Festool with the use of EC controlled motors is creating a tool line in which various tools can use batteries of different voltages and amperages so long as the battery has enough minimum Amp Hours to power the tool.
The 2013 Festool USA catalog had a battery comparability chart in the Drill section.
The Battery Charger TCL 3 - 499412 can charge Festool NiCD, NiMH and Li-ion batteries.
One can check their tool's instruction manual as to what batteries the tool can use.
From the owners manuals:
T15 Batteries:
Battery Pack BPC 12 Li BPC 15 Li BPC 18 Li
Voltage 10,8 V 14,4 V 18 V
Capacity 3.0 Ah 3.0 Ah 3.0 Ah
Weight 0.49 kg (1.08 lbs) 0.58 kg (1.28 lbs) 0.68 kg (1.50 lbs)
C Series Batteries:
Battery pack BPC 12 Li - 1.5 Ah BPC 12 Li - 3.0 Ah BPC 15 Li - 3.0 Ah
Voltage 10.8 V 10.8 V 14.4 V
Capacity 1.5 Ah 3.0 Ah 3.0 Ah
Weight 0.31 kg (0.68 lbs.) 0.49 kg (1.08 lbs.) 0.58 kg (1.50 lbs.)
It looks like the BPC 12 LI - 3.0 Ah and the BPC 15 Li - 3.0 Ah batteries can be used by both drills.