Replaced battery on 2010 Prius and still doesn't start

So, the old battery died, possibly because my kids left a light on. In either case, the original battery was 5 years old and needed replacing. This was confirmed at Auto Zone. so I bought a new one but they had to order it from the warehouse. The next morning, I picked up the new Optima battery and installed it. The Prius did not start.
I thought the battery must have been sitting at the warehouse for a long time (not too many Priuses in South Texas) so no big deal, I just jumped it with the truck. I used the front jump plate and it started right up. I drove it around the neighborhood and over to my Dad's (who has an older model Prius) and I guess I didn't drive enough because it did not start back up.
So, we went to the internet and couldn't find the solution. It was kinda like old times in High school with the two of us working on an old car, but I digress. We checked every fuse, including the one on the actual battery and everything was fine. My Dad said something about the battery in the FOB and I thought, what the heck? so we went back to Auto Zone and replaced the battery in the FOB.
We jumped it with the old Prius, which died for a little bit, which was wierd, but it worked on the second attempt. I drove home and ran it in Park with my foot on the gas for 10 min. turned off and did not turn on. At least it was back at my house under the carport. At this point, even the FOb wasn't locking or unlocking the doors. no beeps.
I borrowed a battery charger from my inlaws because the battery was reading 5.5 volts on 20. I'm not sure if that's 5.5v or 11v but I'm gonna splurge next week and buy an upper end voltmeter. Still nothing. I went back to the internet and noted that one guy had to reset his FOB with a new battery by disconnecting the neg on the battery. The FOB worked no problem. but then the car apparently sucked all the juice out of the battery, which had been charging for 20 min prior to the disconnection.
At this point, I am overnight charging the battery, hoping that it will hold a charge in the morning because it was sitting in the warehouse for too long and I didn't drive it enough today to charge it up. I will let you know what happens, but I am concerned that there is something in the 12V system that is sucking out the juice. At times, I hear a small whir noise that was described somewhere else as the battery charging system just left of my fuse box.

Any and all comments are welcomed. If this thing doesn't start in the morning, I'm gonna have to go to the dealership, which would be a shame because it was working fine until two days ago.

Serge

 

How did you know that? The battery went from 5.5v to 6.5v by morning. by the time I took it back to the store it tested at 4.5v. They charged it for an hour and wouldn't hold a charge. we are ordering another one overnight.