Two many solar panel amps?

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TwoBlocked

TwoBlocked

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Bradford County, PA
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Industrial Electrician
A remote radio tower wasn't working this morning. Solar panels, batteries, and controller installation. Turned out the controller was bad and was able to get one to replace it. But also noticed that only 2 of the 3 solar panels were connected to the controller. Was able to contact the installer and was told that it was not connected because the controller wasn't rated for that many amps. I left it disconnected, but it didn't sound right to me. What say you fine folks?
 
TwoBlocked said:
I would expect any power source to provide only as many amps as are drawn from it.
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No, PV modules are current sources, not voltage sources. (Very) roughly speaking, voltage sources fix the voltage and the load determines the current, while current sources fix the current and the load determines the voltage.
 
TwoBlocked said:
I would expect any power source to provide only as many amps as are drawn from it.
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ggunn said:
No, PV modules are current sources, not voltage sources. (Very) roughly speaking, voltage sources fix the voltage and the load determines the current, while current sources fix the current and the load determines the voltage.
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I deliberately posted "power source" not "voltage source."
 
TwoBlocked said:
I deliberately posted "power source" not "voltage source."
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OK, but current sources and voltage sources are the two types of power sources. See https://www.allaboutcircuits.com/technical-articles/understanding-i-v-curves-part-i/. When you said "I would expect any power source to provide only as many amps as are drawn from it" what you were talking about is a voltage source, like a battery. PV modules are not like batteries; in the linear portion of the I-V curve the current does not depend on the resistance of the load.
 
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ggunn said:
OK, but current sources and voltage sources are the two types of power sources. See https://www.allaboutcircuits.com/technical-articles/understanding-i-v-curves-part-i/. When you said "I would expect any power source to provide only as many amps as are drawn from it" what you were talking about is a voltage source, like a battery. PV modules are not like batteries; in the linear portion of the I-V curve the current does not depend on the resistance of the load.
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And if the load is not resistive, but current limited such as a battery charger?
 
TwoBlocked said:
And if the load is not resistive, but current limited such as a battery charger?
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I was trying to explain how "any power source to provide only as many amps as are drawn from it" does not apply to PV modules because they are current sources, not voltage sources.

If what you mean by "battery charger" is a PV charge controller, it is indeed a resistive load on the PV modules; it is just not a static resistance. It varies the resistance it presents to the module(s) to search for the maximum power point for the output of the module(s), but there is a limit to its range. If you connect too many modules in parallel to a charge controller they will push too much current for the charge controller to control, and that is apparently what you were asking about.

If you are really interested in how this stuff works, look at the link I posted for information about ideal voltage and current sources, and check out some IV curves for PV modules. You will see that for most of the voltage range between zero volts (a dead short) and Voc (open circuit voltage) the IV curve for a given amount of illumination is very nearly flat, which means that in that range it is very nearly an ideal current source and it will deliver virtually the same amount of current no matter what the resistance of the load.
 
ggunn, I invite you to reread my original post and respond to it's obvious meaning: Is there a problem with hooking up solar panels whose total amperage is greater than the controller is rated for? If so, what is the problem?
 
TwoBlocked said:
ggunn, I invite you to reread my original post and respond to it's obvious meaning: Is there a problem with hooking up solar panels whose total amperage is greater than the controller is rated for? If so, what is the problem?
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I hope you dont mind if I respond. It depends. In practice I believe that pretty much every modern charge controller is programmed to not destroy itself even if there is more current available than its rating. One thing that is possible, is that the charging current was too high with 3 panels. For example trojan recommends the max charge rate be no more that 13% of the 20 hr AH capacity. They can "take" more, but it is not good for them. Most/all off grid charge controllers I have seen do not have a programmable max charge rate so they will give the batteries whatever they can take which may not be ideal.
 
electrofelon said:
I hope you dont mind if I respond. It depends. In practice I believe that pretty much every modern charge controller is programmed to not destroy itself even if there is more current available than its rating. One thing that is possible, is that the charging current was too high with 3 panels. For example trojan recommends the max charge rate be no more that 13% of the 20 hr AH capacity. They can "take" more, but it is not good for them. Most/all off grid charge controllers I have seen do not have a programmable max charge rate so they will give the batteries whatever they can take which may not be ideal.
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Thanks for replying. :) The controller is a Morningstar PS-30. Has some pretty sophisticated charging settings. I think there are six 110 Ah batteries in parallel (12 V) The installer is a co-worker, but not an electrician. I haven't been here long, wanted to see if there was something I am missing before I push for connecting up the third panel. My thought is that during full sun, 2 panels is plenty, but when there is not, why not take advantage of a little more power. The load is a radio tower that draws 7 amps max for maybe 10 minutes per hour running 24/7.

