Time to replace 12v battery?

How old is the 12v battery?

You could try using a battery charger and see how the battery responds before buying a new battery.

 

How are you "reading the 12v number."

Basically, if your battery is below 12.4 volts you are living on borrowed time.

The battery needs to hold above 12.4 all by itself to be considered viable.

 

if it's original, replace it with a new oem

 

My 12v battery had similar symptoms to the o/p. Eventually, it could barely start the car after sitting overnight during the New England winter. By "barely" I mean that, after depressing the brake pedal, I would have to wait for the brake booster pump to finish doing its thing before trying to start the car.

Being an experimental type and tired unmanaged lead acid batteries that are easily damaged by over discharge, I decided to go with an automotive 12.8V 50 Ah LiFePO4, currently selling on Amazon for $105:

However, at the time (Jan '23) there was a catch: unless I went with expensive cold weather designed lithium-yttrium chemistry, lithium batteries could not be charged safely below freezing. While the battery above did have a battery management system (BMS) in place, sadly, mine does not prevent charging below freezing temperatures. Perhaps things have changed in the last 18 months.

So, I made a small external circuit for the necessary protection. Battery discharging is always available by way of high current diodes like you would find in jump start battery packs. Once the Prius is powered up, a simple circuit becomes powered and enables charging only when the battery temperature is 5-30 degrees Celsius.

10-30 degrees Celsius is considered optimal to reduce battery stress and extend lifetime. However, for my driving profile of mostly short trips, I suspected the battery wouldn't have time to warm up to 10 degrees C (50 degrees F), so I decided to compromise at 5 degrees C (41 degrees F) which often happens naturally during the winter day for my part of New England.