Downsizing the 12 V battery?

Concerning my prius' little 12 volt battery that's about 4 years old ,that will eventually need replacing. Why not go smaller, saving weight and money in the purchase.

From 2005 Toyota Prius Mobile Installation
The Prius 12 VDC system is a very vestigial affair intended to power just the radio, lights and computer display. It uses a 240VDC-to-12 VDC stepdown converter that can provide about 40-50A but the 12 VDC battery is a 20 AH device that looks like it came from a motorcycle. It's main purpose to power up the computer electronics that need to boot in order to start the hybrid power system. (In the Prius, EVERYTHING is fly-by-wire. The power steering, the water pump, the power brakes and the air conditioning are all powered electrically off the 240 VDC system which in turn is controlled by the 12 VDC powered computer systems.)

I'm wondering about a smaller high quality AGM style battery , sealed spill proof construction. Adapting to the posts should not be a problem for me. Most / all threads concerning replacement seem to involve increasing the CCA's and I just think ,..why? I am the type to use a battery tender for any storage for more than a week.
I suppose my question is, how much smaller can I go safely, and yes my 106k prius is out of warranty.
Thanks in advance for response, northwichita

I disagree with the statement that the 12VDC bus powers only the "radio, lights and computer display." The DC to DC converter can produce ~100A, and the 12VDC bus powers all of the ECUs in the car. It is responsible for powering the steering and braking. The steering motor and the brake pump are NOT directly powered by the high voltage traction battery.

Further, the capacity of the 2G 12V auxiliary battery when new is in the low 30AH range. Now that your battery is 4 years old, its actual capacity is likely to be half of that or less.

If the DC to DC converter fails, you are depending upon the 12V auxiliary battery to power the car sufficiently so that you can safely come to a stop. If you sit in your car when ACC-ON to listen to your stereo, you are drawing upon auxiliary battery power.

I do not recommend that you select an alternative battery that offers a lower AH capacity than the original.

Patrick Wong

The minimum size for a lead acid Prius 'bootup' battery is less than 7 amp-hour; perhaps as low as 3? NiCd power-tool batteries that are even smaller can do the job, perhaps because they have low internal resistance. So there are options that weigh less than a kilo, of that's what you are after.

The major concern as addressed above is how long a downsized battery can support the Prius' parasitic drain and be able to boot the car afterwards. For a daily driver, this might not be an issue.

Why risk over $20,000 worth of car to save 10 or 15 bucks on a battery? Sounds to me like your being penny wise and pound foolish.

There are some errors.
The HV battery nominal voltage is 201.6V.
The power steering, the water pump and the power brakes are powered by 12V line.
The air conditioning is powered by 201.6V line.

Ken@Japan