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

Attention Prius owners,

I believe I have discovered an unknown mechanism of low mpg in the Prius. Gen 2's are 100% effected by this. There is significant evidence it applies to the Gen 3 Prius as well. It may even apply to Gen 4's.

This low mpg mechanism is not complete yet, as my personal Gen 2 Prius has been uncooperative in giving me all the data evidence I need. I only have half! So I will need the help of my fellow Prius owners to help me complete this post by testing their Prii. I have a method for testing this low mpg mechanism, which almost anyone can do, later in the post. The more Prii we can test, the better! This low mpg mechanism can easily cause a 10+ mpg difference in city driving!

Any input and help will be greatly appreciated! At the very least, help blow this post up!

TL/DR: Disconncecting the 12v Battery causes memory loss in the ECU's that negatively effects how EV mode functions by roughly halving it's capabilities. This effects City MPG's by 10+. Hybrid System and Drivetrain behavior at speeds above 42mph remains unchanged.

I call this Halved EV mode the WEAK EV State. In some circumstances the Prius never seems to regain it's full EV capabilities, which I call the MAX EV State. My Prius has been in the WEAK EV State for 1.5 years at this point. I have linked posts of other Prius owners who are stuck in the WEAK EV State on this forum. The point of this post is to prove a new mechanism of low mpg. I'm hoping those who read the whole post and test their Prii will be able to gather the evidence for the MAX EV State and maybe figure out what triggers it. I need YOUR help to complete this low mpg mechanism!

Terminology used in this post:

ECU - Electronic Control Unit

EV - Electric Vehicle

HV Batt - High Voltage Battery

ICE - Internal Combustion Engine

MG1 - Motor Generator 1

MG2 - Motor Generator 2

SOC - State of Charge

So what am I talking about?​

I'm talking about what happens when you do a 12v disconnect/discharge. This resets certain ECU settings, and causes low mpg's as a result of the memory loss effecting how the Hybrid System runs.

I know knowledgeable Prius owners will say - "Yes Mojo, we know the computers need to relearn certain parameters after a 12v disconnect. It's normal for a Prius to temporarily get worse mpg's when relearning parameters after an ECU reset."

Yes you're absolutely right and I couldn't agree more. But I have found that for what I believe is the Hybrid Control ECU (also known as the HV ECU), the parameter relearn process is more complicated then people realize. The relearn process seems to be based on a certain commute style that needs to be consistently done multiple days in a row, potentially in combination with some other factor, to initiate the ECU's optimal state to max MPG's. In my experience, It seems that under certain circumstances, the Hybrid Control ECU may NEVER enter its optimal state and permanently reduce mpg's!

What is the difference between a Hybrid Control ECU that is at peak parameters compared to one that isn't?​

Let me start by saying total power is not effected. If you floored the gas pedal, the ICE, MG1, and MG2 work together to provide as much power to the wheels as quickly as possible. In either ECU state, flooring it, as well as behavior at speeds above 42 mph will essentially be the same.

The main differentiation occurs with how much pedal travel, acceleration, and distance is allowed in pure EV mode before the ICE spins up at speeds between 0-42mph.

I call these two different states of EV operation the WEAK and MAX EV state. In both EV states, EV mode effectiveness directly correlates with HV Battery SOC.

The WEAK EV State Graphic Overview.​

Hybrid Control ECU States: WEAK vs MAX​

In a WEAK EV state Prius, EV mode capabilities are a fraction of its maximum capabilities. Pure EV mode is still possible with exceedingly light pedal pressure, but you have to accelerate VERY slowly. It makes it very unrealistic to stay in EV mode and drive normally the majority of the time in the city traffic. The Prius refuses to discharge the HV Battery aggressively through pure EV mode in any circumstance, even at a full HV Battery charge. Once the HV Batt SOC drops below the 6th SOC bar, the ICE kicks on almost immediately with any amount of pedal pressure. It becomes impossible to use EV mode by itself unless you're at a slow crawl. City mpgs will barley scrape past 35 when in the WEAK EV state..

In the MAX EV state, you get much more acceleration, speed, distance, and pedal travel in EV mode on a much lower HV Batt SOC. EV mode is able to be used aggressively. Easily clearing a mile or more and are able to keep up with the stop and go traffic, accelerating and decelerating completely normally without the ICE kicking on down to 4 HV SOC bars. In most situations, you get about 2-4x the pedal travel/acceleration in pure EV mode compared to the WEAK EV state. However, once SOC drops below 4 bars in the MAX EV state, that's when EV mode will really begin to struggle in normal city traffic and all bets are off as to pure EV mode operation. In my experience, 4 SOC bars in a MAX EV state Prius is equivalent or greater to 8 SOC bars in a WEAK EV state Prius in terms of EV Mode ability. The MAX EV state roughly doubles EV mode capabilities. That's how drastic the difference is, and why city mpg's are generally effected by 10 or more. In the MAX EV State, you should be getting 45+ mpg's during city driving. The MAX EV State is the half of the mechanism I don't have data for.

