2008 Prius dead 12V battery, potential HV battery drain in progress??

My 2008 Prius which has <100k miles had an issue for the last couple days where when I went to turn it on (depress brake, push power button), symbols came up on the dash and the power button was orange (is this accessory mode?). But if I tried to turn the car on again, it would fully turn on with the green LED (ready mode?). It was fine to go to the grocery store, and when I

My understanding is this is a dying 12V battery and diagnostic mode seemed to confirm that. Today, I planned on getting a new battery, but when I went to turn on the car, it did the orange LED/green LED thing again, except the car wouldn't fully turn on. It got stuck in a loop where I couldn't turn it off and I was afraid the car would end up draining the HV battery into damage.

I live in an apartment and was able to flag down someone in the parking garage to give me a jump. This had the benefit of finally turning on the main LCD (the MFD?) but it gave an error about the parking brake that I couldn't fix. As far as I could tell, the engine never came on.

I was able to "turn off" the car, though it beeps when I get in which reminds me of when the car is on but you get out with the key your pocket. I'm fine to walk/Uber to an auto store to get a new 12V battery tomorrow, but my main concern is: is my HV battery getting drained into damage as we speak? Is there anything I can do to ensure the car is fully off until I can get a new 12V battery tomorrow?

 

I can't speak for the previous owner(s), but I've had the car for a little under 2 years and I drive it on an almost daily basis. Thanks for the head up about the HV battery though, I didn't know that.

If the car is in some weird state right now where it's not truly off, is there a chance the HV is being drained? I ask because I thought the HV battery was used to charge the 12V, but it sounds like I'm wrong? I assume it's okay if the 12V gets drained fully dead right now since I need to replace it anyway.

I looked up the price of a replacement battery at Advance Auto Parts and it's apparently $220?! Does that sounds right?

 

The car was always fine until this week when I ran into the problem as described in the OP: I go to turn on the car (brake + power button) and all the dash lights turn on but the power button LED is orange. I "turn on" the car again (brake + power button) and it fully turns on with green LED and the engine starts after a few seconds like normal.

You say the HV battery doesn't drain when the car is off, but I'm not 100% sure it's off because of the beeping when I open the door.

 

I just checked at the jump points under the hood with a multimeter, I got 8.3V. I've left the car unlocked since it's in an underground parking garage. The red security light still blinks, but there's a beeping when I open the door, maybe to tell me the Smart Key system won't work?

1) Is my battery salvagable?
2) If yes, there's a Mr. Tire 3 blocks away. Could I take the battery to them and have them charge it up?
3) Should I try to find a portable jump/charger from a neighbor in my apartment building to borrow?

When I tried jumping the car on Sunday, it "turned on" but it gave me an error about the parking brake not being right, so the engine never spun up.

I have experience accessing the 12V battery area since I installed a cable to power trailer taillights, not on this 2008 Prius but a different one.

Is this because the hatch is opened electronically and will be a pain in the butt once the battery fully dies? My concern is, wouldn't that light up the dash light saying the trunk is open, further draining the 12V? I may not be able to get a new battery/go to the auto shop until Saturday.

Since October, my only driving is 10 min (each way) trips to the grocery store and Costco, maybe 3-4 times/week. Sometimes there was a 45 min or so trip thrown in every month.

These are my options within Uber distance:

$180 NAPA The Legend Premium AGM Battery BCI No. NA 325 A Glass Mat BAT 9851P | Buy Online - NAPA Auto Parts (napaonline.com)
$200 Duralast Platinum AGM Battery 340 CCA (autozone.com)
$220 DieHard Platinum AGM Battery, Group Size S34B20R, 340 CCA S34B20R: Advance Auto Parts
$220 DieHard Platinum AGM Battery, Group Size S46B24R, 410 CCA S46B24R: Advance Auto Parts
$284 Optima Yellowtop Prius Deep Cycle Car Battery, Group Size DS46B24R | 513308 | Pep Boys

 

After some bloodied knuckles, I was able to remove my 12V battery and it's now charging via a neighbor's 87102C Century 10/2 Amp 6/12 Volt Manual Automotive Battery Charger (centurytool.net)

Although the charger instructions say to use the 10A mode for car batteries, my battery itself says 4.6A max, so it looks like I'm stuck with the 2A mode and I'll be charging the battery for quite a few hours.

Two questions:
1) The battery was down to 4.5V when I started charging. This might be because I left the rear hatch open for about 24 hours (I wanted to make sure I could open it yesterday but I didn't have time to remove the battery). Is there any hope my battery will be usable, or should I just anticipate buying a replacement?


2) In the process of removing a mounting bolt for the controller that blocks the battery, the bolt snapped in half, apparently from rust. I still have 2 other mounting points--I should be okay there, right? Unless the bolt that snapped is a grounding point. The metal attachment still makes contact with the frame of the car. Maybe I could use some duct tape?

 

I've been charging for an hour and wanted to check on the progress, so I turned off the charger and disconnected the cables. My multimeter is reading 12.4V now. Surely that can't be right??? Also, the charger showed 6A when I unplugged it even though it was on 2A mode and showed 2A when I first started charging.