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12V battery going dead

Discussion in 'Gen 5 Prius Main Forum' started by Paul Gregory, May 5, 2024.

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  1. Paul Gregory

    Paul Gregory Active Member

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    If you own a Gen 5 Prius Prime, you may be seeing excessive battery drain while parked. I certainly have; I parked for 2 hours while hiking, and when I returned, my car would not start due to "low battery, car can not be started." The battery has been tested multiple times and remains within spec. Frustrating to say the least, but this subject has been covered in other threads.

    I don't know if Toyota ever takes suggestions about design, but here's one. When the 12V battery is dead, why not give it a boost from the much larger traction battery? I understand that the traction battery must be protected from discharging too low, but couldn't that be monitored with simple circuitry? I mean, if the traction battery is too low to allow boosting the 12V battery, it wouldn't happen, otherwise isn't that a very small drain on the larger battery?

    In my Gen 3 Prius, as long as there was enough power left in the 12V battery, it would engage the DC to DC converter, restoring power to the 12V system. This seems not to be the case in the 2024 Prius Prime. All you get is a message saying that your battery is to low to start the car.
     
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  2. bisco

    bisco cookie crumbler

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    the traction battery can never be too low to charge a 12 volt, unless it is broken.

    have you tried toyota customer service to open a case?
     
  3. Paul Gregory

    Paul Gregory Active Member

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    Their website allows you space for a short question, but not feedback per say. It's very hard to put this idea into the 200 character limit allowed.
     
  4. bisco

    bisco cookie crumbler

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    there is an 800 number in your warranty manual
     
  5. Paul Gregory

    Paul Gregory Active Member

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    I have used that number for support, but I doubt they would want to take suggestions there.
     
  6. BiomedO1

    BiomedO1 Senior Member

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    I have a feeling your dealership is lying to you. Can you take the car to another dealership? If your battery is good and there isn't a parasitic drain on your system - you must be leaving the headlights-on or something to kill the battery. Interior lights wouldn't be able to drag the battery down far enough to prevent it from going into ready mode in 2 hours. Any aftermarket 12 VDC accessories installed?

    I'd probably buy a cheap lithium battery jump-pack, just so I don't get stranded again.
     
    #6 BiomedO1, May 5, 2024
    Last edited: May 5, 2024
  7. Paul Gregory

    Paul Gregory Active Member

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    I tried a different dealer, but they don't seem to want to step on each others' business.
    I have been using a jump pack, but it's a bit inconvenient.
    I'm considering wiring a 100Ah Li-Time battery in parallel with it.
    I don't have any accessories that would drain the battery when the car is off. That's the first excuse the dealer used, and they unplugged everything, leaving it on the floor.
     
  8. BiomedO1

    BiomedO1 Senior Member

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    If your going to do that; I'd wire it into a manual multi-battery RV switch. If you hard wire directly into the cars system, the dealership can claim that your modifications is what fried your electrical system. You can switch it on to recharge or jump the car. Switch it off whenever you go to the dealership. If there is a parasitic drain within the car; supplying it with 24x7 power will likely cause an electrical failure - hopefully while it's still under warranty. BTW; if your only going to use it to jump start the car - 5-10Ah battery should be able to do the trick. Much less costly than the big boy...

    In the gen5; doesn't the lighter and USBs go dead when you shut down the car, so it doesn't drain the battery?

    Good Luck....
     
  9. Paul Gregory

    Paul Gregory Active Member

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    I don't believe my parallel battery would be doing any harm, but thankfully, it's easy to remove the ring terminals attached to the battery. I'm sure that would be the first thing the dealer blames for the problem.
     
  10. Mendel Leisk

    Mendel Leisk EGR Fanatic

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    Measure it? Or get someone to do it? Dealership won't, or did but didn't divulge their readings?

    IIRC so far you've put a clamp meter on it, but I believe they're not sensitive/accurate enough for milliampere measurement.

    It should be around 20 milliamperes.

    This is a new car, under warranty; and you're talking about putting a secondary 12 volt in there??
     
    #10 Mendel Leisk, May 5, 2024
    Last edited: May 5, 2024
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  11. Louis19

    Louis19 Active Member

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    Mendel is right , and I am about getting bored of this excessive battery drain case
     
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  12. Paul Gregory

    Paul Gregory Active Member

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    I measured it once, at 340mA, but it may fluctuate widely from that. I'm planning to wire a 100mAh battery in parallel with it. I'm sure it won't cause any problems, but the dealer may not like it, so it will be easily removable.

    I told the service manager about the 340mA drain and she nodded in acknowledgment, but they admitted they didn't have a solution for it. They suggested a battery maintainer, which I installed the week I bought it. It works, but not when I'm not near an outlet, hence the auxiliary battery.
     
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  13. Paul Gregory

    Paul Gregory Active Member

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    I'm bored with you saying you're bored.
     
  14. Gokhan

    Gokhan Senior Member

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    There is another problem here, as only ~ 1% of owners are reporting 12-V-battery issues. You and/or the dealer need to figure out and solve it.

    Charge the 12-V battery from the traction battery? What if the traction battery is low, too? Moreover, with the rate of drain you claim, that would drain the traction battery and brick the car quickly as well.
     
    #14 Gokhan, May 5, 2024
    Last edited: May 5, 2024
  15. Louis19

    Louis19 Active Member

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    Hey last call
    Enjoy
     
  16. Gokhan

    Gokhan Senior Member

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    What he is experiencing is far from normal. He is implying that the parasitic drain is 20 A now, which is necessary to drain the 45-Ah OEM battery in merely 2 hours. Now, that's a big parasite! ;)

    In any case, it is the dealer's responsibility to fix it under warranty. If they can't fix it, then the car needs to be returned to Toyota.
     
  17. Xse

    Xse Junior Member

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    Just saying my prime is only weeks old The second day the windows were tinted hrs doors and hatch opened no issue starting few days later it was front end filmed half a day in the shop
    Plus there is a electronics rust unit forever on(parasitic drain)no issues starting I just today installed a receptacle nearby the PP so now my second battery tender is in use to the 12v battery planning for next winter usage
     
  18. Paul Gregory

    Paul Gregory Active Member

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    Well, I don't think accuracy is the issue. The car won't start in as little as 2 hours being parked. At any rate, I have tested my clamp ammeter, and I found it to be very accurate both on AC and DC.
    Judge for yourself: Budget Multimeters › UNI-T UT210E Current Clamp/Multimeter Review.
     
    #18 Paul Gregory, May 5, 2024
    Last edited: May 5, 2024
  19. Paul Gregory

    Paul Gregory Active Member

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    There's plenty of room in the pocket opposite the 12V battery for another battery.
     
  20. Paul Gregory

    Paul Gregory Active Member

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    Don't need it thanks. It's one of the easiest things to do.
     
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