Odd solar readings

4ncar

100w suitcase(monocrystalline), Victron 75/15 mppt controller, full noon sun(panel deadnuts on it). No load on battery....

when I first fire it up, I get 117 w and ~ 4-5 amps. After 3-4 minutes wattage drops to ~40watts and 2 amps. Why does it do this? Does the solar controller kick it down because my SOC is 99%?

pthomas745

If the battery is fully charged, and there is no draw except the low parasitic drains, that sounds fairly normal.  What happens if you kick the television on or something like that?

4ncar

@pthomas745 we shall see at first light...or sometime there after. I was thinking I might fire the 12v side of the fridge up to draw the batteries down... we’ll see...

pthomas745

That will do it!

4ncar

Yup, that’s it! The controller is smarter then me, and throttles the power based on load.  Turned on the trusty 3 way fridge, 12v mode 10.a draw) and the controller readings shot up and held.  Now I know, and others who read will too!😅

ScottG

Just a curiosity question... Is that what the controller is showing coming from the panels, or is that what is being sent to the battery.

I'm not familiar with the Victron interface but my Renogy PWM controller shows both these parameters and I have at times also noticed a wide variation in readings in full sun conditions.

4ncar

@ScottG it shows both. What was confusing to me was that the initial reading of what the panel was generating fluctuated. So in the end, what is shown is NOT a maximum amount of available PV power, just what is needed based on battery needs.

ScottG

Thanks, @4ncar--I assumed you were probably referring to what was coming from the panel rather than going to the battery. You must be right about it showing what the controller is taking from the panel, but it seems like that arrangement can be a little confounding when you just want to know if your panel exposure is optimized.

I'll have to observe my controller readings more closely at some point. I haven't really paid much attention but your post made me think of those few times I recall observing such variation in the "input" from the panels.

ckjs

4ncar said:

... After 3-4 minutes wattage drops to ~40watts and 2 amps. Why does it do this? Does the solar controller kick it down because my SOC is 99%?

Yes.  Once the battery approaches a full charge, the current drops. If you scroll down in VictronConnect you’ll probably see that it is in Absorption or Float mode, rather than Bulk.

ScottG

@ckjs, are you referring to the output of the controller--i.e., what the controller is sending to the battery?

In @4ncar's case, I think we are talking about the input the controller--i.e., the controller is getting from the panels.

That is what seems odd, since the output of the panels should be consistent under consistent conditions. The operating theory is that the Victron readings in question are indicating what the controller will accept from the panels, based on what the demands of the battery are.

At least that is my interpretation. Someone set me straight if I've got this wrong!  

rh5555

The MPPT controllers restrict the current flowing in from the solar panels if it is not needed.  Generally, the output power (battery volts x charging amps) of the controller equals the input power (Solar volts x solar amps) times the efficiency of the converter.  MPPT controllers are pretty good, and efficiencies of about 90% should be expected.  So if little charging current is needed, then little solar current will be drawn.

4ncar

rh5555 said:

The MPPT controllers restrict the current flowing in from the solar panels if it is not needed.  Generally, the output power (battery volts x charging amps) of the controller equals the input power (Solar volts x solar amps) times the efficiency of the converter.  MPPT controllers are pretty good, and efficiencies of about 90% should be expected.  So if little charging current is needed, then little solar current will be drawn.

This is what I found to be the case, despite reading tome after tome and watching 13,000 useless videos. @ckjs, yes my battery was in absorption. It wasn’t until I put 10ams of draw did I see that the panel was generating measurably more power to the battery.

@ScottG I’m finding that even when the battery is not in any great need of juice, you can see the wattage increase/decrease as you move the panel.  I use a rattle can lid on a piece of white paper, taped to the panel, to cast a shadow to determine exact angle & direction. Once this is done, no need to watch the screen to see if optimal placement has been achieved.

ckjs

ScottG said:

@ckjs, are you referring to the output of the controller--i.e., what the controller is sending to the battery?

In @4ncar's case, I think we are talking about the input the controller--i.e., the controller is getting from the panels.

That is what seems odd, since the output of the panels should be consistent under consistent conditions. The operating theory is that the Victron readings in question are indicating what the controller will accept from the panels, based on what the demands of the battery are.

This confused me the first time my batteries got to “float” — even with my engineering degree. The solar input to the controller closely tracks how much juice goes into the battery. The excess is wasted.  A 100 watt panel is in perfect full sun could generate 20 volts at 5 amps.  When the battery is charged the controller ignores that excess power and just draws enough to keep the battery in the float state. It is similar to the 120v outlets in our homes: a 60 watt lamp only takes as must power as it “wants”, even though the wall outlet could provide 120v * 15 amps = 1800 watts.

The Victron solar input power is what it is currently using, rather than the maximum power that it could provide at that instant.  You can get the max power value by, e.g., turning on the fridge and fans.

rh5555

For the technical junkies out there, MPPT controllers are pretty neat.  They are basically DC to DC converters that transform (almost) any input voltage to a desired output voltage.  The Victron controllers only reduce the input voltage, but the current is boosted by the same ratio as the voltage is reduced.  This lets you charge your batteries with more current than the solar panel produces.  As I commented above: Generally, the output power (battery volts x charging amps) of the controller equals the input power (Solar volts x solar amps) times the efficiency of the converter (about 0.9).