Entering the MAX EV State: What triggers it?​

What causes the Hybrid Control ECU to enter the MAX state? I couldn't tell you 100% for sure in no uncertain terms, but commute style seems to be the most critical factor. I have a general idea of what seems to be required. It seems that a period of interstate driving at 65+ mph, possibly with cruise control on, done multiple times, with a rest period, done 2-3 times in a row is required to initiate the MAX State. I must stress, I'm not 100% sure on the exacts of the commute style required. There may be other factors as well. Doing this was just the commute style that reverted my ECU back to the MAX EV state after over a year of being in the WEAK EV state. Once my ECU achieved the MAX EV state, it stayed that way no matter how or where I drove until a 12v disconnect.

This is where it starts to get complicated...​

I'm not going to go in-depth to it here... but for the whole comprehensive background story to this post, read My Low Mpg Story Post. My occupation as a rural carrier, which requires the use of my Prius, had a lot to do with me figuring all of this out.

But to summarize, my Prius has spent 2 separate long periods of time in the WEAK EV state since I bought it in 2016. I was in the MAX state for the first year or so after acquiring the vehicle. Then I disconnected my 12v battery in late 2017ish and lost EV power and was in the WEAK EV state. This maintained until mid 2019 when I engaged in the commute style above and re-entered the MAX EV state. I disconnected my 12v Battery 2 separate times after this in 2019, which caused my Prius to revert to the WEAK EV state. But as long as I did the commute style above a few consecutive times, my Prius would re-enter the MAX state.. In 2020 I disconnected my 12v Battery again and lost the MAX EV state.

The problem is I haven't been able to get my Prius back into the MAX EV state since my last 12v disconnect in 2020, despite doing similar commutes that triggered the MAX EV state before. There must be another factor I'm missing or something that's different with my Prius now. I believe commute is still a primary trigger regardless. My HV Battery is still at 65% health according to Dr. Prius. I don’t think HV Battery health is a factor in why this occurs as long as it's not failing and seems to be functioning otherwise normally. No other error codes of any kind either. My Prius appears to be in perfect health. Again, read my Low MPG Story Post for the specifics.

To back up these claims...​

I've found several other posts on PriusChat/Reddit that describe this exact low mpg phenomenon of the ICE spinning up quickly with little pedal travel.

First, here is a different post I made a few months ago on PriusChat describing what I described here, just not as fleshed out.

Here are the other posts I found that sound very similar to my low mpg mechanism, even on brand new Prii!:

Beware of Dealer ECU Calibration MPG way down

Significant reduction in MPG between 2004 and 2005 Prius'

2007 "poor" gas mileage

Gen 2 prius; why is my HWY MPG so high and city so low?

Here is one from a Prius v (Gen 3 drivetrain): Possible undiscovered performance and MPG issues with Prius v (lowercase v)

I also mostly confirmed this mpg mechanism existed with another Reddit user.. but they unfortunately deleted their account a couple weeks later! It occurred in their Prius after disconnecting the 12v battery and replacing the Hybrid Battery. He said doing the commute style I described above fixed it!

Now on to the Evidence.

Testing and Evidence​

How do you determine scientifically that your Hybrid Control ECU is in the MAX or WEAK EV state? All you need to do is measure the percentage of gas pedal travel that is allowed while traveling in EV Mode at city speeds before the ICE kicks on. The amount of pedal travel allowed before ICE operation will show whether you have a MAX or WEAK EV state Prius. More pedal travel in EV mode being the MAX state, and less pedal travel being the WEAK state.

Note going forward, pedal travel will also be referred to as Accelerator Pedal Angle due to it being the measurement term. I will use them interchangeably.

The tests below were done using a Bluetooth Code Reader, Smartphone, Torque App, and the Realtime Charts add on for Torque. I'll explain how I did the tests after I convey the evidence.

WEAK EV State Evidence and Testing.​

Pedal Angles are much lower in the WEAK EV state for EV mode. When the HV Battery SOC is between 5-8 bars on the display, Accelerator Pedal Angle is limited to a max of 25-35% before the ICE kicks on when traveling between 10-40 mph. The max pedal angle for EV mode is allowed to be slightly higher at 40-50% when accelerating between 0 and 10mph. But for simplicity, we won't be worry about speeds below 10mph as it's to hard to record.

