Dead 12V Battery, Jump-starting doesn't go into ready mode ("transmission "P" lock mechanism" error)

Hey all,

I have a second gen Prius (2005) that I got at auction (former fleet car for the county), and it has a weak 12V battery from sitting a few months (sometimes reads only 10V in accessory mode). I used it a month or so but it seems to have finally given up the ghost; it stopped going into Ready Mode (rest of the dash lights would go on). On later attempts, only the CEL would appear.

I tried jump starting it (would only work directly on the battery, not via the front fuse box) with an Everstart portable jump starter, but while I seem to get power for everything, I still can't get the car into Ready Mode. I get the "transmission "P" lock mechanism" error on my MFD, which I heard can be related to 12V issues. Green light on the parking button would flash too, if that means anything useful.

Is it possible my jump starter isn't providing enough voltage? I remember also seeing this error when I had my door ajar (don't think this is it, but I could try again). I just want to get the car moving on its own power so I can get it to a battery shop for a swap. I tried pushing down the parking brake lever a few times (suggested on the forums), but no luck there. Do you guys thinking charging overnight with a 3A battery charger would give it enough juice to start one more time?

Or should I just buy a new one and do the swap where it's parked? It's on the street so not ideal...

 

Finally managed to get it into ready mode with a freshly topped-off starter (charge indicator was green but it had sat for a month) and making sure the doors were securely closed (which was probably the real issue).

Now to replace this 12V ASAP... but there's a fairly new Optima Yellowtop (<6 months old) inside. I think it probably just sat while discharged for too long, and I've heard Optimas are not great in recent years. But is there a possibility of something else wrong that's killing the 12V? Some rogue accessory taking excessive load? I'm also thinking about trying a cheap lawnmower/mobility scooter battery as a stop-gap solution, maybe it'll work better than I think.

I have the non-SKS model so fit might be a problem (different bracket?). I like the long warranty on the Toyota dealer batteries, but they want $300+ installed. And the non-SKS battery is only 30Ah.