2009 Prius Hybrid battery dead?

welcome!
corrosion can be the issue. if not, you need to load test the cells. are you in a carb state?

 

Daniel,

At this point, my only recommendation would be to not jump to conclusions. There's no mention in your first post about whether you had codes read to determine the actual diagnostic trouble code. It may be in your best interest to find out what the car is telling you before you start swapping parts. More than just the battery could be causing your problem

 

Or, get the error codes read and see what the warning lights really mean.
Edit: Beat me to it!

 

Spend ~$75 to buy diagnostic equipment
- Android phone/tablet (~$50)
- Elm 327 Blutooth OBDII adaptor (~$20)
- Torque Pro app (~$5)

Then you can see the cell data live as well as many other bits of information.
Plus you can read and clear codes.

Where are you located?

 

Have you checked all your fluid levels? Low oil can give you a triangle and won't do much when you step on the gas

7.4v is pretty low for a running car, I would think it would read 7.6v and above....unless it's been sitting for a while