I think that the type of metering is the answer, not the battery voltage. As good as those meters were - generally - they were inconsistent when it came to differing focal lengths. An inbuilt spot meter in these circumstances would be more accurate so long as the meter was pointed at the same place.
Even my F100 and F80 show different exposures for different focal lengths especially with my 17/35 where the scene contains a large amount of different tone, even using the matrix meter, but are almost parallel when it come to using the spot meter.The simple answer is to meter off a piece of grey card.