Prius battery does not seem to discharge much and has bad gas mileage

Hello all! I am hoping some people can give me some advice on a battery problem I have been having with my 2008 Prius.

Since roughly October I have been getting bad gas mileage on my 2008 Prius (35 to 40 MPG on fairly level roads doing about 100 km/h, about 60 mph). The battery is green about 40% of the time or so, even on fairly level roads, which is weird in itself, and it never lets the battery drop down from green by more than 1 or 2 bars into the blue, so it seems to stay about 80% to 100% charged all the time.

I took the Prius into the local Toyota dealership and the tech said there were a lot of error codes being generated (sorry, I do not know what the error codes are). He asked if I had killed the battery recently. In fact, the battery was killed awhile ago, I can’t remember exactly when but it was roughly last summer to spring the dome light was left on and killed the battery. The car battery was boosted according to the manual and there were no gas mileage issues after that, until roughly October. So, going by that, the tech said he reset some of the hybrid systems. What exactly was and was not reset or done, I don’t know, but I trust he did what needed to be done based on what he saw. The tech said it was also likely caused by the dead battery as that apparently can happen, so I had to pay for 1 hour of labour ($130 with taxes), even though the car is still under full warranty.

I drove the car back home on a level highway, doing about 100 km/h, and there was no change to the problem. Same gas mileage, same battery won’t discharge much issue. Half way home, I took the Prius on a side road and slowed down enough for the gas engine to kick out, and I drove it slowly so the engine stayed off to see if I could get the battery to discharge more than what I was seeing in town and on the highway. The battery did actually discharge into the purple bars, then the engine kicked in. So at least I know the batteries are at least capable of discharging into the purple bars. However, once I got back onto the highway, it charged the batteries back into green, and then stayed green most of the way home (about 20 or 30 min). The battery was fully green, right to the top, as I pulled into the garage. Definitely not right. There were no hills whatsoever on the trip that should have allowed the battery to get fully green, and then stay there for so long.

I called the Toyota dealership, and they said to give it a couple of days for the system to re-learn. Ok, fair enough, and the dealership is treating me well, so no complaints there (except for charging me for something I think should be covered under warranty, but I suppose they are just following Toyota policy). In the meantime though, I do fear the problem is not fixed and I was hoping to get some ideas from some of you. I also wanted to hear if anyone else has had similar issues after they accidently killed their battery, and if this is something that should or should not be covered under warranty. I am on the thought that a car should be able to handle boosting from a dead battery without going completely, well, wonky, for lack of a better term, and costing me $130 for servicing, but maybe this is just the way it goes with a hybrid.

Thank you,

 

Hey there!

1. The gas mileage is bad for this temperature. It does not get too cold where I am, typically -5 to 0 C. Today was an unusually cold day at -10, but it has been doing this since October.

2. Sorry, I should have listed the error codes from the invoice. They are P0852, B1421, C2315, B1200, B1207, B1271. It notes that all codes pertain to low battery voltage. It also notes “checked ISC learn status and it was incomplete. Will not complete with all codes in ECU’sâ€

3. Understandable. However, does not finding error codes in the diagnostic mean that it is a “real†problem?

4. Hmmm… hopefully there is something that can be done about it, I hate seeing my Prius get only moderately better gas mileage than my minivan

 

Hello! Thanks for replying.

1. I have not recorded actual averages for these temps. But I have owned this car for 3 years. 35 to 40 mpg under these conditions is low. That, with the refusal of the Prius’ system to take from the battery below about 75% battery charge under normal conditions is not normal. Running with a green battery too is not normal. It’s like the car doesn’t realize there is more battery reserves that it can take from.

2. Yes. Although it is a US Prius, imported into Canada from brand new in 2008.

3. Ok

4. The minivan is a 2004 Sienna. It’s mileage varies of course depending on driving conditions. It is getting close to the Prius in MPG, although the Prius still beats it in MPG, but really, the Prius usually exceeds the Sienna, as it should, by a long shot. Sorry, I have not tracked specific specs, but I do know my Prius MPG is way out

 

Ok, my MPG is usually 50 MPG approx for this stretch of highway I am comparing to in my above posts, even in the winter. I have had this car for 3 years, and have driven it in much worse conditions than I am describing now. I know about D and B on the shifter. The car has never been in an accident. I have original tires the car was provided with from Toyota. They are inflated to recommended specs. Toyota has looked at the vehicle to check all this, and have checked fluid levels to ensure they are within tolerance, including the oil level. I have seen that the fuel economy gets worse when it gets cold. I use the onboard computer to determine my average MPG, I do not do the calculations manually. I trust the computer is fairly accurate with this… maybe I am wrong with this assumption? I am taking into account it is -5C when I am talking that my MPG is not normal. -5C is not that cold for a Prius’ normal performance from what I have seen in 3 years. I have driven in -35 C temperatures with my Prius. Now THAT is cold I’m not new to what I can expect from this car performance wise. If there were no problems with it, I would expect no error codes… but there are error codes as the Toyota dealer has reported.

My Sienna does get close to my Prius in MPG, as per your Consumer Reports testing reference. 25 to 30 MPG on the highway (which sounds about right)… my Prius gets 35 to 40 MPG under the same conditions right now. But I suppose “close†is objective. I consider that close, considering the Sienna is a 6 passenger minivan (and a rather good sized one at that) with a V6 engine with absolutely no hybrid system at all… and it is 8 almost years old (bought in August 2003). I expect, and normally get, significantly more from my Prius. I love my Prius, and I have watched its performance with satisfaction over the years. There is definitely something wrong now though. Maybe it will clear in a few days as the Toyota dealer hopes. I will keep you all informed on this.

Comparing to the Sienna aside (that was just a supposedly witty comparison, hence the happy face in that post after that comparison), and my original post was never meant to compare the two, as that is like comparing apples and oranges. I was hoping to get an opinion on what might be wrong based on people’s experiences. Is this normal, after boosting a dead battery? Will the system “relearn†after the tech reset certain systems, or is there something more serious going on? If you accidently kill your battery on your Prius (e.g. dome light left on in interior), can you expect that the car’s hybrid system will be affected like this? Maybe these questions are not as easily answered as I had hoped… nothing is ever easy in life, as I have learned too many times! Either way, I’m hoping for some insight… and I will keep you all informed too as I learn more.

Thanks for reading my rambling post… I am trying to make it as compact as possible, yet still give the details needed for this

 

Hello! Sorry, I was not clear. I have had this problem since October, and I just brought it into the dealer today, so now at least I have more information on it, including specific error codes. Yes, yes, I was in denial for a bit there (what, something wrong with MY Prius… NEVER!) Plus, the Prius gets used less in the winter until I get it setup with some winter tires J The Sienna takes over the tough winter jobs with its shiny new winter tires until the weather gets nice for the Prius. All the times I have reported on the Prius’ performance has been on dry, bare pavement.

 

Hmm… 12 v battery issue? I would have never even guessed that might be an issue. But that is why I posted this here! The car has not given me any starting issues at all, but I know that’s not to say the 12v battery is not having issues. Although I would expect the 12v battery to last longer than 3 years, perhaps that time of running it dead has significantly reduced its life, as I know that can happen.

I’ll bring that up with my Toyota dealer for sure. I know they want me to give it a couple of days and see what happens. If the condition remains the same, I will suggest replacing that. Unfortunately I don’t DYI with these things… I will get the dealership to replace the battery if necessary.

I will keep you all updated as to what happens. But for now, I am off for bed! Good night all and thank you for the replies so far.