12 Volt Battery Low

Discussion in 'Prime Main Forum (2017-Current)' started by maiki, Mar 21, 2019.

  1. maiki

    maiki Member

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    Damn! 2018 Prius Prime bought brand new in September. Went to start it a little while ago, and it would not start, got the message "12 Volt battery low". Very unexpected.

    Why would the battery be low on an almost new high tech car? Has this happened to others?

    Called Toyota's roadside emergency assistance, and now have to wait an hour or so for someone to come and jump start the car. Aggravating!

    Driving to the location I am going to from here might not be enough to charge the battery. Will charging the EV battery (now used up, in HV mode) also charge the startup battery?

    Thank you.
     
  2. lextoy

    lextoy Active Member

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    Plugging in the evse to charge the traction battery does put a little charge into the auxilliary 12v battery. But probably not enough to bring it back to full from a discharged state. Likely you will need to recharge the 12v with an external charger for 10 hours or more.depending on what charger your use. A battery tender like 2A could take overnight. Slightly better would be ctek or battery minder at 4 to 8A. Fastest would be full size battery charger automatic or manual at 10 to 30A. At least the battery is easy to access under the hood. What did you do to discharge it? Light on overnight? Extended listening to radio or running accessories in ignition off mode? Door open for a few hours while cleaning interior? Left it sitting for a month or two over the winter? Any of these could kill the battery. It's smallish, doesn't have alot of reserve energy. Modern cars with all the computers tend to kill batteries more frequently if your not careful.
     
  3. schja01

    schja01 One of very few in Chicagoland

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    Yes. Several lengthy threads on this.
    Happened to me but I Drive infrequently.
    Possibly a problem with the 12 volt battery.
    Check the threads for more possibilities.
    Now I carry a Lithium Ion jump starter and monitor the 12 volt battery.voltage with an inexpensive voltage display I plug into the cigarette/accessory plug.
     
  4. schja01

    schja01 One of very few in Chicagoland

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    The manual says 4amp max I believe.
     
  5. drash

    drash Senior Member

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    Yep happened to me. There’s a lot of things that the 12V system supplies power for while the car is not on:

    Battery Cooling ECU and fans (they can turn on and off without warning)
    Activation of the charging ECU for lights (on the front of the dash) and charging light at the Charge port
    Monitors Charging door and cable locks
    Activation of access ECU for the door lock modules (for unlocking/locking) when the key is near
    Activation of the interior lights when the key is near
    ECU that Monitors/activates the remote key access
    ECU that Monitors the Entune remote access cellular connection
    Activation of the DC/DC converter to prepare for ready mode via the ECU
    ECU that monitors all switches in the car
    And don’t forget the delay off of headlights when the car gets turned off.

    This is not an inclusive list by any means. I’m sure there are other things being provided with 12V power.

    I keep a super capacitor jump start which is kind of overkill for the Prime but I actually bought so I could help anybody else since I can’t jumpstart anybody. The best thing about it is I never worry whether it has a charge, because it can actually charge from even a discharged battery.


    Unsupervised!
     
  6. bisco

    bisco cookie crumbler

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    is anyone pressuring their dealer/toyota for a resolution?
     
  7. Rob43

    Rob43 Senior Member

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    I too have noticed large-ish voltage fluctuations on our Prime, I then put my 12v Battery Tender on it overnight to give it a full charge. Since that full charge, I still see voltage fluctuations day to day. Some days it's as low as 12.2v (~60%), & other days its resting voltage will be higher at 12.5 (~90%), average for our Prime seems to be at about 12.3v (~70%) which is only fair in my opinion since it's driven & charged everyday.


    Rob43
     
    #7 Rob43, Mar 21, 2019
    Last edited: Mar 21, 2019
  8. Mendel Leisk

    Mendel Leisk Sand Pounder

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    @maiki what was the build month, vs when you bought? Maybe it ran down some on dealership lot??
     
  9. maiki

    maiki Member

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    What did I do to discharge the battery? Nothing. The car was sitting for a few hours, I came back to drive it, got that low battery message, and it wouldn't start.

    Oh, it is possible that one of the doors was not closed all the way, accidentally. How would that discharge the battery?

    Earlier that afternoon there was one of those Entune apps updates, which I allowed to occur (when the engine was running). I wondered if there could have been some bug in that update that drained the battery?)
     
  10. maiki

    maiki Member

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    Thanks. As said, I bought it in September. Not sure of the build month, but I think the dealer just got it in the week before. In any case, I have been driving it since September, and the battery first was low like that last night.
     
