I've been much more careful about my readings on these petrovoltaic cells here lately. The latest test I've done was to cut off ALL power to my house. So flipped the main breaker and my house had no current flowing in it so that I could eliminate the chance that I was reading stray voltage from CFL's or from the walls. The good news is that these petrovoltaic cells still produce voltage. I even moved the cells around to different place in my yard and drove around and they still give off voltage.
I find the best test that I do is the shorting the cell out test. I never seen any battery of any type able to slowly go back to the original standing voltage after being shorted out for a day or even a week like some of the cells. Removing the short shows the cell go back up in voltage to almost the original standing voltage and over a few minutes it slowly goes back to the original standing voltage and some have even gone up above the standing voltage but I consider to be apart of the temperature and lunar effects. I do believe that these cell can't last forever, they must drain, after all everything dies eventually.
I do find that temperature is a real mixed bag. I remember working on cells that used distilled water as the dielectric and one day the hot water would raise the voltage but next week the cell wasn't affected by it. If anything temperature would raise the voltage slightly but it can be hard to say that these petrovoltaic cells are driven by temperature.
These cells also do act like a low micro farad capacitor when given a charge.
I have made some glue cells that are given power from a AA battery while its drying, as to treat like a electret. It does take the charge but I think High voltage would work better. A AA battery at 1.300 volts would have the glue electret cell at about .400 volts after 3 days of resting and fully dried out. It does hold around .400 volts but I think high voltage would align the cells up better than this low voltage method.
I'm also back at testing some rocks from my drive way again and them too when the power from the house is cut off still give off a very small voltage.