12v battery maintenance when Prius is not driven for several days

Please forgive me but I'm going to re-post this content in this thread as well given the parallel discussion.

Results from 2019 Shutdown for 24 days-- It was seamless!

In a nutshell, the 12V battery was disconnected, charged, and left on open-circuit for 24 days. Upon return the open-circuit voltage had declined from 12.68VDC to 12.48VDC. Once the battery was reconnected, without any charging, the Prius achieved a READY light and started on first attempt. The only initialization step that was required was to set the Date and Clock. I am very pleased!!!
(This is a 2018 Prius, GEN4, with 13000 miles. HV battery (traction battery) indicated 3/4 charge both before and after shutdown. The car was garaged at about 50F.)

Dec: 2019 Shutdown for 24 days

Begin by preparing the car: See Owners Manual pp729 through 733. All of it.
There will be initializations required upon reconnect because everything stored in the ECU is lost. Pp780

• Make sure the HV battery is pretty well charged. I'd like 80% or better.
• Doors unlocked...in my case, because its is garaged.
• Power EVERYTHING down
  • Engine, of course - OFF
  • Lights - OFF
  • Audio - OFF
  • HVAC - OFF
  • Nothing plugged into outlets or USB.
• Lift the NEGATIVE (-) terminal first, pp 733.
• Lift the POSITIVE (+) terminal
• Measure the battery voltage:
  Time:__13:03_____ __12.65__________VDC
  
• After 1 hour measure again:
  Time:__13:54_____ ___12.56__________VDC
  
  
  
• Put the battery on SLOW charge, 2A, for a number of hours…..6 in this case.
• Disconnect the battery:
  
  Time:__18:30_____ __13.25_________VDC
  After several hours:
  Time:__22:45_____ __12.68_________VDC

Upon Return:
Begin by documenting the event for future reference:

• Date and time to start-up:
  Date:__1/9/2019________
  
  Pre-startup or charge voltage:
  Time:__13:05___ __12.48____________VDC
  
  
• If the voltage is <12V consider charging before reinstallation. Was not required....
• Clean the battery terminals
• Reconnect battery cables: Reconnect POSITIVE (+) terminal first, followed by NEGATIVE (-)
• Attempt to achieve READY condition to start the vehicle. See pp 732 for possible startup issues. The doors may not unlock. It may take two or three attempts to get the READY light.
• Reset DATE and TIME if prompted. I was prompted by the vehicle for date but not time.
• After restart several systems require initialization: pp 780. These are not a big deal. It will be interesting to see if my customizations will stick along with radio presets, etc. Radio presets good, maintenance times good, customization features....still customized. It all seems to be in non-volatile memory. Great!

 

One extra thing I noticed with mine was that the media centre forgot its login for the Toyota Online services, and I had to turn Shuffle back on.

 

Those numbers don't look really good.

Assuming that the OEM battery still is an AGM, fully charged voltage typically is more like 12.8-12.9 and it will lose less than a tenth of a volt per month.

This makes me think one of two things:
It wasn't charged enough before starting the test
OR
The battery was "abused" at the dealership causing some loss of capacity, which is fairly common.

 

I believe they're not AGM. Fourth gen battery is in the engine bay.

All this speculation...., just get a Solar BA5 (or similar) and test it.

 

Being in the engine bay is not evidence.
A really LOT of cars have AGM batteries in the engine bay and they do just fine.

 

G'Day, OK, a second data point...53 days without any difficulty. Again I was forced to idle the vehicle in February and again I carefully prepared the car and 12V battery by disconnecting, cleaning, and charging. As mentioned, the battery was charged for 19hrs. Then 8hrs after the charge was disconnected the voltage was 12.87 VDC. The 12V battery was then left in open-circuit for the duration.
Upon return 53 days later, with the battery still disconnected, the voltage was 12.58VDC.
The battery was then simply reconnected and the READY light came up immediately followed by a successful start. I was then prompted to set the date and time but its all good. Accessories may need some resets too but I've not checked.

The 4Runner faired the same. Charged to 12.49VDC before and it was still 12.30VDC after 53days (open-circuit). Started on first crank. Its all good too.