  11. maiki

    maiki Member

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    Is the voltage shown on the instrument panel?
     
  12. maiki

    maiki Member

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    PS I forgot to mention, in case it makes any difference, that I have the Advanced trim.
     
  13. Mendel Leisk

    Mendel Leisk Sand Pounder

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    Just to check, look in driver's door at the decal: it's says and month and year of manufacturer. But yeah probably not a factor.
     
  14. Salamander_King

    Salamander_King Senior Member

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    Sorry to hear your trouble. Yeah, as others have mentioned, this 12V battery problem has been reported by others, and we really do not know why. It may be some bad batch of OEM battery or PRIME being prone to battery drain.
    Having door propped open will keep the dome lights on. It is easy to miss during the day.

    No. You will have to measure it using multimeter or battery tester.
     
  15. lextoy

    lextoy Active Member

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    another guy only drove his prime 1800 miles in 9 months... and was upset his battery was low ;) was trying to see if anything jogged your memory.
    easiest place to get a good measurement on voltage is at the battery terminals under the hood.
    problem is when you open teh drivers door to pop the hood, it wakes up computers in the car, and the voltage reads low...
    if i were going to get a good resting voltage reading.
    pop the hood the night before, when you park.
    let it sit overnight, 12 hours would be better. not charging with the evse...
    then in the morning, get your reading on the battery at the terminals.(dont unlock the doors or open them, nothing to wake the sleeping computers!!)
    this will tell you how weak it is.
    if below 12.6
    i would invest in a battery charger like ctek or batteryminder.
    repeat the same process the next night. except hook up the battery charger overnight.
    then in the am can check it.
    it will probably read over 13 V after a full overnight charge.
    drive for a week normally. evse charging overnight etc, whatever.
    then redo the resting voltage battery test
    and see what the now fully topped up rested votage is
    should be 12.7 to 12.8
    i havent done this yet but intend to.
    when i get results for topped up battery reading i'll update
    one of my prime batteries was at 12.4 on a february cold morning the day after i bought it.
    i want to charge it one more time and test it a week later now that its not sub zero.
    cold winter readings are always on the low side.
    i have had several cars in the last 5 years that loved to drain batteries and kill them.
    cadillac bmw mercedes and now Prime.
    my caddy lives on the batteryminder.
     
  16. maiki

    maiki Member

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    Did you have the same problem?
     
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  17. m8547

    m8547 Senior Member

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    I have a few ideas that could explain these dead batteries.

    When you open the driver's door the car runs the brake booster pump (humming under the hood). That thing uses a lot of power, maybe 40-80A? I wonder if leaving a door slightly open causes that pump to continue running periodically? Someone else on here had a similar dead battery problem, and the only thing that might have explained (besides a random fluke) it is a door left slightly open.

    Or maybe there's a computer that's awake when it shouldn't be, maybe related to a door or maybe not. In Ready mode the car uses about 25A on the 12V system just sitting there (but of course the HV system is powering the 12V system at that time).

    I think all the lights will go off eventually on a timer. But on my 4runner there's a bug (or feature?) where if you turn the headlights on after turning the car off, they won't turn off on a timer like usual, and they won't turn off when you lock the doors twice. Maybe the Prime has a similar bug? One time I did notice the headlights on my Prime not going off for much longer than usual, and that might have been related to a door being open or unlocked.
     
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  18. bisco

    bisco cookie crumbler

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    i don't have a prime, but i have been reading about the problem with interest, and the number is staggering
     
  19. Chazz8

    Chazz8 Gadget Lover

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    Here is the chicken dance to get to the secret menus w/ battery voltage (advanced Prime); Prius Prime Diag & Service Menu 11-in display | PriusChat

    I have been able to get to these screens on all my Prius (see signature for current stable of Prius). You just have to find the right button to hold down while you flick the lights on and off as described in the thread.
     
  20. lextoy

    lextoy Active Member

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    the voltage shown in the hidden menu is a good one to know, once you realize that it is always reading low because of the draw from the electronics in ACC mode, like 12.1-12.4... or reading high because you are in IG ON mode and the traction battery is feeding 13.9-14.1V.
    i can correlate my readings from the battery terminals, lets say 12.7v, to the menu reading of 12.3 and know that my battery is in good range. once the menu reads 11.9-12.1 i know i need to throw on the battery charger overnight.
     
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