So this is where the fun starts.  The controllers have 2 other functions:  They monitor the battery to determine if it needs fast (bulk) charging, slow (absorption) charging, or trickle (float) charging.  The voltage delivered to the battery (and all your 12V fridges, etc) depends on which charging mode it is in.  Values are typically 14.4V for bulk, 13.6V for absorption and 13.2V for float, but will be lower if the controller has reached its maximum charge current.  Now the cute bit:  Solar panels are interesting non-linear creatures, if placed in full sun with varying loads, they will generate an output that follows the blue line:

If you draw lots of current from the battery, you won't get much voltage, if you don't draw any current, you'll get lots of voltage.  Because output power is voltage x current, you don't get much power at these extremes and you do better somewhere in the middle (the red line).  The MPPT controller homes in on the peak of the red line to get the maximum available input power, which it can transform into the maximum charging current.  Hence the name: Maximum Power Point Tracking.  But what if you don't need much output power because your battery is already fully charged?  The controller controller shifts its operating point off the maximum power point to operate in a less efficient area, thereby constraining the input power.  The Victron MPPT controllers do this by shifting to the right (lower solar current, higher solar voltage).

Finally a quick word on the naming of Victron MPPT controllers:  They have names like MPPT 75/15.  The 75 is the maximum input voltage the controller can handle.  This voltage will be seen when no current is being drawn (far right of blue line) and is typically called Voc (open-circuit voltage).  This parameter limits how many solar panels you can connect in series:  The sum of their Voc values cannot exceed 75V (in this case).  The second number is the maximum current the controller can produce to charge your battery.  In this case it is 15 Amps.  We already know that under bulk charging the controller will try to output 14.4V and if this is limited to 15 Amps, then the maximum power the controller can deliver is 14.4 x 15 = 216 Watts.  Assuming a controller efficiency of 0.9, this means that the maximum power the controller can use is 219/0.9 = 240W.  It is therefore pointless to hook up more than 240W of solar panels to a MPPT 75/15 controller if the panels are going to be in full sun; output beyond 240W will not be used.

The downside of Victron MPPT controllers is that they generate all sorts of radio-frequency noise.  If you're having trouble listening to a weak FM channel on your radio, try turning off your solar.  You'll be amazed at the difference it will make!

4ncar

Nice explanation, and easy to read! Thanks!!!

dCliffhanger

Has anyone tried to reroute their T@B's power Usage through the Victron Ouput terminals. That should give the capability to monitor power usage.  I tried via a multi contact switch to draw power A) from the battery as originally designed or draw power thru the Victron.   It initially worked! and then stopped.  I'm not sure why, possibly needing a Victron reprogramming change.

ScottG

Good explanations. I'm getting an unplanned (but enjoyable) schooling on solar controllers.  :-)

Do PWM controllers show the same variation in their input readings? I'm guessing not, since it sounds like the effect derives from the MPPT technology. 

My sun exposure is almost always less than optimal, so I don't get much chance to really pay attention to see if the input variations I've observed are driven by the controller or just subtly changing conditions.

MarkAl

@rh5555 Excellent summary! You made an interesting point on the RF noise. I do some work mitigating RF emissions on our products and assumed they'd been reduced but as sub-components and RV use items they may not be. I looked at their site for approvals and only saw safety related ones, but I'm not regulatory proficient.  In which case their radiation may be one reason why the Bluetooth capability is so poor. Can't shield the blue boxes but some serious filtering on the wires may improve this. Such as good ferrites placed in different ways? Thoughts?

rh5555

I've tried ferrites of various kinds, both straight through (clip-on) and multi-turn (torroids), but nothing made an appreciable difference to the reception of a weak FM channel.  My conclusion was that the Victron is radiating right out of the box, which seems to be plastic.  I haven't tried mounting the box in a Faraday cage;  I just turn the controller off using the app when I want to listen to the radio and the reception is noisy.

Marceline

rh5555 said:

I've tried ferrites of various kinds, both straight through (clip-on) and multi-turn (torroids), but nothing made an appreciable difference to the reception of a weak FM channel.  My conclusion was that the Victron is radiating right out of the box, which seems to be plastic.  I haven't tried mounting the box in a Faraday cage;  I just turn the controller off using the app when I want to listen to the radio and the reception is noisy.

I use the weather bands a fair bit, but the FM hasn't been very useful for me. I downloaded music to a tiny thumb drive (like a Sandisk Cruzer Fit) and just leave it plugged into the Jensen all the time. My older Jensen chokes on a thumb drive with more than 999 tracks; I don't know about the newer ones.

ChanW

Thanks @rh5555. Good details info.

ScottG

I'm still curious if anyone knows if PWM controllers can also modify the input from the panels similar to what was described for MPPT controllers.

rh5555

PWM controllers do their regulation by alternately connecting/disconnecting the solar panels to/from the battery.  This is done very fast (tens of thousands of times per second).  The effective charging rate is determined by the ratio of times connected to disconnected.  If the battery is fully charged, the controller will have very little connected time, and lots of disconnected time.  The voltage seen at the panels will be the time-weighted average of the solar panel Voc (when disconnected) and the battery voltage (when connected).  So if the connected to disconnected time ratio is 1:9 then the voltage on the panels will be 0.1 x Vbatt + 0.9 x Voc.  So the short answer is that the apparent voltage on the input panels will rise (given constant sunshine) as the battery gets to full charge.

Let me caution everyone reading this that the above is a simplification of the operation, as various smoothing/filtering stages are usually incorporated which I have totally ignored.

Finally, if using a PWM controller then you should be using a 36 cell panel, as the extra power from a higher voltage panel will largely be wasted.  If you're using 2 (or more) 36-cell panels, connect them in parallel.

ScottG

Thanks, rh5555--a simplified answer works for me!  ;-)

I'm not sure how many cells my panels (2x50W connected in parallel in a suitcase) have, but they top out just north of 18V in full sun.