Engine revving High? Not switching to EV mode? Check your 12 volt battery!

My 2013 Prius V started revving High when I coasted stop and even while I was stopped for a period. It also would not switch to EV mode when it should. I noticed this as soon as it started happening last week and after the second day of this Behavior I started searching online and have yet to see a post linking these behaviors to the 12-volt battery. In a nutshell let me tell you that was my problem.

I saw suggestions ranging from the traction battery to the inverter to the spark plugs to the mass airflow sensor, Etc. The car was in good General Health as well as the traction battery.

I have recently changed the PCV valve because of oil consumption. The old PCV was bad and my oil consumption has been reduced significantly. Following that trail down the rabbit hole I figured I did need to do some cleaning Downstream because of all that oil loss through the PCV tube into the intake manifold. So I did all that cleaning along with the EGR tube and found quite a bit of sludge and oil so I was glad to clean things up to let the engine breathe. However this did not cure my revving or EV mode problem so I pulled the mass airflow sensor and clean that to no avail.

I found and unrelated topic talking about the 12 volt battery and also found the post on how to pull up the onboard diagnostic on the radio. Lo and behold after sitting overnight my 12 volt battery was reading 10.6 volts. Very bad indeed! I also verify the voltage with a multimeter which showed 11.2 volts. Inconsistent between the two readings but both indicated a bad battery.

So I found a local Auto Zone with an Optima battery for the same price as the lowest I could find online. After swapping it out I saw an immediate Improvement. The high-revving stopped, the car switches to EV mode when it should, and my mileage has improved drastically.

I figure the engine was running to try to charge that bad battery on. I had noticed my mileage dropping off somewhat, by about 5 - 7 MPG before the other symptoms started.

On a side note it's easy to pull up the radio diagnostic screen. You can do it with the engine running or with the engine off and just in on mode or auxiliary. Hold down the power button to the right of the radio screen and at the same time cycle the parking lights on and off rapidly four times. The servicemen you should have here. Press the function check setting button then vehicle signal button your battery voltage will be displayed amongst other things. Obviously if the engine is running or the car is in ready mode then you should see the charging voltage going into the 12 volt battery. When the car is in on mode the display will show you the resting voltage of the battery which is what you want to see.

A second side note, I thought I was experiencing the morning cold start up shudders. That turned out to be misfiring which didn't last long enough to throw a code but it did Scare the bejesus out of me. New spark plugs solve that problem and she's purring like a kitten.

Good luck and I hope this helps some people avoid chasing unnecessary service items.