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Hi @John,

@M.Lange answer is correct, and I would like to use the opportunity of your question to explain even further.

Solar panels, unless heavily shaded have a remarkably high and consistent voltage output. It is the current output that decreases.

Have a look at these I-V (Current vs Voltage) and P-V (Power vs Voltage) charts for a 300W solar panel.

You can see in the P-V curve, that as the solar radiation decreases, from 1000W/m2 to 200W/m2 (20%), that while the power drops from 300W to 60W, the Voltage output range remains nearly constant, with the Maximum Power Point (MPP) voltage identical at 33V, and the maximum open circuit voltage only dropping from 43V to 38V.

If the voltage is pretty constant regardless of the intensity of the light, then the Current must be changing.

You can see this clearly in the I-V curve, where the output current is dropping from 9.8 A at 1000W/m2, to 2 A at 200W/m2.

What this means is that the input voltage in a correctly designed and installed system free of obstacles should nearly always be within the acceptable voltage window of the MPPT for optimum performance (even when its cloudy).

Where voltage DOES vary is during heavy shading, like a tree branch obstacle, and due to temperature.

Due to the nature of the semiconductive silicon in PV cells, the effect of shading on the solar panel is so severe that if a single cell (of which there can be 36 to 144 in a panel) is completely shaded, then it will completely restrict the flow of electricity through it.

Solar panels use built in bypass diodes to then skip that troublesome cell group (usually a several horizontal columns of cells) to allow the energy from the other unshaded cells to flow.

The Victron MPPT itself is a buck DC to DC converter. It can only reduce the higher PV side voltage to the lower battery side voltage. It cannot boost the voltage from a PV panel up to charge a battery.

The MPPT can accept, and with good efficiency (ie very little loss of total power) convert any PV side voltage up to its maximum PV input voltage limit. This varies with the Victron models between 75V and 250V and is clearly printed on the unit itself, and all associated documentation.

On the battery side, it is the battery that is setting the system voltage. The MPPT will be converting the panel voltage and outputting a higher than battery voltage, to get the current to flow into the battery. But the battery chemistry will be dragging that MPPT voltage down at the DC bus level, and that electrical work is going into the battery chemistry to charge it.

Once the battery is full, and reaches a target voltage, the MPPT will adjust the conversion to maintain the float voltage. Still higher than the battery voltage at rest, but not enough to allow for meaningful current to flow, or the battery would be overcharged.

I hope this helps you understand a bit more about the equipment and the process.