As stated before, SOC effects the max pedal angle in both EV states. For the WEAK EV state, 7 bars of charge will allow closer to the 35% max pedal angle in EV, while 5 bars SOC will be closer to 25% max pedal angle in EV before the ICE kicks on. Also note that up to 11ish% pedal input is "glide". So no force is being applied by MG2 or the engine until past 11% pedal angle.

Below is a screenshot of a of a trip that I graphed with Realtime Charts via Torque in my Prius using my Phone:

This trip was done while my Hybrid ECU was in the WEAK EV state. The various lines relate to the legend in the upper right. Their instant value and meaning are displayed to the right of the corresponding color square. The 5 yellow dots hitting the data lines in the center of the graph are attached to the instant values displayed in the legend. So speed that has a value of 16.16 corresponds with the white line where the yellow dot is in the photo. Some graph lines are scaled to make the lines fit the graph better which is noted in the legends as [1.0x] or [0.1x] etc to the far right. The instant value reads 1.0x for all graph lines despite it being scaled when graphed. Instant mpg's are hidden from the line plot because it's very erratic and makes the data look sloppy. The light blue MG2 line is the force the electric motor is applying. This graph is being used to represent what the WEAK EV State looks like. I slowly pressed the gas pedal in succession until the ICE kicks on. This shows what pedal angle activates the ICE in relation to HV Batt SOC.​

Here is a zoomed in portion of the above graph showing the WEAK EV State in action:

Notice how every time I slowly press the accelerator pedal and approach the 25% pedal angle (blue), the engine (red) kicks on without fail? This is indicative of the WEAK EV State. HV SOC (green) is around 60% and is what causes the Pedal Angle to max out at 25% in the WEAK EV State. Also notice how electric motor torque (light blue) immediately drops off when the ICE engages? See the yellow dots on the graphic for an example of what I'm talking about.
​

Here is the same thing on a higher HV Batt SOC of around 66%:


See how pedal Angles are allowed to be up to almost 35% before the ICE kicks on, but never higher?

MAX EV State Evidence and Testing???​

Pedal Angle and MG2 Torque are much higher for EV mode in a MAX EV state Prius before the ICE kicks on. The problem is, as stated in the beginning, I haven't been able to measure the MAX EV state because my Prius has been in the WEAK EV state for the past 1.5 years. I still have not been able to reinitiate the MAX EV state, even with trying experimental commutes.

This is where you guys come in!​

To help complete this low mpg mechanism I will need you guys to test your Prii so I can gather the MAX EV state data, prove the MAX EV state exists, and maybe discover why my Prius and others are in the WEAK EV state. I have shown what the WEAK EV state is, I'll need test data from YOUR PRIUS to prove the MAX EV state. I know there are many Prii out there in the MAX state, and maybe we'll get a better idea of how many are effected by the WEAK EV state. There are at least a few as evidenced by the linked posts of other Prius owners above!

How to test this low MPG mechanism yourself.​

I will list multiple ways to do these tests. I have made everything as easy and straightforward as possible. With a few $ spent on an OBDII adapter and couple of apps, the testing will be a breeze.

For the preferred method you will need:​

1.) An Android Smartphone (Apple users skip these steps, alternative method below)

2.) A OBDII Bluetooth Code Reader to connect to your Smartphone. Any OBD II that connects to your Android phone should work.

3.) Torque Pro App

• To get access to the Prius sensors I used to make the charts, you'll have to add the Custom Prius PID's in the settings.

4.) Realtime Charts add on for Torque.

5.) Download my Hybrid Control ECU Analysis Chart Setup for use in the Realtime Charts App. The app will automatically import the chart setup file when you open the app after downloading. All you have to do is then load the chart setup via the app and record data with it. You don't have to go through any trouble creating the chart!

6.) (Optional) - Download my WEAK EV state data file to view in Realtime Charts and compare against your data file. Again, the app will automatically import it after being downloaded and you just have to open it via the Realtime Charts file opener.

7.) RECORD THE DATA!

Max sure your Prius is warmed up with as minimal accessories on as possible and has a relatively high HV Batt SOC. You can force charge the battery by putting the Prius in drive and fully depressing the gas and brake pedal at the same time. Then while recording the data, drive at city speeds between 25-40mph and slowly press the gas pedal in succession to measure when the ICE activates in relation to Pedal Angle.

If the Pedal Angle maxes out at 35%ish or lower for EV mode, you're in the WEAK EV state.

If Pedal Angle maxes out significantly higher than 35% for EV mode, you're in the MAX EV state.

Again, make sure this is done at speeds between 25-40mph and you have a relatively high HV Batt SOC, around 60%.

8.) POST YOUR RESULTS

Then post your results here or at this post!

You can do this by screenshotting the data or uploading the actual Realtime Charts data file for everyone to view. You will have to upload the chart file through something like OneDrive and share the link here.

