12 Volt Battery/MPG Relationship

2015 Prius owner. 147K.

My 12 Volt battery was manufactured in February of 2019. Not sure when it was installed as I bought this Prius used in March.
Starting to show symptoms of death. Twice now it has either shown flaky computer images and one complete failure to start situation this morning. Voltage reading from under the hood and at the battery showed 7 volts. Hooked up a second battery to jump the system which worked instantly. The engine started and ran for about 45 seconds sending the voltage readings from under the hood to 14.6 and at the battery 14.0. This does not seem possible in just about 1 min of engine run time? An hour or so later voltage readings now 11.84. Drove car 2 miles or so and voltage back up to 14. Seems it is failing to me. So, I have two questions. Why does the weak battery reduce fuel economy? How is this possible? Which battery is a good replacement? Optima is mentioned quite a few times here and there on this site.

I will say I found nothing turned on or obviously left on from the day before. My wife says she may have triggered the ACC mode and left it there. Not sure. Doesn't the Prius turn off ACC mode after about 20 min? 7 Volts is just not a good thing. Thanks.

 

Thanks, I believe the current battery is a TruStart. I will check out Walmart and see if it's available around here.

 

Drove a few miles today, and the charge seems to be holding now at 12.78 volts. Will check it in the morning. Hopefully, we drained it somehow. I do know that a battery can't take that kind of discharge without some damage. Maybe a couple of times then it's probably lights out for good. Best to have a new battery in the wings.

 

No go with Walmart. They claim nothing for the Prius.