I thought my C12 batteries had simply died. However, after getting new batteries, it turned out the C12 itself was dead. Festool service said my almost 5 year old C12 needs a new electronic card. (Good thing the possible sale a couple of months ago did not work out. That would have been embarassing). The cost will be about $100 plus shipping.
I have a T 15 +3 (kit) and a CSX (along with a couple of old pre Festool devotion Dewalts). The C12 batteries will power the T15 and the accessory chucks would be useful on a new drill I believe.
So, should I get the C12 repaired or put the money towards something like a new T 18? I am leaning towards repair, but I am also nervous that something else could go wrong.
I also have the T15 and CSX and find them to be a great combination. I'm surprised how much I use the CSX with the T15 for backup and big stuff only. I don't think I will need the T18 based on my own needs. Like you, I'd lean toward repair.