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David avatar image
David asked

"Voltage or current limited" - What does it mean?

Looking at a days data from the Portal for my setup, I see a small number (89 out of the 1524 records for the 24 hours) that have for MPPT Controller State the entry "Voltage or current limited".

I would have thought this entry would occur if the PV voltage or the PV Current was over the rated value (so for my 100/30, a Vpv of >100V and a Ipv of >30A) but those values have not been exceeded so cannot see why this would appear?

This is the MPPT Info as seen by VRM:


I have a small extract shown below -

Vpv and Ipv are well below the limits; it encompasses all charge states and is not linked to any band of battery voltage - Below shows the other extreme of battery voltage -

What values in the data above would Voltage or Current limit a SmartSolar 100/30?

It is only a very small time within the day but on checking the logs it is a daily occurance but with no apparent reason. It would be good to know what the reason for this entry is.

MPPT Controllers
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Alistair Warburton avatar image Alistair Warburton commented ·

I dont know anything about your unit but the top set of data looks like the battery is full and the charger has backed out... What is your float set to? 14.4 looks high but that would depend on the battery of course.

The last entry looks like the MPPT trying to get the last bit of current out of a week sun, and reaching the point where it has failed to do that. I suspect if it attemped to actually draw any current the string voltage would crash and shut it down.

Just guessing but both seem plusable to me.

Al

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David avatar image David Alistair Warburton commented ·

My float, from memory is 13.6V. There could well have been a mains charger on at the time there which is why the voltage remains high when the MPPT was in float.

As you see from here, the voltage changes when going from Absorption to Float in an expected way.


As you know, the various chargers don't tend to know about each other, but that is a separate discussion.

What aspect of the data makes the controller think it is voltage or current limited? and why are not similar patterns not voltage limited. Is it saying the BATTERY is voltage or current limited rather than the CONTROLLER is?

If it is the Battery then fine, but it would be good to get confirmation on that that MPPT State actually can mean.


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David avatar image David Alistair Warburton commented ·

Maybe this sample might be more representative? I have also added in the column for the battery SOC....


So PV Current is ~3.5A and PV Voltage is right down at ~36V (This was when the panels were setup in Series/Parallel combo - I changed later to all series, hence the higher voltage in the previous sample). The MPPT is putting in 131-113W (dropping as expected as battery fills) but the MPPT State is still "Voltage or current limited". Why?

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David avatar image David David commented ·

Maybe it is saying that the panels could provide more current than the batteries are able to take?

But if that were the case, then just about any time during the day, whenever anyones MPPT controller gets to "float" mode, the PV panels are very likely to be able (if required) to put more current than the batteries will accept so that doesn't really make sense.

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Mark avatar image
Mark answered ·

Fistly it is normal and generally the sign of a well designed / operating system when the MPPT is operating in 'Voltage or Current Limited' mode.

It means that the MPPT is NOT operating at the Max Power Point of the solar array - in order to intentionally limit charge power.

So yes it typically means that the MPPT could provide more solar yield at that point in time/during the particular conditions.

There are a few reasons why the MPPT will operate in 'Voltage or Current Limited' mode;

  1. The battery voltage target setpoint for the particular charge phase that the MPPT is in has been achieved (or is approaching / close), so charge power MUST be reduced to avoid the battery voltage increasing over the setpoint.
  2. The programmed or hardware limited MPPT charge current limit (not PV current) has been achieved, so charge power must be reduced to avoid charge current increasing over the limit. This will typically only occur with oversized PV arrays (in relation to the MPPT) and during good solar conditions.
  3. If the solar array Vmp does not have a good buffer to the battery voltage and solar conditions are average/poor, resulting in Vmp at the time/in the conditions actually being below battery voltage. In order for the MPPT to 'attempt' to keep the PV voltage above the battery voltage, charge power is to be reduced. Otherwise the MPPT would simply turn OFF.

With MPPT charge controllers it is good system design practice to use a PV array wired in series with significantly higher Voc/Vmp than the battery voltage. The exception to this is setups with partial shading, which may require a compromise.

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Alistair Warburton avatar image Alistair Warburton commented ·

I hadnt thought of that, good to know.

Also good to know I wasnt far off base with my guess, V/I limit being normal!

Stuff I will be encountering soon with my new system.

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David avatar image David commented ·

Thanks Mark. I could not find anything about "voltage or current limited" on an (admitted brief) search and it sounded a fairly negative thing rather than a normal occurance.

Out of interest I usually have my 4 panels in a combination Series/Parallel configuration - in order to reduce the effect from potential shading (never know where I may park) whilst still upping the voltage to improve the Voc. I decided to change to all 4 in Series as a test to see if the higher Voc/Vmp would make any real-world difference (it should do even if just due to slight reduced losses from Panel to Controller with hgher voltage and lower current). I'll probably move back to Series/Parallel quite soon due to shading from trees where I usually park up.

Out of curiosity I just looked at another installation I did and that has the same "voltage or current limited" status entries and much more frequently - but in that case the ratio of Battery Ah/PV Panel W is much closer than to mine and a lot more time is spent at close to full batteries compared to mine, so that makes sense when applying your explanation.


Cheers :)

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Alistair Warburton avatar image
Alistair Warburton answered ·

OK... I am not a Victron expert, still working on that. but I do work on control systems for a living and I am now wondering if the actual question should be what 'Voltage or current limited' avtually meand and also why you think it is a problem.

My point was that in both cases, in your origional post, the MPPT backing out seemed reasonable. Case one because the battery is full and in case two because there wasnt sufifciant energy in the input to do anything else.

I will admit that the SOC being less than 100% is odd, for case one at least, but I suspect that the terminal voltage takes precidence over that.

My guess is that 'Voltage or current limited' means that some external influence/signal is being applied and overirding the MPPT algorythm in the solar achager, hence the vagueness of the message. If that asumption is correct then the issue is likely with a setting, or instrument, external to the charger.

Again, I am not saying I am correct in fact I have been looking for information about how the Victron systems interact myself, and have thus far been frustrated.

I will be hanging new PV tomorow morning and right now I have no idea how to configure it for may battery because I cant find any info.

I suspect it will work as it should but right now I dont anticipate understanding why! Perhaps you are in the same boat... No pun intended.

Al

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