PV Voltage and Charging

[RobertA]

Hi all, a couple of questions re: the attached image.


So last night I had to run the AC for the 1st time since I installed my 1900w PV/24v 690ah system and I woke up to 47 SOC (not great, I know). So today, also for the 1st time, I was fully expecting to see the majestic powers of my Classic 150 juicing my battery, but the most output amperage I've seen is 65~.

When I isolate my panels from the Classic I'm getting 84v @ combiner (VOC 44.6). After tying the Classic back in and on a restart I get 77v from Classic screen, but then it drops down to the 65~ shown in the attachment. So the Classic is throttling back available voltage. If it instead applied accurate voltage to current available of 28.4a it should be feeding 90.7 amps to battery, but it's not even close.

Could someone explain why? If this Classic has a max output of 98A why am I not seeing that today?

[Westbranch]

there is something not right here you should NOT be in EQ with only 72%...  something is not set correctly, you should only EQ once 100% SoC has been reached....  that said your 1762W is a blazing ~93% of the PV rating you posted   it should normally be ~ 72% so I don't know if those are NOTC # or not since you did not post all the panels details...

Not sure but is your max V set to 27.1 V for an EQ or the global max V?  .... 
1762W  / 27.1V = ~64.4 amps into the battery which is what you are getting...

Tell us the details of your battery and Classic settings, that way we don't have to guess...

[ClassicCrazy]

Yes - it would be helpful to know all your settings ,PV and   batteries specs and configurations , etc.
Like Westbranch said - something isn't set right if it is in Equalize at that voltage and SOC.

Larry

[keithwhare]

VOC is Voltage Open Circuit which is what you are measuring when you check the voltage at the combiner with the charge controller off. The Voltage maximum power (Vmp) will be lower than VOC. The Classic 150 is going to run things closer to the Vmp because it is tracking the maximum power point. As Westbranch says, 1762 watts out of 1900 watts of PV panel is pretty good.

But double check all of the settings in your Classic 150.

With 1900 watts of PV panel and a nominal 24 volt battery bank, you will never hit the Classic 150 maximum of 98 amps. If the Classic 150 is charging the batteries at 27 volts, you would need 2646 watts of PV panel input to get to 98 amps.

[RobertA]

Thanks for the replies.

That EQ reference is just the Classic. I have EQ set to auto and I guess yesterday was the 28th day, but EQ was never hit, nor was absorb. I got to 91% SOC before the sun vanished.

All settings (attached) have been confirmed by Deca.

As far as PV output being at 93%, I agree but have noticed that these Topoints have often exceeded their ratings. I would think their true output is as tested with voltmeter and that's 40~v. If yesterday I test at combiner and get 84~ volts, and Classic indicates it's receiving 28.4a, that should mean that in yesterday's environment my array was outputting 2400~ W, no?

Why is the Classic throttling the array's voltage down?

Edit: Panel ratings (PDF)  http://sepbatteries.com/media/add_info/Topoint_JTM%20190w_mono-min.pdf

[RobertA]

VOC is Voltage Open Circuit which is what you are measuring when you check the voltage at the combiner with the charge controller off. The Voltage maximum power (Vmp) will be lower than VOC. The Classic 150 is going to run things closer to the Vmp because it is tracking the maximum power point. As Westbranch says, 1762 watts out of 1900 watts of PV panel is pretty good.

But double check all of the settings in your Classic 150.

With 1900 watts of PV panel and a nominal 24 volt battery bank, you will never hit the Classic 150 maximum of 98 amps. If the Classic 150 is charging the batteries at 27 volts, you would need 2646 watts of PV panel input to get to 98 amps.

Thanks for that. So you're saying that even though my panels are consistently outputting VOC (is that normal?), the Classic's logic is to tone that down. Makes sense, I guess, but how does Classic know my VOC and VMP if they're not settings I input? Does it just automatically reduce any array's voltage 20%?