But your point about overcharging may have been the case before this controller and new batteries were put in. The inside of the battery box I was in showed definite signs of batteries being overcharged - heavy white dust, SS boxes.
 
TwoBlocked said:
ggunn, I invite you to reread my original post and respond to it's obvious meaning: Is there a problem with hooking up solar panels whose total amperage is greater than the controller is rated for? If so, what is the problem?
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I answered your question in some detail; the short answer is "maybe" and the potential problem is that the current supplied by a PV module is not dependent on the resistance of the load. My advice is to read the specs on your charge controller and abide by them or risk damage to it and/or voiding the warranty.
 
TwoBlocked said:
ggunn, I invite you to reread my original post and respond to it's obvious meaning: Is there a problem with hooking up solar panels whose total amperage is greater than the controller is rated for? If so, what is the problem?
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In general, yes, there is a problem with hooking up solar modules to equipment that isn't rated for the highest current the solar modules may produce. For example with Enphase or Solaredge they state a max module Isc which you can't exceed. However, after a brief look at the PS-30 manual I don't see such a limitation, only a max load current, which is the scenario electrofelon described. The spec sheet does say it has overload protection on both sides, which I assume means it will shut down if too much current is drawn. If this was happening then electrofelon's post makes the most sense.

This is a bit similar to some string inverters that state a maximum input current but will clip the current on arrays that can produce more current. Sometimes they state a different max Isc spec. Sometimes they don't state one and it's a bit ambiguous. The morning star is a bit like the latter, except that I presume that since it doesn't control the load current then it's response to overload is to shut down instead of clip?
 
TwoBlocked said:
A remote radio tower wasn't working this morning. Solar panels, batteries, and controller installation. Turned out the controller was bad and was able to get one to replace it. But also noticed that only 2 of the 3 solar panels were connected to the controller. Was able to contact the installer and was told that it was not connected because the controller wasn't rated for that many amps. I left it disconnected, but it didn't sound right to me. What say you fine folks?
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I looked at the datasheet and manual for the charger. The input limits are a 60V Voc and the 30A the charger is rated for. In the troubleshooting section, it says that if the PV amperage exceeds the charger current rating it will disconnect the PV until the current is less than the rated current. No MPPT on this charger so it can't regulate the PV current. It holds the PV at whatever voltage the battery is charging at and if the current goes over 30A it disconnects the PV.
My guess is that adding the 3rd module will either exceed the Voc or exceed the 30A rated current. So you have a spare module.
 
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Hmmm it is highly unlikely each panel has over a 10 amp current, so regardless of the "will the charge controller protect or destroy itself" debate I don't see what the issue with too many amps is (and clearly not a charge rate issue with that big of a battery Bank).
 
electrofelon said:
Not saying you are wrong about this controller, but just wanted to point out that even without MPPT, a controller could use PWM to control the current.
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True, if it's designed to do that. I don't see anything in the documents that indicate any current control on the PV side with this product. PWM is being used to control the charging voltage to the battery. If at the charging voltage, the battery can draw more than 30A and the PV array can supply more than 30A the charger will shut down. What we want in this situation is that the PV array can't provide more than 30A and it keeps charging.
 
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TwoBlocked said:
A remote radio tower wasn't working this morning. Solar panels, batteries, and controller installation. Turned out the controller was bad and was able to get one to replace it. But also noticed that only 2 of the 3 solar panels were connected to the controller. Was able to contact the installer and was told that it was not connected because the controller wasn't rated for that many amps. I left it disconnected, but it didn't sound right to me. What say you fine folks?
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Did you get the PV panel model number and manufacturer, or take a photo of the label with the specs? Really need that before we can speculate on if you can use the third panel. Also, are the two connected panels in series or parallel?
 
pv_n00b said:
True, if it's designed to do that. I don't see anything in the documents that indicate any current control on the PV side with this product. PWM is being used to control the charging voltage to the battery. If at the charging voltage, the battery can draw more than 30A and the PV array can supply more than 30A the charger will shut down. What we want in this situation is that the PV array can't provide more than 30A and it keeps charging.
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I was just saying that with a PWM controller, PV current and charging current are one and the same, and you can control them with a few lines of code if the designer chooses to. I suspect some of the these controllers may have "indirect" control by temperature, instead of a specific current value. I agree that often manufacturers aren't super clear on "over paneling".
 
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