During a production, a wireless mic required "0" input gain at the console preamp and still had plenty of signal in the level meter (acting like a line-level input requiring -20dB PAD). Very odd. Then, the batteries died. Once they were changed out of the transmitter, the gain staging went back to normal and (as it should) the mic required input gain on the console to get good levels. We use Pro Cell batteries. Is this a normal characteristic of these batteries when hitting end-of-life? If so, what really causes this?