Series panel question

[lifer]

Hello,

I have an array of 8 x 230W identical PV panels. They are working fine for about five months now, direct charging (parallel connected) a 24V battery string.

I want to reconfigure the battery string for 48V mode so I first tried to reconfigure the PV panels.
Well, after putting two by two panels in series (then paralleling the four groups) and connect them to the same 24V battery string, I found out that in every particular series string, the measured voltage on every panel is different. For example, even the series string has a voltage of 24V, inside a string I've got 16V on one panel and 8V on the other. I've got similar results on all four series string.

I know that PV panels are working as current sources mainly but doesn't it look quite strange? Like I said before, all the panels are identical - I mean the same manufacturer, same model and bought them at the same time. All the panels are mounted side by side in open sky (no shadows).

[Flux]

They may share better when connected to the 48v. I have never chrcked the sharing of series strings but assumed it was much better than that.

Try measuring the open circuit voltage and short circuit current of each panel, they should be close if from the same batch.

You could also measure open circuit v and short circuit current of each pair. The voltage ought to be double and current the same. If there are significant variations they aren't identical panels. If they behave in these tests then results over the proper working region may improve into 48v.

Flux

[lifer]

Actually, at the time of my previous tests it was a cloudy/foggy weather. The open voltage of every single panel was almost identical (32V) but I barely got any current from all of them (paralleled) together. So it might have been an inconclusive test after all.

Next time I'm going to make all the test as you've been suggested (and in full sun conditition). Thank you very much!

[madlabs]

Solar conditions are very important when measuring panels. Even on a sunny day output can vary quite a bit. I prefer to check current as well as voltage, but a dead short/voltage check in sunny conditions should tell you if they are OK. So if it was foggy/cloudy then for sure your measurements are meaningless.

Jonathan

[lifer]

Thanks, Jonathan! There will be no sun on my street these days - that's why I need to finish my wind turbine (VAWT) project in the next few days. I was living happily during sunny days with PV panels only.. but now I have to revive the old gas generator quite often.

Thank a lot, guys! I'll keep you informed about measurements. The panels seems OK - maybe was just the non linear region of their characteristics at the time of testing.