question

Is the MPPT PV voltage target constant?

Hi, I have an installation including a Victron MPPT 75/15 connected to one 12V 14AH battery and 4x10W solar panels (2 series x 2 parallel). I've discharged the battery and let the system charge the battery. When I measure the energy output of one of the series with my INA226+Arduino, while the mppt in bulk charging (yellow led blinking fast), I am getting about 2xVmp as the voltage output as expected. But I am getting the same voltage output even in low irradiance and even in partial shading situation. The current decreases but the voltage tries to keep stable. Does it mean that the mppt is working with the constant voltage algorithm?

Vmp of a panel is 16.8V, so Vmp of a series is 33.6V and I am getting a 31-34 voltage range in nearly every situation even in low irradiance and partial shading.

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Guy Stewart (Victron Community Manager) answered ·

This is the I-V (Current - Voltage) curve of a 24V solar panel.

Each coloured line represents a decrease in solar energy landing on the panel.

This sample starts at 1000W / m2 - which is roughly the peak sun that falls on earth, and the standard test condition.

It ranges down to 200W / m2 - which is a heavily overcast, or very low sun angle condition.

You can see that across this very large difference, the solar panel behaves remarkably similar in its voltage output and maximum power point, while it is the current that varies significantly.

This is the same behaviour you are are observing. It is not the Victron MPPT that is dictating a fairly fixed voltage, but the behaviour of the panel.

Even under very bad conditions, the maximum power point voltage is still very high and consistent, the inflection of the curves begins at nearly exactly the same point of about 38V (for this panel).

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Your answer is correct when solar radiation is uniform. But as I mentioned in my question when a partial shading occurs on solar panels for example when I put a semi-transparent obstacle on one of the solar panels, one of the panel in the series will get 1000 W/m2 solar irradiance for example but the second one in the series will get solar irradiance lower than 1000 W/m2. In this setup what should the output voltage of a series should be?

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