I don't believe from all I have read about batteries that cell voltage is a good indicator of anything but cell voltage. Would be super nice if voltage alone would be a good predictor of charge, battery condition, service life or anything besides 'voltage'. I would like to see some data collected to determine if the voltage correlates with anything. Sure would make life easier. But from what I think I know the best indicator is battery energy or watt-hours. That means amps x volts x time to charge (input watt-hours) and same for output watt-hours. That becomes much more complicated to measure though some of the chargers do measure mAh which is close but doesnt take into account the cell voltage.