Dead battery over the weekend.....why?

Shot-in-the-dark....
Without getting into all the "charging rate" specs...And in the absence of having "volt meters"; you may do a very close inspection of all the wiring/terminal/junction connections..even to-the-point of unloosening the connections, inspecting for corrosion/cleaning and reconnecting....
I gave up on working on my cars when you could throw a bucket of water under the hood and it wouldn't reach the pavement below....

 

Several on here have reported similar problems with new Optima batteries that were not completely charged. Assuming that you replaced the battery correctly this could be your problem. If you have a voltmeter check the battery voltage at the battery terminals. If it is less than +12.5V then the battery needs to be fully charged with a battery charger.

 

This is another shot-in-the-dark, but before I bought the 2012 Prius we went out and purchased a beautiful new Camry hybrid. It was dead on the lot before we had the first test drive, but the salepeople assured us that someone must have just forgotten to turn the car off.

The car was dead in the driveway the first morning we had it, and after a trip back to the dealership, dead in the driveway the next morning, too. They ended up finding that the inverter was eating up all the energy in the low -voltage battery. They ended up taking that car back so I now have a Prius.

So the inverter is another potential culprit, if the simple things aren't the problem.

 

Could have simply been a bad cell in the battery. Had the original battery in my 2010 replaced under warranty at about 30k miles due to a bad cell.

- Spindifferent

 

I bought my Prius off main dealer, it is a limited edition model here in the UK so is above top spec model, 4 months old and dealer had owned from new.

We picked up the car after work and drove home. Went out after dinner to play and battery dead! Called out car assistance company and mentioned to me it was about the only thing he had ever been called out for on a Prius, so good news realy!

Phoned dealer next day and they didn't believe me, but told them I didn't care and was bringing it back right away for a new battery.

Been happy ever since.

 

Very small chance you have a bad out stock Optima.
Sounds like your car is drawing current while in power down.
If you had a good multimeter, like a Fluke, you could measure current draw while all is powered down.
You need to know how to use a multimeter while measuring amps.
It is true, all batteries should have a topping charge before being put into service, but this is a minor thing related to life span of the battery. I have a experience with a 6 year old Optima, still going strong in a TDI of my daughter's.