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12v battery issue

Discussion in 'Prime Main Forum (2017-2022)' started by John roden, Dec 3, 2023.

  1. John roden

    John roden Member

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    my son borrowed my 2021 prime and it was displaying a check engine light but running OK and throwing a p0a1f code. he lives in a city and runs on internal combustion very frequently and was away for a week so the car was parked. It was giving him trouble last night the traction battery was dead. I figured he could drive it and recharge the battery but I was wrong as it started getting flaky and threw a u019b code and quit.

    It’s been towed to the dealer at this point. It has 55 k miles. Does this sound like a bad or discharged 12v? my son did not have a multimeter so I don’t know the voltage of the battery unfortunately.
     
  2. bisco

    bisco cookie crumbler

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    How did he determine that the traction battery was dead?
    If that were the case, you will have a lot more problems than a 12v replacement.
    That being said, it could certainly be a weak 12 volt.
     
    #2 bisco, Dec 3, 2023
    Last edited: Dec 3, 2023
  3. John roden

    John roden Member

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    Sorry wasn’t clear, the traction battery was just discharged
     
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  4. bisco

    bisco cookie crumbler

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    Hopefully, it’s just a low 12v from sitting for a week
     
  5. BiomedO1

    BiomedO1 Senior Member

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    More than likely the 12V battery is weak and needs a recharge, since it's getting cold - that doesn't help the battery either. When he's running around the city, are they usually "short-hops". The traction & 12V battery doesn't get recharged, if your only running the car for 10 minutes at at time. Has the dealer/mechanic, checked and topped off the electrolyte levels in the battery over these years? I had to put some distilled water in mine last year. Did he leave the car plugged in for the week he was gone?
     
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  6. John roden

    John roden Member

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    It seems odd that it started and ran okay on the ice then died after a while. Those have an alternator for the 12v system? He was driving with a discharged traction battery at the time.

    I’m just not enthusiastic about the dealer getting into it. I don’t want this to turn into a $1000 battery recharge.
     
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  7. BiomedO1

    BiomedO1 Senior Member

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    I believe MEG1 in the transmission is the starter/alternator. If your 12V battery is low, <10.5VDC - funky things happen to electronics. They get "delirious", and trigger all kinds of phantom error codes. Charge up the 12V battery and plug-in the traction battery. Bring them both to full, clear all error codes and go from there. The cars' internal systems are pretty good at protecting itself from damage.
    Put a DVM on that aux. battery, while the traction battery is plugged in and charging or the car is in "ready mode" - you should see 13.5-14V. With the car off, and unplugged, battery should read >12VDC.

    Hope this helps......
     
  8. ChapmanF

    ChapmanF Senior Member

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    You almost never see a Prius with a traction battery that is truly discharged. When you do, there is no way to start it at all, because the traction battery is what starts the engine; you need an appointment with a high-voltage traction battery charger if that ever happens.

    The battery charge level display on the dash just covers a normally-used portion of the battery charge range. When that display looks "discharged", it just means the traction battery charge is at the lowest level the car normally likes to use; there is still plenty of charge left for starting the car. (People do get into trouble sometimes if the engine has a problem making it hard to start, and they try a bunch of times; do that enough and you can indeed end up with a truly discharged traction battery, and need a high-voltage charger to save the day.)

    There isn't an alternator for the 12 volt system. Whenever the car is READY, the 12 volt system is being supplied by the high-voltage system through the "DC/DC converter" that lowers the voltage, and that's how the 12 volt battery gets charged. Even when the dash shows the traction battery "discharged", this is still what's happening.

    So what keeps the traction battery charged? Other than plugging it in (for a Prime), the two motor-generators in the transmission take care of that. Whenever either one of them is generating more than the immediate propulsion needs, the surplus is going into the traction battery. MG2 does that when the car is regeneratively slowing, and the car can arrange for MG1 to do that any other time the engine is running and the traction battery wants charging.

    Reports of electronic delirium and "phantom" codes can be greatly exaggerated. Several of the ECUs in the car have specific trouble codes they will set if the supply voltage dips below 9.5; that isn't delirium, they're still non-deliriously doing their job, and part of the job is to set these codes to say "hey, just letting you know voltage got below 9.5 here".

    It never hurts to make sure the 12 volt battery is well charged if there's any doubt—the Toyota repair manuals are full of little notes to make sure it's at least 11 volts before working harder on a diagnosis—but it also doesn't hurt to make notes of what codes you do have before just assuming they are "phantom" and clearing them. There will be times they are telling you something worth knowing.
     
  9. BiomedO1

    BiomedO1 Senior Member

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    Well the good news is that your covered under warranty - EXCEPT for the 12V aux. battery. That's covered under the 3yr/36K mile bumper to bumper warranty - which you've surpassed by 20K miles. NY is a CARB state? If so, traction battery and associated EV electronics are 10yr or 150K mile warranty; otherwise 8yrs or 100K - Your covered either way. I almost guarantee you the dealership will charge you for a new battery and possibly a tow-in diagnostic fee. So worse case, bill shouldn't exceed $700.

