car trouble

I have been having trouble with my Prius since middle November.  Sometimes when I shut off the car by putting it in park and then pressing the start stop button I try to restart my car and the computer says no.  The error message is weird.  It says transmission (P) is abnormal.  No matter what you do it will not start.  Well until.... I was so upset the second time it happened that I got out and shoved the car.  When I  looked into the car the computer message was gone and I could start it and go.  Well it has happened several times since and rocking the car fixes it.   Friday I went to the Toyota dealership and talked to a man I was introduced to as the head technician.  I explained what was happening and he knew immediately what was wrong and why my rocking the car solved.  It is kind of a funny visual of me outside rocking the car first sideways and then front to back.  But anyway for the explanation.  It only has been happening when it is really cold.  Remember my car is 8 years old.  Something I didn't know was that I have a battery separate from the high voltage batteries that run the car.   That battery is used to start the electric motor that is is used in turn to start the gasoline motor.  Well when that battery is low it cannot move the cog that fits into the cogs that start the motor.  If the cogs aren't meshed the computer sends the error message and keeps the car shut down.  Hence my rocking the car is enough to move that  cog into place to make it mesh.  Then the computer takes over again and removes the message.  If I knew I had a battery in this car I would have been asking when I needed  to replace it.  After all it is now 8 years old.  I got  it fixed on Monday.They were amazed that the battery lasted that long because they normally change them at five years.