Apple/Alternative/Cheap Method:​

For the alternative method you will need:

1.) Apple/Android Smartphone

2.) A WiFi OBDII Code Reader or a more expensive Bluetooth OBDII Code Reader due to iOS using newer Bluetooth. Android users can still use the cheap Bluetooth Code Reader

3.) A compatible free or paid OBDII scanning app that allows you to create graphs or display sensor data similar to the Torque screenshot below:

You will need to add the Prius specific OBDII PID'S to whatever app your using. There should be an option for this in the settings if the app is reputable. You will need to find out how to setup the sensors/data yourself.

It will take some fiddling around to get the settings exactly how you want them, I'm not going to explain step by step how to do it as that will take too long.

Once you get it setup to record the appropriate data, see step 7+ of the preferred method.

If you made it this far...​

Thanks for reading and hopefully together we can discover an unknown cause of low mpg. Test away and help me prove my theory!

Any other questions or comments, please ask!

Thanks guys!!!

 

I threw together a TL/DR at the beginning for ya.

I made the post as in-depth as possible to prove my point effectively. It had to be that way because all of you would call me crazy and ship me off to the loony bin if I didn't.

I really think this is a major cause of low mpg in many Prii, and it could potentially effect every single Gen 2 and Gen 3 out there.

 

If you want to try it out go for it.

Any data will be useful.

But what I really need is a Realtime Charts data file to view so I can make the comparison between the WEAK and MAX EV states.

Regardless, it would be interesting to see what EV state your Gen 2 is in.

Posted via the PriusChat mobile app.

 

Your mileage does look very steady.

It doesn't discern what EV State your in, but if you do a decent amount of city driving and get those mpgs you probably are.

Of course this is assuming the EV states apply to the Gen 3 in the first place.

The only way to tell if disconnecting the battery does anything would be to do my tests in the post before and after a 12v disconnect.

Posted via the PriusChat mobile app.

 

The WEAK EV State generally effects city mpgs by 10 or more.

In my experience, city mpgs will be around 35 miles per gallon.

Highway mpg's are not effected.



Posted via the PriusChat mobile app.

 

Read the main post a couple times, it will start to make sense.

No herb smoking for this one... gotta be completely sober!

Posted via the PriusChat mobile app.

 

I've often worried one of the ECU's has been damaged and that's what is causing this.

But a couple of links in my post above show this exact same thing happening on brand new Prii, and even someone who fixed their WEAK EV State Prius by doing the commute style I suggested.

I've also personally experienced my Prius going from the WEAK EV State to the MAX EV State after 1.5 years of getting low mpg from being in the WEAK EV State. This was even on a 12v that was suboptimal and wouldn't hold a resting charge past 12.14V.

I have since replaced with a new 12v that charges to 12.6+ volts and still have not attained the MAX EV State again.

It seems to me like it's some sort of learned parameter that needs to be triggered.

Posted via the PriusChat mobile app.

 

I've still been investigating possible reasons for why this low mpg mechanism happened to my Prius and still has not resolved. I'm thinking it actually might be related to the HV Battery ECU having to remap the HV Battery capacity/charge level after the 12v disconnect.

Reading this thread about a Gen 3 Prius experience what sounds like that I've described makes me believe this is possible. They talk about how after engaging in a highway commute like I describe, something shifts in the HV Battery and they get more power and travel in EV mode at city speeds. Gives even more credence to the fact this may be a multi-generational problem rather than limited to Gen 2's

Going down this line of thought further, I found that when you disconnect the 12v, the HV Battery ECU resets HV SOC back to 60%, then recalibrates itself based on pack voltage and actual coulomb counting. It's not quite as simple as it seems. I believe something possibly goes wrong with this process in certain circumstances.

Notice in the below photo of my Prius HV Battery readings via Dr. Prius, pack voltage reads 215 volts at 59.5% SOC:



Then compare that to a very similar Dr. Prius reading screenshot from another Gen 2 post where the pack voltage reads 240v at 58% SOC:



What exactly is going on here? They are both at very similar discharge rates, yet a pack voltage difference of almost 30 volts between the two reads out virtually the exact same SOC of 60%?!? How is this possible? Could this be the source of my low mpg woes due to a poorly calibrated hv battery ecu? Or maybe this is because the OP of the post used their prolong charger and HV Battery ECU isn't calibrated yet?

Would @jacktheripper have an explanation for this?

Going off of the link at the beginning of the 3rd paragraph in this post, I am going to disconnect my 12v tomorrow to reset the battery ecu back to 60%, force charge to 80%, then do that one more time. Maybe this will get my Prius HV Battery ECU recalibrated.

But I have no idea if I'm correct in my thinking on all this and need further clarification from the experts.

Also, See this preliminary Video explanation for what is occuring in my Prius.

Posted via the PriusChat mobile app.