The days when G6 TRANSMITTERS could be restarted and run until their battery died are long gone. G6 transmitters will not start a sensor more than 99 days after first start, and then will no longer start a new sensor. You can get a maximum of 99 days plus 10 days, as described in this thread: Oldest G6 trans to start new sensor? - #15 by Jag1
Voltages are now irrelevant. If you look in System Status page, on G5/G6 Status, Transmitter Days field shows how many days ago transmitter was started, so watch that to see when your transmitter will be expiring.