Check the 12 V battery voltage while operating a window or just see
how much the lights dim ...

With the car in READY? With the car ON, the 12 VDC system gets the power
from the inverter. But if you notice the speed change with the car not
READY, so you're running on just the battery, this does sound like it
might be getting a bit weak.

The battery is rated at 35 Ahr and I think AutoZone will test it for
free.

But before doing a battery swap, you might check over in "Prius
Technical Stuff" and probably even PriusChat about how to add water.
This is not a difficult task and a little distilled water goes a long
way.

Sounds like a good idea but not something I worry about. In my case, I
don't have much 'stored' in the ECUs.

I've heard a lot of good things about the Miata battery. But I need to
point out I live in North Alabama and our batteries suffer more from
heat than cold. So for me, a lawn tractor battery is a good alternative.

Bob Wilson

I made no comment on the expense--because neither did the original
poster.  I responded directly to his idea that somehow, some way, the
time to replace the battery should be specified, that he should never
randomly come out to the use the car and find out it won't start, that
he should never have any SURPRISES in life whatsoever, that it's
Toyota's responsibility (!) to prevent him from ever coming out to find
that the car won't start.

Apparently, he's never owned a car before.

As for the price--well, a smart buyer investigates all aspects of the
car before buying it.  A smart buyer looks at regular maintenance costs
and finds information on reliability, in order to evaluate operating
costs.  A battery is a maintenance item.  'Nuff said.

Said smart buyer also investigates insurance costs, for example.

One hates to be called "not a smart buyer," but if the shoe fits...

BTW, Chrysler pulled a similar stunt with their cars awhile back--put
the battery way down such that replacing it meant pulling the front
wheel and pulling off the fender liner.  Sure, it was a standard
battery.  But the labor costs added up.  All so that it cost Chrysler
less to manufacture.

Such is the Prius.  It was convenient for the manufacturer.  Screw the
customer.  Let him pay for it.  Toyota needed a certain price point on
the sticker.  That's all.  Toyota doesn't care if the car needs $5000
worth of maintenance 5 years later.  That's later.  (The car doesn't,
but even if it did, Toyota wouldn't care.)

You know, there's enough information out on the net nowadays that
EVERYONE spending $25,000 on a car (or even $10,000) should know
EVERYTHING about that car prior to beginning the purchase process.

The problem is, the world is full of idiots who happen to have (or think
they have) $25,000 to spend.  They don't bother to KNOW anything about
that $25,000 purchase; they go out, get caught up in the emotions of
buying a new car, and just sign on the dotted line right then and there.

Stupid people let emotions rule the decision.  Smart people don't let
emotions into a business deal.  Notice how the car salesman is a
businessman, and has no emotion in the deal whatsoever.

Guess who wins?

And if you find out 5 years later that the tiny CMOS battery in the
trunk costs $335 to replace, so be it.  You COULD have spent a couple of
months lurking in any number of Prius forums, and found out everything
about the car.  But you didn't--because you made it an emotional
decision, not a rational business decision.

Warren Buffet doesn't buy things on emotion.

It's a car.  The battery is good for ABOUT five years, but....frankly,
it goes dead when it goes dead.  Replace it.

That's how the world works.  There's no specified time limit, no
maintenance schedule for the battery.  Either it works, or it doesn't.  
When it doesn't, replace it.  That's how car ownership is, with every
car.

If you've had the car (or the battery) for 5 or more years, you should
not at all be surprised to come out and find the battery dead.  You're
on your own as to how to deal with that.  If you don't like surprises,
then just have the battery checked every time you're in for
maintenance--they'll know when it's on the downhill slope, and you can
take care of it right then and there.

In short:  no, Toyota should not have specified maintenance replacement
every X years or months.  Not at all.  That's not how car batteries work.