Edit: Thinking more about that, it has only been recent that the Classic has throttled my array's voltage down like this. A few weeks ago it was consistently 75~v. I thought that was because it's been heating up around here so PV voltage dropped, but yesterday my voltmeter indicated that was not the case.  So in a matter of weeks, Classic has throttled my input voltage from 75~v down to 65~v (presently at 68.2v)

[ClassicCrazy]

If I take a typical wall wart cube that you plug into power something it may say 9 volts but if I measure the voltage it may show 12 volts. But as soon as you put a load on it and measure the voltage with the load it drops down to closer to what the rating is.

Pretty much the same deal with the PV - unloaded you get the open circuit voltage but when you connect the battery the voltage is going to drop . It still has to be higher voltage than the battery to force any amps into it  . 

So when you check your Voc  you can also check the Isc ( think that is what it is called ) current short circuit. You take the positive and negative wires from a panel and short them together and check the current . Of course you don't want to do this with a whole array because it is too high voltage and can arc. But one panel at a time and if you have a clamp on DC amp meter you will see the current potential of that panel at that time of day . 

If you look up MPPT there are plenty of explanations of how they work to get more power out of the panels than typical PWM controller

Are you using ending amps to stop absorb or just time ?

There are so many considerations to a system - voltage drop of wiring, connectors, and breakers or fusing, etc.  Hard to diagnose a system without seeing or details of the whole thing. For example - how many batteries are in series and series parallel ? 

Larry

[RobertA]

If I take a typical wall wart cube that you plug into power something it may say 9 volts but if I measure the voltage it may show 12 volts. But as soon as you put a load on it and measure the voltage with the load it drops down to closer to what the rating is.

Pretty much the same deal with the PV - unloaded you get the open circuit voltage but when you connect the battery the voltage is going to drop . It still has to be higher voltage than the battery to force any amps into it  . 

So when you check your Voc  you can also check the Isc ( think that is what it is called ) current short circuit. You take the positive and negative wires from a panel and short them together and check the current . Of course you don't want to do this with a whole array because it is too high voltage and can arc. But one panel at a time and if you have a clamp on DC amp meter you will see the current potential of that panel at that time of day . 

If you look up MPPT there are plenty of explanations of how they work to get more power out of the panels than typical PWM controller

Are you using ending amps to stop absorb or just time ?

There are so many considerations to a system - voltage drop of wiring, connectors, and breakers or fusing, etc.  Hard to diagnose a system without seeing or details of the whole thing. For example - how many batteries are in series and series parallel ? 

Larry

There you go. Great explanation that I should have realized.

So I assume I'll see input voltage displayed rise once EQ is finished and in float.

"Are you using ending amps to stop absorb or just time ? "

Yes, I'm not certain of all the settings but I have ending amps set at 5. I had absorb at 3 hours on Deca's recommendation but changed it to 2 hours after noticing a bit too much water escaping batteries.

"how many batteries are in series and series parallel"

https://www.youtube.com/watch?v=8YDiAO9W-Qc

[keithwhare]

From the diagram of your system, it looks like you are using Top Point JTM190-72M PV panels. The data sheet I found lists the following characteristics:
Vmp: 36.5
Imp: 5.20
VOC: 43.8
ISC: 5.83

The VOC of 44.6 that you quoted in your original post is from the Top Point JTM195-72M PV panels, which have the following characteristics:
Vmp: 37.2
Imp: 5.24
VOC: 44.6
ISC: 5.88

In any case, the important number is Watts, not Volts.

Under standard test conditions, the panels will produce 190 watts per panel at 36.5 volts and 5.2 amps. Standard Test Conditions (STC) are 1000 watts of sunlight per square meter. Real life almost never matches standard test conditions.

[Vic]

Hi RobertA,

First,  regarding calculating the power that the PV array is producing at any one time,  you must use the actual String Vmp (the Max Power Point that the Classic has found,  in its most-recent Sweep).   The voltage that you have measured at the combiner often has no relation,  at all,  to the Vmp that the Classic has found for the conditions of PV Cell temperature,  and irradiance ( the effective Solar intensity),   from the last Sweep.

The Classic has a very accurate current Shunt to read the Classic's OUTPUT current,   and also has an accurate voltage meter.

