Check PCS System and Error Code B15EC

I have an error that says Check PCS system. Cruise Control is disabled. For the last year this error has been intermittent. I took it to a Toyota dealer. They indicated that there was an error code B15EC. They want to replace what they referred to as a "Mayday Battery Pack". (They want $300 used from ebay, I see it on ebay for $35. Plus $700 labor including removal of seats to get to it.) It was described as NOT being either the hybrid battery nor the 12 volt battery. However, when I research the error bode B15EC, it seems to refer to yet a different battery referred to as telematics Backup battery. So this raises several questions in my mind.
1) Is the Mayday battery pack the same as the telematics Backup Battery?
2) Would this error code really be the cause of the PCS system being inoperative?
3) I can't seem to find any documentation or videos on where either one of these batteries are located. Can someone point me in the right direction?

Thanks,
Tim
2015 Prius 5