Sudden 12V battery failure

All,
It's always good to refresh some of these threads with new information. I experienced an interesting (albeit temporary) failure on my 2007 Gen2 Prius related to this issue. My car has 250k miles on it...still with original everything...including the 12V battery and gets an average 50mpg with regular gas (Chevron) here in CA.

When I started it up for my daily 20mi commute to work, it started fine...and I flipped on the wipers to clear the dew from the windshield...and noticed that they operated surprisingly slowly (about 50% normal speed). I looked down at my USB plug in the 12V power port and it read 10.2V (not good)...even though the engine was running. No sooner had I noticed that when the red triangle of death came on, followed by a battery and low sensor voltage warning on the center console. The engine then shut down (ignition still on...all the warning lights on). I then hit the engine ON/OFF, and restarted the car. All problems fully cleared up. It appears that a relay or some other process that engages the 12V DC charging system did not turn on...and my 12V battery was clearly low. I confirmed this with the following process:

1) With the car off, press the ON/OFF button twice to turn on the ignition.
2) Press and HOLD the INFO button on the lower right of the center console for 4 seconds; while holding it down, turn on and off the headlights three times.
3) This brings up a diagnostics menu on the center console.
4) Select "menu", then "display check", followed by "vehicle signal check"
5) The display will show the voltage level of the 12V battery.
6) If the voltage is below 12V...and you've been operating the car daily, the battery is likely worn out and needs to be replaced.
7) Turning the vehicle off and on again will clear the menu screen.

If you're a competent tech person, you can use a voltmeter to check the battery itself (located under the right-rear trunk compartment) and compare it to what the computer shows. There is a chance that there is a weak battery connection (possibly at the post, or in the fuse box). There are other sources (youtube) where you can see how to check/measure those problems. Note that you should remove the trunk cover and trunk to remove the panel without any undue prying and bending of the tabs. You'll need to do this (for sure) if you're going to replace the battery.

Keep in mind that YOUR Gen2 Prius may have DIFFERENT battery sizes, depending on whether it has the keyless unlock and start feature. Vehicles with that feature are shipped with an S46B24R sized battery (which is slightly longer). You can order these online from a variety of sources (Amazon, etc). I used https://shop.advanceautoparts.com/, since they have free shipping and often offer a 25% discount when you provide an email address. You can also return your old battery to Car Quest (Advance Auto) and redeem the $23 core charge. At the end of the day...free delivery for about $160 (still requires a trip to return the old battery). TIP: place another online order for filters and oil for pickup at the store...with another 20-25% discount...and return the battery when you pick up those goodies.

I hope this helps anybody else that experiences the eventual failure of the 12V AGM (Advanced Glass Mat) battery. If anybody knows why my engine was running, but not producing a charging voltage when I first started the car...I would like to know why. My subsequent start-up went without a hitch (12V power socket showed 14.0V) and I drove to work (knowing I needed to replace my 12V battery...heck it's 12 years old...so it's lasted more than twice as long as my other car batteries).

Keyslammer

 

When I had that problem it was dirty battery terminals. Went away as soon as I cleaned them. Got a new battery anyway but the old one worked fine until I did.