Which 12v battery to get.?

Seeing threads from a few years ago saying the Optima isn't as well regarded, and that the Toyota Truestart is the one you'd want now. I'm in the same predicament as you and just missed a great Autozone coupon. They had a 15% or 20% off coupon on an item $125+, would have been great to put towards the yellow top Optima there.

 

I got a new 12 v at autozone after the new year. Was seeing the original in our 2010 slowly degrade over time. But it was in service for almost 9 years:



Here’s that we got:


After the in store gift card and coupon, it came to ~$160.

Hope this one lasts just as long.

 

Whether or not the 12V battery needs replacing would not affect your mpg.
A traction battery going weak can affect mpg. If you suspect that, the recommended solution is the Prolong Reconditioning System from Hybrid Automotive. The owner, @jeff652 is fairly active here. @Raytheeagle and others have used that system with great success to extend the life of the traction battery.

I am assuming you have already checked tire pressure & other possible issues that can affect mpg.

 

Can you add water to that Duralast? I thought I glimpsed caps. partially hidden by the label.

 

Probably can (there are caps there).

 

stick to the oem

 

They're nercessity, for safety in the event of a crash, since the battery's inside the cabin. And it should be vented to the outside, to avoid possible gas build-up.

There are several compatible options available. With various retailers you can enter your car's data and find batteries that'll work. Pep Boys for example has this, conventional cased AGM, compatible vent port, proper posts, for around $140~150:

Bosch Premium Performance Battery Group Size 51
Part # 51-440BAGM

 

Well, if MPGs are involved engine air filter, engine air filter, and engine air filter. A dirty one WILL drop mileage and confuse the system (you're always running a little rich). From experience. Yes, the computer tries to compensate. It appears it has trouble doing so.

Oh, and it's a "cheap fix".

 

I'll second this. 12V battery has nothing to do with your MPGs.

I did just have to change mine though. 6-7 years is about normal for the 12V. I have a 2011 that I bought in April. I didn't quite make it 7 years. I suspect yours may be due if it wasn't already replaced. If it dies, you won't be able to turn the car on, even if your traction battery is full charged. You can have a shop test it for you, usually for free. I noticed it getting weak when I drove at night. The lights would be pretty dim until the car turned on... more than normal. I put a trickle charger on it, and that held me over until I replaced it.

If you do replace it, I recommend just sticking with the OEM Yuasa or the Optima. In my search for other options, many showed up, but none of the reliable brands seemed to have the proper vent connection you need for the Prius application, and some required minor mods.. which I didn't have time for. The off-brand options didn't come at a huge cost savings either. Like this one:

. I paid $10 more for the exact replacement, Made in Japan, Yuasa OEM battery from the dealer 5 min away.

Both the Yuasa and the Optima are plug and play. Both are stupid expensive (>$200). Dealer will stock the Yuasa. Nobody will stock the Optima. You will need to order it. Just make sure you buy from them directly or from an authorized dealer or they won't warranty it. Product Finder | OPTIMA Batteries

All that being said... you probably don't need a 12V battery. Check the top of the battery in the trunk for a date. <5 years, you're probably A-Ok.

 

Not all batteries have a date; the stock Yuasa does not, has only a coded date in a number.

I thought an ailing 12 volt battery could impact mpg?

 

My OEM battery lasted 7 years. My new yellowtop lasted 3 years before it went dead overnight out of the blue. The guys at Autozone were pretty cool about and gave me a new battery for $20.

But if I had to do it again I will buy the OEM battery.