Hybrid System Less Efficient After 12V Disconnect/Battery Change? ECU's?

Has anyone else noticed that whenever they disconnect their 12v battery that the hybrid system behaves more tepidly as a result of resetting the ecu's? This drastically affects city mpgs.

The main thing I've noticed is that whenever I disconnect the 12v, my Prius refuses to use EV mode aggressively. The amount of pedal travel I can get before the ICE kicks on is probably a 1/4 or 1/2 of what it was before the disconnect. Also driving on EV is basically impossible when using any sort of acceleration if the SOC drops below the first blue bar.

I went for almost a year thinking my Prius was just getting old and the Hybrid Battery was dieing as I was not getting the same MPG and pull from EV with it on mail routes. Going from up to 44 MPG to almost never breaking 35.

Then last year I started working in an office that was about 20 minutes of country road driving (55-60 mph) and 30 minutes of freeway driving (65-70 mph). After a couple days of this commute, it was like my Prius Hybrid ECU's had relearned how to work at max efficiency. I would generally have much more pedal travel and power in EV, and this could be sustained on a lower HV Battery SOC.

My route MPG jumped like crazy too. I consistently almost always got over 40 and set a record of 47 mpg. This was after a year of never breaking 39 mpg no matter the weather, tire inflation level, or driving style. It was the Hybrid System doing more work.

I then disconnected the 12v mid summer last year when working on the vehicle and my mpg and hybrid capabilities dropped off again. Within a couple of days of traveling to the same Post Office above for work, the hybrid capabilities returned.

It appears that a regular commute with freeway driving of 65 -70 mph is required for the Hybrid System ECU's to relearn certain parameters to maximize efficiency. And once it relearns therm, it doesn't forget until a 12v disconnect.

It also might be necessary for a pause in between commutes for the ECU to learn, like a 9-5. One trip in the morning. One in the evening.

My MPG and Hybrid capabilities maintained up until early this year when I had to replace my passenger side CV Axel and Wheel Bearing. I disconnected the 12v for this and have not gained my hybrid capabilities back since.

I also haven't had any sort of regular commute with freeway driving either. It appears this is major in the relearn process.

I had a week where I had an hour commute. All non-freeway driving (55-60 mph) and the hybrid system never changed functionality.

I have gone on a couple of free way trips but it was nothing that was consistent for more than one day. It took 2-3 days of doing the exact same road trip before for the hybrid capabilities to maximize.

Has anyone else experienced this?

Or someone who would be willing to try and replicate this with their Gen 2?

This also may explain why some people have sudden unexplained MPG drops after doing some maintenance or a dealership service.

I will update this thread if my Prius's hybrid capabilities maximize again after I start a regular commute with freeway driving.

Also, my Prius is an 09 Gen 2 with 170k on it. Original HV battery that still has over 50% health according to Dr. Prius.

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Update: I have found another thread from 2005 describing the drop in MPG/Hybrid efficiency after a ECU recalibration rather than a battery disconnect.

Here is the thread.

If anyone can provide more information on the relearn procedure to get the Hybrid ECU back to max efficiency please post.

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I described in the original post on this thread how the relearn procedure does not happen overnight. It takes rather specific driving circumstances. My prius still has not returned to having the same power from EV mode that occurs when the relearn procedure is complete.

EV mode lost significant power after my last 12v disconnect and it still hasn't returned. But I also haven't had a commute that's is freeway driving at 65+ mph or one that involved a 8 hour rest period between driving like a 9-5 commute, which is what was similar to the above commute.

My prius had suffered from poor EV power for months as described above, then when I started this commute in 2019, EV power came back.

When continuing this same commute 1½ years ago, I disconnected my battery twice during that period. My prius suffered from the same loss of EV power. After a couple days of doing this commute mpgs and EV power returned to their maximum state.

Now I've stopped the above commute and EV power still hasn't returned for months after my last 12v disconnect for car maintenance.

The only reason I linked the above post was because the ECU recalibration describes the exact same condition with loss of EV power that a 12v battery disconnect does, which confirms I'm not just making this up. He describes the loss of pedal travel in EV mode just as I did.

I drive a mail route daily with my Prius that's 106 miles and 400+ mailboxes which means 400+ starts and stops. The ECU still has not relearned. This is the difference between 35 and 45+ mpg on my mail route .

I just wish there was some exact information on how to get the Hybrid ECU to relearn. I know if I were to do the above commute I described it would relearn, but I have no need to do the above commute right now.

I spent 4 hours one evening driving back and forth between gas stations on the freeway twice trying to initiate the relearn procedure. I took a 15 minute break at each gas station with the car off to kind of do a soft simulation of the above commute but the ECU did not relearn.

Just figured someone might have some exact information about how the final relearn procedure works. Someone has to know somewhere.

I also wonder when people post about their Gen 2 Prius getting barley above 40 mpg may be because of this and their commute never allows the Hybrid ECU to relearn to max efficiency. I've gone almost a year now without EV power returning to it's nominal state.

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