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Dave avatar image
Dave asked

Tricking an MPPT 100/20

Hello,

I would like to use an MPPT 100/20 primarily as a data logger to test the peformance of a new type of panel. I want it to track the maximum power of a single 300W panel and to record the power output of the panel as a function of time. My question is can I just connect a resistive load to the Battery output of the MPPT to dump the energy? I have a resistor bank which can dissipate 300W. I have tried this with some success and the MPPT is in bulk mode for most of the time. It stops periodically and all 3 lights flash (which is not one of the cases explained in the manual). I am wondering is the MPPT searching for a minimum voltage on the battery output? A key thing is that I want the MPPT to remain in bulk charge mode, as if it goes to absorption or float it will artificially reduce the output of the panel. Anyhone ever try something like this?

Thanks,

Dave

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2 Answers
wkirby avatar image
wkirby answered ·

As you know, the MPPT has been designed to charge batteries. Therefore the battery on charge is always expected to be available as the source of energy to power the circuits and CPU which control the charger.
An MPPT charger will periodically perform a scan of the solar panel or array. When it does this sweeping scan, the MPPT will load the panel to such a degree that it'll not produce much energy at all and at that point the MPPT will not have enough power to run it's self if there is no battery connected. Forthat brief period of time the MPPT loses power, resets and the PV goes open circuit. Then there is enough PV power to run the MPPT again, so it reboots. This is when you see the three LED's flash at once which is what happens as it boots. Then the cycle repeats on the next MPP scan.

I appreciate what you are trying to achieve, but the MPPT simply can't run continuously without a constant power source.
You'd need to keep enough load on the battery during the day to prevent the Absorb Voltage being reached, then the MPPT will stay in bulk all day long.

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Dave avatar image Dave commented ·

Thanks, Very helpful yes I first assumed that the MPPT would power itself off the battery, but was then surprised to see it working well off the panels alone. It looks like I will need to incude a battery so. I do like the software, GSM communication hardware and the VRM so it might be worth it.

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Dave avatar image Dave commented ·

Hello WKriby, I've noticed that my proposed setup works pretty well with two 20V, 50W panels in series and the charger set to 24V. I monitored it from 8am to 8pm yesterday and it did not seem to restart. Previously I was using only one 20V panel with MPPT 100/20, and I think the voltage was dropping below a critical value when it was cloudy. With the higher voltage it seems the charger stays in bulk mode, as I can set the absorption mode to 37V which it never really reaches. Even with the panels only producing 3W late in evening it seems to be enough to keep the MPPT powered up. The only issue I can see is that at night the MPPT will shut off and when it restarts in the morning it seems to add the new energy generated to the previous days energy (i.e. it doesn't know it is a new day.). I am waiting of a Cerbo GX and then I can use the VRM to more accuratly monitor the power versus time. Do you think the MPPTs tracking algorithm will not work well without external battery power? My issue is that if I connect a battery it will continuosly drain to the load. Yes I can have setpoints on the load output, but this would not be great for the battery, and if I can remove the battery altogether this would greatly reduce my cost as I need to monitor 10 panels for the trial. Thanks

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ripper avatar image
ripper answered ·

Put a small battery on there and use the resistive load across the load output. Also shut off the load at 12V, and on at 13.8 while aiming for 14.4V you she ll stay in bulk forever. that ll get you through the night till the battery is buggered.


Easier and cheaper is using a 10 to 20W panel in the desired direction and poly or mono crystalline as the actual panels and measure the short current via INA219 logging that. I am under 3% off my actual array using this setup as an irridiation measurement.

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Dave avatar image Dave commented ·

Thanks I will try that

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