The temperature of the PV solar cells has a large impact on the output voltage,   and this results in a fairly large reduction in the PV power available.  This is primarily due to the heating of the PV cells by the sun.

Typically we use a rule-of-thumb,  that for a typical day,   the PV power production will be about 75% of the STC power rating of the PV,  at solar maximum,  with the PVs oriented at right angles to the sun.   But,  on cool/cold days,   especially with considerable wind,  the Vmp of PVs can exceed  STC Vmp ratings.   This can cause the PVs to exceed STC output power ratings.

The STC ratings of PVs are measured values,  using a Flash Tube for illumination of the PV Module,  with the Cell temperature held at 25C.  The Flash Tube creates no real heating of the PV.   This is one of the main reasons that we typically do not see STC power outputs from PVs in most applications.

It is a good idea to EQ Flooded batteries after they have been fully-charged,   or, very close to it.   For generally unattended systems,  Auto EQ can be good,   but if possible,   Manually starting and monitoring the EQ process might be better.

YES,   when the Classic is in a charge stage where it is regulating the battery voltage (Absorb,  Float, and EQ and not showing -MPPT)  you should see the Vin to the Classic rise.   The lighter the load on the system,   compared to the amount of PV power available,   the relatively higher will be the Vin to the Classic.

FWIW,   Vic

[RobertA]

As told to expect, voltage did climb once float mode was achieved. That's good!

I did make another observation today that I'm not too thrilled about. I think the Classic's logic should be re-written to respect the Whiz-bang better. For example, because of the depleted state of my battery yesterday (47 SOC) it took 1.5 days to fully charge, but I had to restart the Classic in order for it to get there, otherwise it would have moved to float mode and left my battery undercharged. I think it should have instead realized, thru Whiz-bang, that I wasn't fully charged and continued in absorb mode until battery hit full capacity.

Sure, I could increase absorb time again but I'd rather not fry battery, not should it be necessary. Classic should change modes based on Whiz-bang amp stat, I think.

[Vic]

Hi RobertA,

There may be one (or more) settings in the Classic that you may need:

There is the Auto Reset function,   that when is set to ON,  begins a new day at 23:59,   each day,   and "ReBoots"   the Classic.   Most of us run Classics with ARST =  ON.

You might want to check in the Tweaks menu,   to see if   " ARST "  is set to ON.

Without ARST enabled,  the Classic will begin the each day in Float-MPPT,  and avoid fully recharging the battery each day,  UNLESS you have set ' Skip Days ' to some value other than ' 0 '.

If I understand what you are saying about what was happening today ...

Vic

[RobertA]

Well, kinda, except I think its behavior on restart is actually the opposite. If the sun is up, it will always start in absorb mode and for the time period configured. I know this because I've had to restart in the past to top battery off, and had to do so again today. Whatever the case, my point is that it should instead read the Whiz-bang data and, even on restart, jump to the mode reflective of amperage readings. Just because I have absorb set for 2 hours shouldn't mean Classic moves to float after 2 hours even if my battery is only 80% charged. That needs to be addressed.

[ClassicCrazy]

If you only have 2 hours for Absorb time and the batteries need 3 or 4 hours then it would take multiple days to fill up the batteries. That is why ending amps when done correctly is a better way to go . But to set it up with flooded batteries you need to watch the amps and find the point where they start to taper off as the SOC comes to 100% and confirm that with Specific Gravity readings of the electrolyte.  Then when you are pretty sure you have the correct ending amps you set the absorb time up to 4 or 5 hours and let the ending amps setting take it from absorb to float.

Not sure what you meant that the voltage climbed when it was in float - the voltage should climb in Bulk to the Absorb setpoint , then hold it at the Absorb voltage and the Classic will cut the current back to maintain that . When it gets to either ending amps or absorb time which ever comes first then it will go to Float and the voltage will stay in Float unless you put on some huge load to draw it down above what current the panels are providing.

Also make sure you have your temperature compensation values set - that will change the setpoints higher or lower as needed according to the battery temperature via the external temperature probe you have on them.

Larry

[RobertA]

I have ending amps set at 5 but I thought the Classic overrides that setting in favor of Whiz-bang?