This test has been on going for quite a bit now, 211 days ago to be exact. Been running a 900mah Zeus as only a PT for that time to see what kind of effect this has on the overall battery life, and once a month using it wireless and keep track of how long it lasts with a 1.5ohm DC tank. For control purposes have also been running a Zeus under the same conditions without ever using it as pt.
900mah Zeus never used as a pt-
First run time- 4 hours and 33 minutes
After 100 charges- 4 hours 31 minutes
After 200 charges- 4 hours 32 minutes
All in all no real change to speak of yet. This battery has gone on the charger as soon as the usual dramatic voltage drop was detected (in other words have deep cycled it) then charged using the usb port.
900mah Zeus used only as a pt-
First run 2/18/13- 4 hours and 33 minutes
3/18/13- 4 hours 28 minutes
4/18/13- 4 hours 25 minutes
5/18/13- 4 hours 20 minutes
6/18/13- 4 hours 11 minutes
7/18/13- 4 hours 3 minutes
8/18/13- 3 hours 8 minutes
9/18/13- 2 hours 29 minutes
This Zeus was left on the charger whether in use or not and vaped for at least 2 hours every day as a PT. As we can see keeping it on the charger 24/7 and only using it as a PT does hurt the cell (which didn't come as any surprise). While it still works, know it's getting close to retirement.
In conclusion, if you want your Zeus to last as long as possible, don't always use it as a PT. Remember lithium batteries don't like being kept at a full charge, shallow cycled, or deep cycled.