    Since we're getting so many aux. battery questions. I decided to use my car as an example.
    1. I plug-in every night and burn through my 25 mile EV range almost every day, plus 10-15 minutes on ICE to get to my destination.
    2. If the ICE is running when I"m parked and reached my destination; I'll let the ECU complete ICE cycle before powering down.
    3. I do biannual aux. battery inspections and top-offs, if needed
    4. Car is parked outside, so exposed to the small temperature swings in my area.
    Last night was in the low 40's with cloud cover, holding in the heat, and the car completed it's recharge cycle. Battery was at 11.5VDC, read on a certified Fluke meter. I placed the battery on a 10A charger. 15-20 minutes and levels came up to 13.75VDC, after another 10 minutes battery voltage settled at 12.65VDC. My battery is behaving normally, didn't disconnect it from the car or do a load test.

    Hope this helps someone gauge their aux. battery performance.
     
  10. bisco

    bisco cookie crumbler

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    when the o/p refers to the hybrid battery being discharged, he means that the wall charge is depleted, and it is running as a hev.

    there is no 'altenator'. the engine charges the hybrid battery, and the hybrid battery charges the 12v battery. as long as the car is in ready, and also when plugged in.

    this is a classic example of when triple a or the like pays off.
     
  11. vvillovv

    vvillovv Senior Member

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    Both codes are related to the traction pack system.
    here is a link for the toyota dtc u019b

    and a link for the toyota dtc p0a1f
    The 12 volt battery ? possible, With these cars a lot of things are possible.
     
    #11 vvillovv, Dec 3, 2023
    Last edited: Dec 3, 2023
  12. Mr.Vanvandenburg

    Mr.Vanvandenburg Active Member

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    It was discussed that charging the traction battery externally does not charge the 12 volt. I charge my 12 volt at least once a week, did it today. Of course if one has a battery monitor on the battery so much more voltage info is there than a multimeter, and a state of charge value. Plus temp on some.

    The reason I charge so much is very little driving time episodes, usually 10 mins sometime 5 mins. The traction has no time to fill the 12 volt like an alternator would. Learned and now adapt to the situation.

    The check engine light on at the beginning of the story is surprising on a 2021. Check engine light on but running fine, I don’t know that seems like don’t do that, go to a dealer.
     
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  13. bisco

    bisco cookie crumbler

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    it is at the dealer, that's the frightening part :p
     
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  14. Doug McC

    Doug McC Active Member

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    No the REALLY frightening part is he is turning to PC Egineers (mis spelling intended) for advice!
     
  15. vvillovv

    vvillovv Senior Member

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    Most of us here at Priuschat are not engineers, only like minded DIY ers that may understand some of the hoops that need to be jumped through when doing DIY repairs on a Prius.
    Of course there are also a LOT of cynics providing their own unique input as well.
     
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  16. bisco

    bisco cookie crumbler

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    It’s ironic that people come here to learn or teach, yet criticize those who come here to learn or teach
     
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  17. John roden

    John roden Member

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    [ QUOTE="bisco, post: 3409483, member: 5889"]How did he determine that the traction battery was dead?
    If that were the case, you will have a lot more problems than a 12v replacement.
    That being said, it could certainly be a weak 12 volt.[/QUOTE]

    thank you for the help on this. I am sitting in New York and the car is sitting at the Toyota dealer in New Orleans. My son is not so mechanically inclined and I don’t want the dealer to take him for a ride especially since it is coming out of my pocket. I think there is a pretty wide range of honesty among the dealer population and I guess I come at it with a little bit of skepticism. My hope is the car simply needs a recharge and the codes cleared but once they get around to turning off their off hours phone message maybe I can speak to a human being. So far they are an hour late and the voicemail box is full none of which inspires confidence that the service department has their act together. But maybe I’m coming at this with the sense of cynicism right from the start
     
  18. Mr.Vanvandenburg

    Mr.Vanvandenburg Active Member

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    Engineer is not even the top dog class. It’s a wide ranging mish mash of inequality and sometimes self promotion.
    He needs to go to a professor of engineering, tenured and published with awards, to get the real low down. Of course, how is he going to do that? Would the professor know about his own 12 volt issues in his ev? Probably not.
     
  19. ChapmanF

    ChapmanF Senior Member

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    If they're anything like the demographic studied here:

     
  20. Washingtonian

    Washingtonian Senior Member

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    My experience with my 2017 model has been extremely positive. With the local dealer not so much. Mine still has the original 12V battery. I carry a charged 12V jumper box in the car and have a Fluke DVM at home to check the battery. I know I will eventually have to replace it and found that Costco currently doesn't stock it and the local dealer wants $315 for a replacement. This is the same dealer who wants $59. for and engine air filter and $79. for a cabin air filter. I am not an EE, but did retire from the Navy as a Master Chief Electronic Technician, so I do understand a little about the car's electricity and electronics.