Author Topic: Classic Aux 2 PWM configuration options  (Read 2925 times)


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Re: Classic Aux 2 PWM configuration options
« Reply #15 on: October 20, 2018, 01:03:53 AM »
Thanks Will for weighing in here.

It appears that we do have a working system with Aux2 PWM voltage configuration at +0.5 and +1.0 for the two configuration parameters respectively.

The system does not kick on in early bulk stage, ramps up in absorption, and we haven't seen the system get to float yet.
It does ramp down and stays off at the end of the day. It appears to bounce back and forth between bulk and absorb which is troubling. Is the absorb timer going to be screwed up?

In the TWEAKS menu, there is a setting called DvrtCnt  which, if ON, allows the Absorb counter to continue counting down when you see  "Bulk MPPT"  and the Aux2 is active... Also counts down when "Absorb" is being displayed of course.  This is what you want to happen rather than seeing Absorb being displayed because that usually means that excess power is being wasted.  i.e. the Classic is backing off by raising its PV input voltage.

One reason you might see the word Absorb momentarily could be because the diversion is taking a moment to react or your dump load is smaller than the available power at that time.

Another reason you might see Absorb is that you have it set to a POSITIVE relative voltage.   Your +0.5V is overlapping and should be OK since Aux 2 will start being active slightly below the Absorb Set-point voltage (Temp comp'd) because of the 1.0 volt PWM width.  So, about -0.5V to +0.5V relative range.But you will be seeing Absorb from time to time because of the positive relative +0.5V.   I think I like this setting !  I would probably set it slightly lower than this but whatever works for you is great.

I guess that as long as you have a nice big diversion load, the PWM at 50% or less should regulate the voltage without the battery voltage rising above the 0.0 relative point smack dab in the middle.

I'd like to see the system get into float mode, but we've got a 5 hour absorb time getting interrupted by bulk mode shifts.

Check the DvrtCnt TWEAKS setting to make sure that when divert is active the Absorb timer is still counting down while displaying Bulk MPPT....  Poorly documented at the moment we agree.

This is probably a great place to change the display from the simple "Bulk MPPT" to something  that indicates that even though the Classic is actually in MPPT mode due to the diversion, it is really an equivalent Absorb mode.   It has to fit in allowed amount of characters too of course.

I have a small analog multimeter with some resistors and capacitors hooked up to get a rough figure on the duty cycle, which helps immensely with getting a feel for the behavior since there is no metric reported by the Classic.  I've got a 10k resistor in series with the aux2 out at the tap point of twisted pair, then a 0.68uF capacitor paralleled to a 1mA analog meter with a 100k 10 turn calibration potentiometer in series with the meter, which as I recall is way down near 5kOhms to get the 100% duty cycle at full scale, which is found by manually turning on the aux2 output. That 100k pot is more appropriate for a 200uAmp full scale movement meter, but works fine temporarily with an old analog multimeter set for 1mA scale. The output after the 10k resistor is also available to view with an oscilloscope by pulling a double BNC jack which has the other parts mounted to it. No damage can be done to the Aux2 signal due to the 10k resistor being at the tap point; including shorts in the wire thereafter. 

The meter is great idea !   Perfect.

If you have a good eye, you can also notice the brightness of the LED behind the Classic's air slots if the LED mode is set to LED 1

IMO the problem is not a standard diverter algorithm when implemented properly. If the MPPT algorithm can sense when excess power is available, and when that excess power dwindles then the option to have quite a different algorithm is apparent.

There is no easy way that I know of to tell when there is excess power or how much power.  At least not with varying DC side loads and varying solar radiation.

That's why the Classic Waste-Not mode uses an Auxiliary output to run some kind of load to keep the battery near Absorb voltage rather than allowing the PV input to rise and losing that power to keep battery voltage regulated at Absorb or Float.....  Just make sure there is somewhere for that extra power to go. i.e. the diversion load and stay in MPPT.   Unless you have a diversion load that is undersized for the amount of power available to keep the battery stage in Absorb, you won't be able to have "Absorb" voltage regulation at the same time as the Aux output is trying to regulate the battery voltage.  The actual voltage that is being diversion regulated has to be slightly below the actual set-point voltage that determines when the Classic has to start raising its PV input voltage to reduce power.  I think that your +0.5V/-0.5V setting may very well be optimum for this to happen. 

This mode is not perfect and has compromises and takes a bit of playing and tweaking as you are finding out.

If you have a 5 hour Absorb time and the DvrtCnt is enabled and have enough power during the day, you should at least see some Float time.  Sun-hours are getting shorter now so it won't get better until next spring.

Have you checked the Absorb countdown timer in the MNGP display ?  It should give you an idea how well this is all working.

The charge controller could run clean w.r.t. absorption or float voltage with some minor overshoot or undershoot as the environment fluctuates. I do not observe a 0.5 volt drop in the setpoint. It seems to be a bit smaller than that.  Possibly the complexity of the algorithm has been abbreviated so as to translate back to the way things used to be. I'd have to study the firmware to understand.

To me the point of having the charge controller as master of the diverter is clean and is the engineered solution that prevents quite a few possible errors.
Caring for the batteries is paramount.

We are still verifying the system and it has been a struggle to get it going. That said it does appear to be working properly though the math is unclear.

Yeah, it's not perfectly smooth, but if your batteries are being charged properly and you can use some of that extra power, you're doing good.

Keep us posted


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Re: Classic Aux 2 PWM configuration options
« Reply #16 on: October 20, 2018, 02:29:11 PM »
Thanks Bob.

DvrtCnt:I don't think I'd find that without your help. I have passed over it many times in the Tweaks menu but stayed away from it.
It does show up in the glossary of the manual but I think that is about it. I'll look for the countdown to float on a status screen I suppose.
Sounds like we'll get to float.

So does the Aux2 PWM diversion algorithm run independently of the MPPT algorithm?  I guess this would be the simplest implementation. The parameter value ranges suggest otherwise. Incidentally the default figure was a large high value.

Real glad you are here and appreciate your time and detail.

24 SURRETTE 2-KS-33PS 2V 2500AH @48VDC, 7kW STC PV  (three arrays) 3 Midnite Classics, one with Waste Not High diversion driving 3kW of heating element at 48VDC through three SSR circuits, Two 6kW XW inverters


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Re: Classic Aux 2 PWM configuration options
« Reply #17 on: October 20, 2018, 07:16:18 PM »
Great ! Glad I can help but you are absolutely correct about the manual and lack of education.

Yes, it is independent of weather it is in MPPT or not.  It is dependent on the temperature compensated Absorb, Float and EQ voltages is all.

If the battery voltage is much lower than this voltage set-point, then the controller would be in MPPT mode and obviously would not have any extra power to do anything with, unless you want to limit the amount of current going into the battery....  That is also a possibility with the Whizbang Junior if it is connected but typically systems have too little PV source in the first place.