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, I am getting about 2xVmp as the voltage output. 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 one series is 33.6V and I am getting a 31-34 voltage range in nearly every situation even in low irradiance and partial shading.
The Vmp and Voc voltages of a solar panel only change very slightly with irradiance. Most of the power loss arises due to the current (Isc and Imp) falling in proportion to the irradiance. Therefore, what you are seeing is not really unexpected, regardless of the MPPT strategy. Hope this helps.