Not measuring/getting full wattage

photonboy

Folks,

I have a victron 30 Amp mppt charge controller that I am using with a folding 200W panel (100W per panel).

I am measuring ~ 20V and 5 amp on the cabling going into the charge controller, but the controller is only measuring ~105 W (~15V & 7 amps).   Any thoughts on the disparity?  I will likely in either case put the panels in series versus parallel for the next round of experiments?

Appreciate the help.

Chris

MichaelK

Numbers look exactly like what they should be.  The MPPT controller is acting like a transformer, taking the raw higher voltage solar input, and transforming down to the voltage the battery wants.  As the voltage gets transformed down, the charging amperage is going up. This is exactly as it is supposed to be.

BTW, if you utilized an older PWM controller, the battery would drag the voltage of the panel down to that of the battery, and then only the 5A coming in from the panel would go back out to charge.

photonboy

Thanks for the fast reply MichaelK, but the app shows both the solar input and what is actually being " transformed" and going to the battery. I would expect the solar input to measure close to what I am getting with my multimeter, and the battery input to be the "transformed" value.  In either case I am about half of the wattage of the panels?

Photowhit

photonboy said:

I am measuring ~ 20V and 5 amp on the cabling going into the charge controller, 

Your measuring of the panels and their output seem fine. Solar panels will very rarely be even close to their rating under Standard Test Conditions (STC).

Solar panel rating under Standard Test Conditions (STC) is based on a 'flash' test were the solar cells never heat up. Once left in the sun panels produce closer to NOCT Normal Operating Cell Temperature values. These tend to be around 75% of the STC. Some manufacturers even post the info on the back of the panels or in their spec sheet. Here are 2 examples;

littleharbor2

How are you measuring with your multimeter? Open circuit voltage, VOC, and short circuit current, ISC, are not the true output of the panel.

BB.

Just to be clear...

• 20 volts * 5 amps = 100 Watts into the charge controller
• 15 volts * 7 amps = 105 Watts out of charge controller

More or less, in solar, 10%+/- accuracy is not uncommon for controllers (and non-laboratory power meters, etc.). So 100 Watts +/- 10 Watts... Pretty close.

Actual power transfer should be something like 85-95% MPPT controller efficiency:

• 100 Watts in * 0.9 (approx MPPT eff) = 90 Watts out

If the question is 200 Watt panels and 100 Watts out... Yea, as talked about above, that is the nature of the beast.

More or less, first approximating, solar panels+controller losses are closer to 76-77% of their rated STD (standard test condition) output on a hot sunny day, around solar noon, panels pointing directly at sun. 75-77% is the "best" you will typically see on a warm day in full sun.

• Power = Voltage * Current
• Vmp (voltage maximum power) falls as the panels get hot (Hot day, full sun, Vmp can drop to almost 80% of Vmp(std)

Add a bit of moisture/dust in air, panels not pointing directly at sun, dust on panels, very light scattered clouds--You could be looking at 50% harvest very easily (our eyes--50% sun and 100% sun is almost an undetectable difference--Need to measure solar energy with solar cell+meter to see "real" energy available).

Lastly, as batteries become charged, their voltage rises and they "accept" less current. I.e., if your battery bank is at 15 VDC (sounds a bit high), then the charge controller should be regulating at ~14.75 volts and the battery accepting less than full charging current.

For Lead Acid batteries--They (more or less) accept full charging current at less than 80% State of Charge... Above that level, they accept less and less current (even though there may be more solar energy available).

-Bill

BB.

There are other "inaccuracies" that can get into a system too... The standard digital volt meter (i.e., cheap type) does not always accurately read voltage and current. They assume "clean" Sine Wave or DC power.

There are True RMS reading meters (Root Mean Square) which are more accurate when measuring "noisy" DC voltage and Non-Sine Wave AC voltages...

https://www.fluke.com/en-us/learn/blog/electrical/what-is-true-rms

When trying to get down to 2% or better accuracy--There are lots of things that "get in the way" of better accuracy. Besides the right tool for the right job--$$$ and calibration, etc. are also issues (and don't overlook weak or near dead batteries in a DMM either).

-Bill

NANOcontrol

This hasn't been mentioned, but 20V going into the controller indicates the panels are nowhere near operating at power point which would be closer to 17V in sun.  That clearly indicates the charge controller is not asking for all the power due to batteries nearing full charge.

littleharbor2

NANOcontrol said:

This hasn't been mentioned, but 20V going into the controller indicates the panels are nowhere near operating at power point which would be closer to 17V in sun.  That clearly indicates the charge controller is not asking for all the power due to batteries nearing full charge.

It's not clear how he got 20 volts and 5 amps in his first post but in post #3 he says he used a multimeter. This is is why I asked how he arrived at those numbers. 20 or so volts, in parallel  in an open circuit situation on warm panels sounds about right. Current is quite low which could indicate a shadow on the panels, poorly orientated panels or overcast skies, or, a combination of all three.

OP, definitely wire the panels in series with your mppt controller and be sure to test in good sun, shadow free and properly aimed panels.

RCinFLA

A common mistake folks make is not realizing if the load demand is not there the PV charge controller will back down production to just meet the battery charging and AC load power demand.

When a PV charge controller backs down power output it allows array voltage to rise above normal Vmp point to allow the panels to absorb unused PV power.