Prius battery died today

Flattening the 12V battery wont have any effect on the HV battery, so don't worry about that. Yes you should try charging the 12V battery. Do you currently own a charger?

With the voltage measurements, try measuring them first thing in the morning. Be sure not to allow the car to go to ready (at all on that day) before taking the first two voltage measurements.

 

I'm thinking you are probably low on oil or overheating. When your car is running, it will not drain your battery.

 

The newer style "smart" chargers are the best ones to get, as you can leave them connected for a long time (overnight or even 24 hours) without risk of overcharging.

It sounds like it's ok to drive it. Driving it around for a few hours will help the battery, but it probably wont fully recharge it. As I said before, the very best way to check if the battery is truly "holding up" is to test it first thing in the morning before it's been started or ready for that day.

BTW. I'm assuming that you flattened it originally by leaving it in accessory mode while you were waiting (for someone), is that correct?

 

Should be ok, just don't run the car without being in READY. In the Prius there is "off" where nothing is lit up, READY mode and ACC mode. In ACC mode the car is being powered off of the 12v battery. There are lawn mowers with bigger batteries. It is not meant to do that. ACC mode is for a quick check of the odometer reading or ejecting a CD. Not to sit and wait. READY is the default mode of the car. If you are in the car and want anything to be on, put it in READY. That simple, no questions asked. The engine already shuts off when not needed, you are not saving much in ACC mode. Technically you are saving about 300W to 400W of power that is "wasted" keeping the car alive and ready to move when not moving, but that's pretty small in the grand scheme of things. Also lets you run whatever you want like AC or the giant subwoofer you might have in the back.

If this is the first time you killed the battery, you are probably OK. If this is a chronic problem, then you might as well replace it. If this is your grandmother's car and you never want to risk her being stranded ever, then replace it.

The act of jumpstarting a Prius will not in itself damage the car. However, it is extremely easy to damage the car by blowing the inverter by connecting the jump leads backwards. This happens more often than you think, especially with quick roadside type mechanics who connect without thinking. In a normal car you might get some sparks and a laugh. In a Prius you will get a small pop, some smoke, and a $4000 bill from the dealership. Get yourself a pair of smart jumper cables and keep in the boot. Those will not allow wrong connections.

 

Not enough information.
How old is the battery ?

If the answer is: "More than 5 years".....or....."I don't know", then you probably should consider a replacement soon.

You also could have a load test done on it after a good charge but those aren't always conclusive either.