I've compared the output voltage of 2 Victron solar panels I've just purchased and in lower lighting conditions my digital voltmeter reads approximately 6volts different for the same lighting conditions. In bright sun they both read approx 22 volts (the maximum open circuit voltage stated on the Victron data sheet). Should the readings in the lower lighting conditions be similar or is this an unfair test?
Voltage comes from the 'presence' of light, Current from the 'intensity' of light.
In lower light conditions, the reduced presence of light will reduce the voltage accordinly. See the voltage (blue line) increase in the morning and decrease in the afternoon at this site:
This is an „unfair test“.
Like @Warwick Bruce Chapman already mentioned PV planels need quite some light to separate charges and therefore create a difference in potential (OCV) or a current flow in case of a closed circuit. By testing in „low light“ conditions you can get pretty much any reading between zero and (full) OCV.
The reason behind this is that there are always also some internal losses within your panels. These losses are incredibly small and therefore not affecting the OCV or the general function under normal conditions / bright light since the charge creation is several magnitudes higher than the internal losses. If you now (heavily) decrease the light intensity and, in consequence, the charge separation there will be a point where internal losses and charge creation are in the same order of magnitude and therefore compensating each other.
Many thanks for your answers but I've now checked a total of 4 panels and 3 out of 4 are approximately the same output voltage and the 4th is 5/6 volts lower than the others. Is this just coincidence?
Many thanks, Brian
@BrianP If the measurement was done in (full) sunlight, than this is a serious issue.
The measurement was still done with a multimeter without anything connected to the panel, right? There is no sticker (or anything else) on the front glass that could have created partial shading?
From my point of view you should contact your seller for warranty replacement.
And even if you shouldn‘t have warranty anymore, please don’t use that panel anymore! It‘s only a question of time, but at some point this panel will probably slowly start burning (at the foil on the back-side). If you own a thermal imaging camera you should already be able to see a hotspot.
