Does a weak 12v battery affect the Hybrid battery system?

I have a 2013 Prius with the original battery. Purchased July 2021. Over the past 6 months, mileage has decline from 5.5 L/100 Km (42.7 mpg) to 7.5 L/100Km (31mpg). I charged the battery with a low current charger a couple of time (3 amp max). Each time the mpg recovered to the original for a couple of hours to a day. Finally bit the bullet and purchased a new AUX 12V battery for $380 Canadian all taxes environmental fees etc. After a couple of weeks with the new battery the car seems to be settling in working well. Today was my first 40 Km (24 mile) commute with 4.9 L/100 Km (48 Mpg).

It was an expensive fix, but I think worth it. I have read that running the car with a low 12V battery can damage the computers and cause other long term issues. Not really worth the risk once you know it needs to be replaced.

 

There are things you can read that are true, but there are also things like that.

If it's low enough, you can get some trouble codes logged in some of the car's computers that tell you the voltage is low. If it gets a lot lower than that, you won't be able to start the car. If it is chronically low and the car is charging it all the time, you may see an MPG effect. None of those things involve damage that persists after the battery is replaced.

 

I would like to report a crime. I do not drive my Prius everyday. I was not using the battery maintainer during the winter months enough. My loaded with ethanol tank of gas was gettin 47.8mpg. That is criminally low for a Gen 4 Prius. I juiced the 12v battery and now it's at 48.3MPG for the tank that is 400 miles or more used up. Still a crime but like being born, I blame myself for the MPG.

 

Definitely!! Our 2014 Prius was having this exact issue. Dealer was stumped AND insisted our 12v was good. This was the car's original battery by the way. So we were pretty much sent on our way and told to keep an eye on the issue and take it back if it happened again. It has continued to happen but I'm done with the dealer. We tried researching a little more to make sure it was the 12v but today the car refused to start at all so we bit the bullet and got a new battery (($280 after tax) and voila! The car started!!. I must caution, please be sure to read your user manual as there are a few little moves you need to make with the gear shift once the new battery is installed but before you press the Start button. At least that was what we had to do with ours. It may be my imagination but our a/c flow was acting a little weird and that seems to have improved. Also, we had definitely noticed our MPGs declining. We have a full tank of gas and after replacing the 12v, our mileage went from 365 to 428. I read somewhere here that if your Prius has any issues at all pertaining to the 12v you might as well get a new one and save yourself alot of hassle. I definitely agree! Hope this helps someone, especially if the dealer makes you feel like you're crazy and your car is fine.

 

Snag one of the electronic, automotive battery testers, and you don't need to rely on them any more. I'm using an older Solar BA5, there are newer itterations, and lots of other brands. You connect it's leads directly to the battery posts, set a few parameters related to your battery's specs, push a button and it tells you the as-tested CCA, voltage, and gives a verdict, say "good", "good but charge" or "fail". Typically, any time the CCA is below spec, it's time.