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Need help designing load testing methodology for MPPT controller
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cbc02009:

--- Quote from: max_torque on October 23, 2018, 12:12:14 pm ---surely a practical MPPT controller needs to be a two stage device?

the "Input" stage keeps the panel at the optimum voltage and the "output" stage keeps the output at the set voltage (say 13v for a LA battery charger, or 240Vac for a single phase ac output etc)

Just keeping the panel at it's optimum voltage seems pointless?

--- End quote ---

Optimally it would. Because this is a school project, I was looking for ways to cut costs. I came across this application note from microchip (http://ww1.microchip.com/downloads/en/AppNotes/00001521A.pdf) that suggested a single stage device controlled by a state machine that would alternate what it was tracking (panel power or battery current/voltage) depending on what was needed. So far it seems to work pretty well, but if I were to start from scratch I would probably just add a second stage to the system.
Hydron:

--- Quote from: max_torque on October 23, 2018, 12:12:14 pm ---surely a practical MPPT controller needs to be a two stage device?

the "Input" stage keeps the panel at the optimum voltage and the "output" stage keeps the output at the set voltage (say 13v for a LA battery charger, or 240Vac for a single phase ac output etc)

Just keeping the panel at it's optimum voltage seems pointless?

--- End quote ---
Actually you only need a single stage as long as the input voltage (to a buck MPPT) from the panel is always higher than the output to the battery. The MPPT will adjust the duty cycle until the maximum power transfer occurs; the output voltage is basically irrelevant other than for overcharge protection etc.
max_torque:

--- Quote from: cbc02009 on October 23, 2018, 12:20:55 pm ---
--- Quote from: max_torque on October 23, 2018, 12:12:14 pm ---surely a practical MPPT controller needs to be a two stage device?

the "Input" stage keeps the panel at the optimum voltage and the "output" stage keeps the output at the set voltage (say 13v for a LA battery charger, or 240Vac for a single phase ac output etc)

Just keeping the panel at it's optimum voltage seems pointless?

--- End quote ---

Optimally it would. Because this is a school project, I was looking for ways to cut costs. I came across this application note from microchip (http://ww1.microchip.com/downloads/en/AppNotes/00001521A.pdf) that suggested a single stage device controlled by a state machine that would alternate what it was tracking (panel power or battery current/voltage) depending on what was needed. So far it seems to work pretty well, but if I were to start from scratch I would probably just add a second stage to the system.

--- End quote ---


For our non-mppt panel, assuming a purely resistive load, then more current = more voltage, so you can just use a fixed resistor of a suitable rating and integrate the total power that comes out (measure V & I to calc power, integrate to energy over time) The choice of resistor will effect your results, as if you chose it to have a resistance that gives an input voltage close to mppt under your average conditions then you will get a greater total power transfer (chose it to be optimum for conditions you never see, and it'll be really bad!)


For your mppt controller, assuming you have a "current controlled" architecture (ie there is an inductor in the system that stores charge) then your output resistance, as long as it is low enough (ie  sufficiently below the minimum panel optimum voltage at max system current) shouldn't matter.

ie

target panel voltage is always 17.5v, and system current is modulated to keep to that target voltage as solar intensity varies, then the duty cycle of the buck will be modulated to get to that target panel current:

 When solar intensity is low, and current is low, the voltage across the fixed load resistance is low (V=IR), so there is a larger Vdrop, and switch on time is short.

 When intensity is high, and current is high, the voltage across the fixed load resistance is high, so there is a smaller Vdrop, and switch on time is longer

As long as the load resistance is chosen to be suitable to maintain the duty cycle within the system limits then it should work. And you can just measure and integrate output V & I in the same way as for the non-mppt panel to get a comparative performance figure

It is the inductor, which acts to limit the rate of change of current, that allows the system to move energy between systems of different potential at a reasonably constant current (ie low current ripple)
olsenn1:
I have two folding solar chargers:

        1. Instapark Mercury 10 -- 10 Watt dedicated USB charger
        2. Powerfilm F12-750 -- 12 Watt 15.4V charger

When I connect the Mercury 10 to my electronic load at CV (5V) I get 1.0 Amp of current (5W) and this is the most I can reliably push out of this. However, when I connect my Powerfilm charger via a switching buck converter (8-24V to 5V USB) to the same e-load under the same lighting conditions, I get 0.5 Amp (2.5W) at CV 5V. This is in line with what my devices charge at when I plug them into this USB port. However, I can switch the e-load to CC 1 Amp and get the same 1A@5V output of this solar panel this way.

Obviously my solar panel is capable of outputting more power than it is. Does anyone know what I could do to fix this? Would a MPPT charge controller do the trick (or is this just for ensuring against overcharge of lead-acid batteries)?

Thanks!
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