Original factory 12V battery dead with zero warning

Unfortunately, this has been a Toyota problem since they came out with hybrids.
We have been trying to figure out why there isn’t a reliable low battery warning for over 20 years without success

 

"eleven-and-a-half years old", that's not a warning? Occassionally checking the voltage with a digital multimeter is good practice. Anything under 12.6 volts, you want to start watching closer, researching replacement options.

Better: get one of the new generation of battery load testers, they're electronic, will assess the battery's current Cold Cranking Amps*, give a verdict. If I was shopping today for one it'd probably be Solar BA9.

* Someone may be along shortly, to say that CCA is irrelevant, since the Prius 12 volt battery doesn't "crank". Basically conflation: it's still a relevant barometer of the battery's viability.

 

You may need more than one.

I had 2 G3's as work cars - and I never troubled myself to learn all of the ins and outs of the power split device - because I don't care.
'Goofy' (my first G3) had a battery die on me one time, back before jump packs became so cheap.
I just used a pair of jumper cables and my POV.
It was in the manual, IIRC.

 

I tried to watch some earlier transaxle deep-dives, soon after buying our '10. I believe it was John Kelly, before he was wheelchair-bound. Something about the mass of technical detail, coupled with his sonorous voice; it was more effective than general anesthetic; I could never stay awake.

 

I have an electronically controlled 8-speed automatic transmission in my 'newly socially acceptable' pickup.
I have NO BLEEDING IDEA how it works, nor do I care.
I don't have to know how it works to service it.
In a few years I will drain and fill (NOT flush!) the fluid and see what the old stuff looks like.

YMMV

 

What’s the miles on it?

 

You’re in good shape then (barring odometer roll-back). I would look into EGR cleaning, soon; there’s links in my signature. (On a phone turn it landscape to see signatures)

 

These make it easy to keep track of your 12 volt. I have the Ancel BM200 on two cars. Use it every day.
Best Battery Monitors

 

Ahh, an optimist.

 

Depending on conditions -- heat and vibration are battery killers -- six or seven years can be pushing your luck.

 

So the battery was somehow kept in good condition by the previous owner? checking the battery status with a monitor can avoid a run down dead battery and a trip to the hatch area again. Many hybrids and evs have the 12 volt problem. Tesla started using a lithium ion16 volt special battery that should last a long time. Weber state auto did a video on it. Good idea they had to fix a problem area. I think tesla may have said it should last the lifetime of the vehicle or something.