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

 

Dealer also told me Toyota did not make the part. But, they can order a used part off eBay for $300. Bad news is that you can get the same part off eBay for $30!!! I did not figure this out till after I got home and had already paid for the part!!! I do NOT believe this will fix the Check PCS problem, it will fix the error code that happens to exist on your car. This battery only supplies power to the telematics system which does things like calling for help if you are in a crash and the main battery has disconnected. I don’t know this yet, but plan on asking the dealer if they are guaranteeing it will fix the Check PCS error. I was hoping someone on here might verify my suspicions.