I have a C15 and a CXS drill. I also have some experience with LiIon batteries in general, so here are my thoughts:
It sounds like either you have a defective battery, or perhaps more likely, the battery's internal "fuel gauge" chip is not yet calibrated to where the battery is truly at. Usually LiIon systems benefit from an initial calibration charge: several hours initially on the charger, followed by usage until shutdown. It isn't memory effect, but more of a calibration of the electronics.
LiIon is different from previous rechargeable battery chemistries in that it is not straightforward to measure capacity. LiIon maintains a nearly steady voltage through most of its range until the very end. So, the manufactures add an internal chip that keeps track of energy IN vs energy OUT as well as maximum and minimum. With a new battery, this "fuel gauge" chip may not have calibrated itself to the true capacity and state of charge of the battery. This takes at least a full charge/discharge cycle. I don't know how exactly Festool may or may not implement this setup, but it is pretty typical.
According to the manual, a solid green light on the Festool charger doesn't necessarily mean the battery is full... only that the charger "thinks" the battery is more than 80% full, so presumably it slows down the charging rate to a trickle in order to not overcharge. You may need to leave the battery on the charger for several hours with that solid green light for the initial charge. At some point, the battery will truly hit "full" which should also calibrate the top end of the internal fuel gauge.
The other end of the fuel gauge would be "empty" ... which is typically only calibrated when you draw down the battery to its "empty" state. The electronics should detect the battery is rapidly nearing empty and shut down to protect the battery. At this point, the internal "fuel gauge" chip should have a good reading on the battery and things will match up.
Another quirk of LiIon batteries... for long-term storage, they are best stored at partial charge, say 15 minutes on the charger.
LiIon have both a cycle life and a calendar life. The latter has been improving... 10 years ago, we were seeing LiIon batteries last at most two years regardless of usage. Now it seems you can get many more years out of them. My experience comes from seeing tens of thousands of LiIon batteries in an industrial setting (handheld bar code scanners).
One last point to keep in mind on LiIon drills... unlike NiCd and NiMH, all LiIon devices must protect the battery. The electronics absolutely must stop the drill if you try to draw too much current, heat the battery up to a certain point, or reach empty. Any of those conditions can lead to bad things happening, such as thermal runaway and a fire (just search YouTube for LiIon Fire for some dramatic examples). With the older chemistries, you were allowed to punish the batteries because at most you were just shortening the life cycle of the battery. LiIon batteries have a more dramatic failure mode, as Boeing has found out the hard way.... monitoring and protecting the battery has to be rock-solid, and I would expect Festool to not have cut any corners in that department.
My C15 drill really does feel as powerful as a brushed 18V drill, so I can't imagine being disappointed with the T18... it should have gobs of torque. I've had no trouble with a 1.25" auger bit through old growth fir... it just goes and goes. However, unlike my old NiMH drill, the C15 will stop and beep if I jam the bit and stall the motor. That protects both the battery and the motor (and my arm).
Hope that helps!
[edited for clarity]
