The Festool batteries / tools are designed to be backwards compatible and higher V tool with lower V battery. At least within the same battery shape / style. In fact the LiOn tools will operate on a NiCd battery. Must be the electronics?
I am not certain this is true for all tools. But most I have tried are "keyed" to fit or not fit.
I have been running my 15v Impact on 12v (1.5 Ah) batteries and my TSC55 on 15v batteries.
One thing that doesn't work though is the TSC55 won't run on low Ah batteries even though the voltage is OK. EX- the 1.5Ah 12v and the compact 3.0 18v won't run the TSC55. Need to get over the 3.0 hump.
Seth