharing more about your Arduino setup? I'm interested in doing the same things with my system.

Sure, I'll start high level, but if you want me to get into the details, I'm happy to do it. I've also put my solar set up below so you know what I'm working with. Also, I'm just getting into electronics and Arduino too, so take all of this with a grain of salt. Also, if you share your solar set up, I can tailor this approach or point out challenges. Let me know what other questions you have once you've been through this. Also, the arduino forum has a lot of much more experienced people who can help you with this kind of stuff.

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Overview
Detail is below, but here is the overview of the system. Basically, for both inputs and outputs, you have to measure voltage and current in order to understand power. If you capture this data every few seconds, then you can extrapolate statistics and usage data. Eventually, I will be able to calculate current battery levels from this set up. Personally, this recording set up is really what I wanted to achieve with my system, so I think it's a really cool way to generate data on your system. Especially if you are tracking investment and time to recoup the upfront costs.

One thing to note: an Arduino uses about 30W, so I'd thought about powering it from my battery, but because my system is 1) experimental 2) in Chicago and 3) somewhat blocked by a building, it would have taken most of my power. Not a big deal for larger systems, but it was surprising to me.

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There are five components to the system
1. Arduino Yun (code is attached below)
2. Current Sensors (ACS712 Sparkfun.com has a breakout board here https://www.sparkfun.com/products/8882)
3. Voltage Dividers
4. Breadboard
5. Website (http://thelibraproject.com/Solar/solar_power.php)

1. Arduino Yun - Arduino's are essentially mini computers that have a lot of interfaces for sensors. Typically, a sensors takes a reading, converts it to a voltage and that gets read by the Arduino. The Yun has Wi-Fi capabilities, so it can send data as long as it's connected to your home Wi-Fi. You could also get an Arduino, load data onto an SD card and upload it periodically.

2. Current Sensors - The ACS712 sensor is connected in between the battery and the inverter (to measure usage) or between the panel and the charge controller (to measure generation). Note the one referenced in the site above is a 5A sensor, so it may not be able to measure currents higher than that.

3. Voltage Dividers - There are pretty basic circuits within the electronics world. Essentially, you use resistors as reference points to calculate a voltage. An example can be found here: http://www.facstaff.bucknell.edu/mas...t/Resist3.html ( Go to the bottom for the breadboard visual)

4. Breadboard - Again, basic component of electronics. I don't have the equipment to actual create my circuits, so I'm going to stick with the breadboard. It's also good for experimenting

5. Website - I have a website that my Arduino sends data too. It gives me real-time stats on the system as well as lifetime stats and trends. It might give you some ideas about how to visual the information. Just an offer, but I'd be happy to keep your data too if you want to just use my website.

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Solar set up (I'm really experimenting with solar, hence the small size of the system):
- 100W Solar Panel
- Morningstar PWM charge controller
- 34 Ah battery
- Cobra Inverter 400W

[SunEagle]

Here is a link to that 30 amp ACS712 board. I can't seem to find the one for the 20 amp.

http://dx.com/p/30a-acs712-current-sensor-module-158589?tc=USD&gclid=CKz3ktr30roCFWpk7AodEyMAXA

Just found the one for the 20 amp sensor.

http://www.amazon.com/dp/B008B656PO/ref=dra_a_cs_lb_hn_it_P1400_1000?tag=dradisplay-20

[zcapozzi]

Here is a link to that 30 amp ACS712 board. I can't seem to find the one for the 20 amp.

http://dx.com/p/30a-acs712-current-sensor-module-158589?tc=USD&gclid=CKz3ktr30roCFWpk7AodEyMAXA

Just found the one for the 20 amp sensor.

http://www.amazon.com/dp/B008B656PO/...=dradisplay-20

Thanks for sharing. I got one of those for my first attempt but either it was broken or I couldn't figure it out (basically it just reported out my Vin voltage rather than the current measurement voltage. Weird). Sparkfun has a board that's much easier to use, but unfortunately, they only sell the 5A. Issue with that one is that you need to solder a capacitor onto the filter pin where the Sparkfun board has a connector for it. So my novice self has trouble with the three pin versions. But thanks for sharing.

[SunEagle]

Thanks for sharing. I got one of those for my first attempt but either it was broken or I couldn't figure it out (basically it just reported out my Vin voltage rather than the current measurement voltage. Weird). Sparkfun has a board that's much easier to use, but unfortunately, they only sell the 5A. Issue with that one is that you need to solder a capacitor onto the filter pin where the Sparkfun board has a connector for it. So my novice self has trouble with the three pin versions. But thanks for sharing.

Those boards are probably designed for RC type uses. You can connect it to the battery cables on your plane and then send the telemetry to the Arduino board or receiver using the 3 pin wiring setup. This sensor would give the person flying the plane a readout of how many amps the electric motor is drawing.