12v Battery Dead Instantly After Driving,

There's no alternator in the Prius you're charging rate is very low compared to an alternator car your battery must be dead and not be accepting of the charge that is being provided it simple new battery time

 

Or you're inverter 12V charge system is not. It should show 13.7 to 14.5 wen ready ...

 

Put a charger on it, or leave it in ready for 8 hours

 

How did the new battery go dead?

 

Normally I would have said that your battery has a bad cell or two. However, since you were able to start the car and the state of the 12V got worse it may be that your inverter is putting out too high a voltage and it is indeed killing the 12V. Long, long ago my mother had a Buick which was eating 12V batteries, and the alternator was putting out something stupid high, like 20V. Even then though, it took a couple of weeks to trash a new battery. Since yours is sooo fast maybe it is something more mundane, like a loose connection at the battery? It happens to be in contact when you start, works its way off during the drive? The 12V only has to have enough oomph to start the car and after that it can be detached and the car will still run. I don't recall who did that experiment, I think maybe Hobbit?

 

Okay, sounds like you just need to charge it up

 

Possibly so you want to take a digital or mechanical volt on meter and see when you're in ready what's on your battery terminals at ready should be 13.7 to 14.5 or so then when you turn the car off all you're seeing is battery voltage whatever the battery has stored in it so if you turn the car on and make it ready and you read say 14.1 volts then you turn the car off and you leave the meter connected to what was just showing 14.1 and then all of a sudden that display turns to 9 volts that's what the battery is stored of the 14-1 that was being sent to it a minute ago and/or on your 30 minute drive the amp and hour circuit of that rating is nothing like an alternator or your battery charger so it'll keep a good 12 volt battery charged up because it's not being taxed very much a bunch of LED lights and some computers everything that really runs on anything is running off the HV battery the air conditioning the electric motors for propulsion braking etc that's why our Toyota battery is really small etc. So there's not much headroom in our little Toyota battery once it starts to fail it plummets very quickly but it's also not under that serious of a load it doesn't have a starter to turn none of that at the most some halogen headlights if people haven't been smart enough to change those to something more efficient.