Replace 12v Battery?

SIGH.
Not true.
IF all systems are functioning normally and you turn off the climate system AND only leave it in N for the short time that it has to be......there should NOT be a problem.
Without the AC running, there should be very little draw on the battery.
I do it with my C ALL the time.
My Ford too.

 

going back to your mpg problem, there are so many factors, especially in a 12 year old car. without trouble codes, its a process of elimination

 

all the best!

 

A small battery will lose a percentage of rated power more quickly than a larger battery, given the same load applied. However, a fully discharged 12V battery is typically in the range of 11.5 volts with a minor load on it, and perhaps as low as 9.5 volts at full discharge with 1/3 of it's rated load on it.

With just a few things running, the 12V battery in your car should be well over 10 V. In the chart below, C is the max capacity of the battery, and C/x is that fraction of the max capacity.

 

That’s not true. You can run a Prius thru a car wash. You just have to turn off all the electrical accessories. With those turned off the car doesn’t draw that much power. The AC is the big draw.

 

If you remove the back deck and the storage tray under it like you were going to change a tire, you can see the side of the traction battery. What color is it? Dorman paints their cases. Check your spare tire pressure when you are in there also. Those tires lose pressure over the years.

 

That’s a Dorman. My condolences your new battery is probably bad, or going bad.

 

Not necessarily true. Something caused the code to come up in the first place.
That reason very likely is still there.......and will come back later.
This is true of ALL vehicles, not just hybrids.

 

Well you have several options. You could try to rebuild your existing battery @PriusCamper might be able to assist you if you can drive to him. He’s around Seattle you could get new batteries from @2k1Toaster for around 1600 and perhaps leverage your existing battery modules for the labor to install them, or you could get a new factory Toyota battery and have someone other than the dealer install it. The online price for a new battery is 1950 or less. 3500 is high.

 

Well, You're about 4.5 hours drive from me... Not so close but maybe I could coordinate it with other travels? If you buy a new battery pack it's $1650 from most dealerships. If you buy it in Oregon you could avoid sales tax too... As for installation I'll do it for you at $40 an hour and it takes about 2 hours.

Other options for saving money is to buy a rebuilt pack with warranty from one of the best rebuilders like Hybrid Battery Conditioning | Reconditioned Hybrid Car Batteries Service or I could rebuild&recondition your existing pack down in the $400 range plus parts with the expectation that you'll need more work in the $200 range in the next year or two and no warranty.

Lastly, If you like learning and find yourself enjoying working on your Prius you could work on it yourself and hardly have to spend any money.

 

This is a good deal. Hustle up there and take it.

 

A dammed good deal. Quite worth a 4 1/2 hour drive. Plus he’s gonna teach you while he helps you. Worth 400 dollars an hour.

 

I'd recommend getting familiar with Dr. Prius app... If you have to pull over with warning lights in the future, that app is good at grabbing and clearing the hybrid battery subcodes, plus gives much more impressive graphics of pack data in real time.

Beginning to think temperature readings are the most important data points if you're working with a problematic pack and you want to be warned before its about to act up again.

Also try the Hybrid Assistant app. This app not only is the easiest way to turn the battery cooling fan up to full blast when needed, but it has a companion app called Hybrid Reporter that even in just a 3 mile drive will give you 10 pages of charts and graphs and sensor readings to nerd out on... Sooner you get that baseline data, sooner you'll have something to compare new data to in the future.

 

Yeah... Those temperature readings are in red, which means Dr. Prius has color coded them to warn you!

If I were you I'd watch some YouTube videos about how to clean your hybrid battery cooling fan. That's the least expensive solution... It could also be that you have a module that's about to go bad, or simply some other part of the venting system is clogged. This is the time of year that people's hybrid batteries fail because the packs are most vulnerable when it gets super hot out.