2006 Prius Warning Lights, please help!

I just purchased this vehicle from a private owner, and the first day everything seemed fine. After a few days I get all these warning lights - Master Warning (Red Triangle !), VSC, and the Brake Light, plus the NAV screen shows the red car with ! in it. The car seems to run just fine though. I don't notice any performance issues. One thing I did notice when I purchased the vehicle, the previous owner had a battery charger in the trunk before he took it out. Now I've check the battery using the LAN Monitor and it shows like 11.7 - 11.9 volts when I turned the car on (press power twice without brake). When the car is READY it shows 14v. I decided to give the battery a deep charge overnight (2 amp charge) after disconnecting NEG terminal from car. It cleared the codes for a little while, but then they came back. My ODB II tester shows no codes, only codes I found on the LAN Monitor was for NAVI - 01-e3 sub 00-5. I really need help trying to troubleshoot this. I don't want to bring to dealer and get soaked for some easy fix.

Do I need to replace the 12v Battery? They cost like $200, so I want to be sure that's the problem.

Thanks