Attention Prius Owners, I believe I have discovered a previously unknown Mechanism of Low MPG.

I know I've been MIA on this thread for awhile... but I'm still working on this. I'll be picking up on working on this during the Spring. During the winter I don't drive my Prius much and get busy with work.

The reason you see one attached photo having a resistance reading of 19 mOhms even at 58% SOC is because the default of the Battery ECU is 19Mohms... the Battery ECU uses a sliding average to calculate resistance between 77-116°F... outside of this range the battery ecu does not display resistance as a viewable OBDII parameter. My guess is the Battery ECU in the 2nd attached photo had recently been reset causing 19 mOhms across the board as well as potentially a different State of Charge % reading.

The post your replying to was very early in the thread working on this low mpg mechanism... I have a much more solidified understanding of what's happening now and specifically how temperature can effect EV Mode behavior.

I'll post again below the preliminary YouTube video I made describing the Low mpg mechanism and how it effects EV Mode:

The final video will be much more comprehensive and describe how EV Mode is effected much clearer. I threw the above video together relatively quickly to lay groundwork that this WEAK EV State Low mpg mechanism could be widespread.

 

A very small update to this thread:

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I am beginning to work on the final write-up for this low mpg mechanism again. I made the short video above. The EV Mode button video on the left is simulating the MAX EV State and how my Prius behaved before a 12v Disconnect in early 2020. The Video on the right shows the WEAK EV State and how my Prius has behaved for the past 4 years. The videos above were taken on the same mail route one day after the other after delivering a package to the same church parking lot so terrain and acceleration metrics are nearly identical... but I did accelerate slightly faster in the video on the right. I changed the audio pitch and lowered the volume in the right video so the sounds of each can be distinguished better. You can see how much greater the EV Output is in the left video compared to the right even on a lower State of Charge. This shows how City Mpg can be effected by 5-10+ given the right circumstances when in the WEAK EV State compared to the MAX EV State.

Also, I believe I have discovered this issue is also present in the Gen 4 Prius/Toyota Hybrid Platform. I messaged with a Redditor recently who seems to be describing the WEAK EV State after a Software update:

 

I have made another short Graphic Video to describe the EV States:

5x Speed Version:

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This video is probably the most condensed yet informative piece of information I've made yet. If you watch it several times, the average Prius owner should start to understand the WEAK EV State low mpg mechanism I am trying to describe.

Doing the same as the last post... this is a Business Park section of my mail route synced one day after the other. The right is me driving normally in the WEAK EV State... the left is me driving in the WEAK EV State but using the EV Mode button as frequently as possible to mimick the MAX EV State. You will see how the Prius uses significantly more EV Output to propel the vehicle on the left side compared to the right. For this portion of the mail route, the EV Mode button equates directly to how my Prius behaved years ago when in the MAX EV State. I will have another video uploaded shortly with dozens of different mail route sections synced just like this one.

I recommend watching in 4k if possible as it makes it easier to read the text. The video is purely graphical in nature and there is a lot to read and understand on screen... but if you watch it several times taking more information each time, the difference between the MAX and WEAK EV States should become clear. If you're trying to compare the two video graphics at the bottom while they're playing... keep whatever device your using to watch a decent distance away from your face as it's easier to process that way.

 

A fairly significant update -
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I am now all but certain the WEAK EV State has the potential to effect every single Toyota Hybrid on the road. I have found a post on the Rav4 Hybrid Subreddit that describes the WEAK EV State Hyperaccurately after hours of scouring. It's a 5th generation 2020 Rav4 that the poster owns... the most recent model/generation:

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The description of this post is excedingly similar to how @davyk described the WEAK EV State almost 2 decades ago for their Gen 2 Prius:

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The only reason this issue has remained in the dark for so long is that 99.9% of Prius owners do not describe it accurately and it is otherwise completely phantom unless you know how to describe the behavior. If exposed this truly could lead to a 100+ Million Dollar lawsuit against Toyota for reduction in Hybrid City fuel economy for the past 2 decades.

Also a couple more graphic video updates to add to the information in this Thread:

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