06' Prius SOC at highway speeds

Hey everyone, needing some advice on a issue with my 06' prius that I believe to be abnormal behavior. Just took the car on a 360 mile round trip. After about an hour of driving (Interstate 78mph) I happened to notice on the Energy screen that the car stared to use the electric motor steadily for about 5 minutes of driving on flat terrain (Kansas). I watched as the SOC went from 5-6 bars down to 1-2 (purple). At which time the car started experiencing bouts of high rpm on level terrain/small hills. It stayed in this state for over 20 minutes or so of driving. It acted like it knew it needed to charge the hybrid battery, however the level never increased until such time as I exited the interstate and started braking. Am I wrong in assuming that the HV battery should rarely discharge that much traveling at highway speeds on flat terrain? And if it did, shouldn't it recharge somewhat quickly again at highway/Interstate speeds?

 

I just bought the car used a couple months ago with 101K on it. Had an excellent service record a thru Toyota. The dealer I bought if from (not Toyota dealer) just had a "new" (reman) battery pack installed. I don't have any issue whatsoever around town or even shorter highway speed trips (less than an hour). Just noticed this behavior as this is probably the second longer trip i have taken with it.

At first i thought it maybe was a battery maintenance feature where it bled off battery charge to allow it to recharge going down the road as a means to exercise the batteries or something. I did notice that if I hit the breaks or had to slow down that at some point it started charging the battery back up to the 5-6 bar range. I'm kinda leaning towards a possible overheating of the HV battery. The batteries seem to be performing as they should (charging/discharging) but a some point after about an hour or so of driving the car steadily starts using the electric motor and drains the pack. Strange

 

The outside temp was about 97 degrees. The car was in "auto" A/C re-circulation mode, set to 79 degrees cabin temp. Obviously while going down the road i cant hear the battery pack fan running, but i did check to see if it was running while parked this morning. I put my ear up to the intake vent and hear fan moving. I don't know how fast that fan should be running at any given time, however, it was running. I doubt when the HV battery was replaced the mechanic took the time to clean the fan or filters. It would probably be worth doing for several reasons, but mainly to check to see if the fan ducting was put back in place correctly when the battery was replaced. I'm told there are a # of service bulletins regarding both the car and battery ECU.

What makes this issue so tough to diagnose is the fact that I have no warning lights illuminated, the car acts and drives perfectly around town and the problem only arises after approx. 1 hour of continuous driving at Interstate speeds (that i know). It may not be a "Problem" at all and just circumstance and/or perception, but I definitely don't believe the behavior to be correct.

 

JimN, thanks for your reply. I'm hoping that isn't the case, but obviously can't rule it out. I'm not sure how having a bad cell in the pack would cause the car to drain it's entire pack for no reason going down the road, but we are talking electronics and batteries here. I do agree it seems to be something that "heats up" during extended driving that is causing the issue.

Just had a base diagnostic at my local mechanic and he said there were no codes in the car and everything passed the diagnostic on the scan tool. Of course the HV pack was not load tested during this so there is no real way to see if there is a problem with the battery pack. His advice was to monitor it in different driving conditions as best I could to see if it did the same thing under other circumstances. He stated that generally speaking if there is a gremlin hiding in the battery pack, ECU, etc. it should show itself at some point by throwing a code.

 

Thanks for all the advice guys. As this is the first Prius I have owned it is nice to get the opinions of those who are more experienced. I have service appointment for an oil change/inspection this week. I am going to mention the problem to them and see if they can diagnose it. I'm hoping that they will be able to give me an answer, or at the very least test the HV pack to see it's condition. It very well may be something like a ECU or the like. It seems to me to be computer related of some sort as I'm not getting any warning lights, car works flawlessly around town, and the pack seems to charge normally if I just hit the brakes on the Interstate once and awhile. I will post what I find out from Toyota. I'm hoping its not a long and painful road to fix.

 

Will do, I have a couple on my phone. I will post them up.

 

JimN

Not sure 100% on the warranty on the battery, as we bought it from a small dealership with the reman battery already installed by the local mechanic. I did get some paperwork (work order, invoice) for the battery so I will have to see if it mentions any warranty. However, given that the replacement was done for the dealership and not me, I wouldn't be surprised if whatever warranty there was wouldn't transfer to my ownership.

I guess we will see what Toyota has to say. I'm hoping that they just don't "punt" on the issue knowing the battery is a reman and blame the hybrid pack. If the HV pack is the problem, fine at least I know that's what is causing the issue. I just don't want them pointing the finger at a part they didn't install simply because they don't know.