[kragen]

Is that the MPPT voltage or the open-circuit voltage? I was thinking of a near-open-circuit voltage (which is what you have if you're powering a 2-watt webserver from a 50-watt solar panel), but MPPT will vary a lot more. Also, covering 6% of your cells will drop your voltage a lot more than covering all your cells 6%.

[lightedman]

Open circuit. This is one specific behavior we looked out for when testing panels before shipping, after the EL test, lamination, and junction box installation.

[kragen]

Huh, I guess I must be totally wrong, then. Thank you for the generous correction!

[lightedman]

You're welcome. Even without MPPT inverters/chargers in the mix, you can take a 1,000V (maximum US NEC allowed IIRC) string of panels (obviously in series) fully black out one cell out of hundreds, and your entire system will do nothing (assuming your panels do not have bypass junction boxes and are straight-series.) That blacked out cell essentially acts like a full clog in a pipe - nothing will pass because of that 'dead' cell, until you remove that whole panel and replace it or bypass it.

We test by shading different cells at a known light level, and seeing how far the panel as a whole deviates from our expected baseline. Usually what we'll find is that a cell hadn't had its ribbon bars properly soldered while in the tab/string machine, so the shading of the cell dramatically reduces performance. You can't catch this kind of defect easily in an EL tester, as usually you've got enough contact between ribbon and cell to get the entirety of the cell to emit light, thus hiding defects from soldering. So we implemented that shading test to root out bad cells/bad soldering jobs. Quite proud of having thought of that testing methodology, because I've not seen any other company test panels in that manner.