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

well it seems the system relearned my driving habits after around 2.5k miles...so happy my car is back to normal but afraid of next time.

 

I the great scheme of things, you are talking about a drop in the ocean.

 

I had my HV battery replaced a few weeks ago, and my mileage has seen a significant decrease. Reading this long thread to figure out what I can do about it.

 

To update this, my mileage finally returning to normal after several long trips. Power, performance and battery use returning to previous baseline. Got 51mpg on a recent trip, after being in the low 40s right after battery replacement. Still tends to baby the battery, but it's so much better than it was.

 

I've been following this thread because 100% my 2008 Prius (67,000miles) is stuck in the Weak EV state after I replaced my hybrid battery with a new OEM toyota battery and the 12v was disconnected, I can tell a dramatic difference in the way the EV performs after this. I can very very slightly use the electric only state where previously it was pretty aggressive. I can creeeeeep up to about 12-20mph and the motor will fire up and i'll watch the MPG instantly drop from the 99 to like 25. It's very frustrating and I hope that we can find a solution to this. If there is anything at all I can do I'm 100% willing to do it.

Thank you so much Mojo for doing all this work and putting all this together. I was pulling my hair out about this trying to figure it out before I came across this post.

 

What are y'all mail people or something,? I've never noticed my car driving under all the electric conditions ever backing up . Pulling out driveway till wheels hit blacktop. Never anywhere else I've got brand new battery in all. Prius here. I can gosilbox to mail box on elec carefully . That's about it . I've never had the cars newer where this was much different now I've other brand hybrids . That can but they're series systems .

 

This behavior is normal for a gen2, gen3 and most non-lithium gen4s.

Instantaneous mpg is a worthless metric. Average mpg over many tank fulls is more accurate but not perfect as it usually over estimates by a couple mpg. If you see average mpg drop and not recover in a week or two, then the hv battery may be getting weak, but this takes 8 years or more with a Toyota OEM hv battery replacement. Sometimes 15-20 years.

More recent Toyota hybrids can do slightly better particularly on top speed in ev mode. Range is still a little over a mile with a feather foot.

Obviously recent plug in hybrids are totally different - much more power, dramatically better mpg-e and all speed ev operation within their 40-50 mile battery range.

So the best answer is to get a recent plug in. A Rav4 Prime/Plugin has over 300 hp, over 100 mpg-e and is the fastest accelerating Toyota. Apparently the improved 2026 model will start at $41k. If you really want extended ev and performance and are "100% willing to do it" .

 

Cheap Chevy volt 1st Gen dirt cheap . Can find em good to . Ours go 90 mph for 40.miles almost forever generator comes on . They're kinda cool n cheap because folks scared . DY got everybody scared of electrics or at least his folks are scared they selling them off like hot cakes . Saw a volt running for 1350 the other day pretty clean to .

 

Read this article: 5 modes of Prius Operation. (which is linked in my signature) and see if it fits your driving scenario. Using the instantaneous MPG is not at all a suitable way to gauge your consumption. Look at the average MPG on the bottom of the Consumption screen after doing > 500 miles.

As a summary, it takes about 15–25 minutes of driving to reach stage 4. Stage 4 give the greatest fuel economy.

If you have a driving scenario like the OP (postal worker) the hard acceleration/hard braking with no meaningful cruise will kill MPG—Period.

 

Sorry, Fitch, that's the problem with this topic - you get reflex responses from people who haven't actually understood the fundamental behaviour change here, and make statements like "it's normal to not run on electric all the time". Well, duh, yes, thanks for that contribution.

I think people just like assuming other people are stupid and asking a stupid question, and then once they've assumed that, they focus on the bits they want to answer, and ignore the bits they don't have an answer to. (And indeed, almost the whole thread.)

In this case, "a dramatic difference in the way the EV performs after this".

What's going on, and that there is a issue hitting some people, is crystal clear in the videos and graphs the OP posted back in the thread (see page 6 for the video).

The "limp" behaviour of the Prius where it avoids using the battery is well known, and that seems to be what's happening here - cars getting stuck in the limp state or something close to it, apparently due to a failure to recalibrate something after losing some state when the 12V was disconnected.

@Fitch - it does seem there's still no definite "cure" here. Some have suggested replacing the 12V battery with a new one, but I've seen no reports of that working. Otherwise, wait and hope seems to be the best bet at the minute. Eventually a car get what it's looking for in the recalibration, even taking a couple of thousand miles.

How long has yours been in the "weak" state?

 

I wonder if can be related to how long the 12v battery was disconnected?
As in, how long has that capacitor thingy been in existence?