That 12V Battery...

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by f18cmech, Sep 3, 2019.

  1. f18cmech

    f18cmech Junior Member

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    Wow guys and gals,
    My Prius is a different car now.

    Aside from my keyfobs doing nothing but holding the emergency key in place, my HV battery was all over the place, topped off to green in like 10 seconds, bad milage, ICE always coming on, I was starting to think it was the beginning of the end for the little tan bullet.

    But...

    As a last ditch effort, I decided to try dropping a new 12V battery in. At least, to get the fobs to actually lock the car. And whatever else might be effected. So, at autozone I went to get it tested, and sure enough, it was bad. So, bought a Duralast Platinum with the right specs, dropped it in,

    and....

    SKS fully functioning(keys never leave my pocket, kind of sweet), battery bars hold solid at 6 blue even on long inclines or long declines, couldn't get it to move! Also sweet.
    Gas milage is creeping back up.
    The takeaway is, when the 12V is failing, this Prius really gets thrown out of wack.
    Just wanted to share my experience. Tell me what you all have had happen with the 12V!
     
  2. JC91006

    JC91006 Senior Member

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    The only purpose of the 12v battery is the start the electronics in the car and possibly lock and unlock your doors. It won't affect all the other things you've mentioned. This is a case of a placebo effect that will soon wear off
     
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  3. f18cmech

    f18cmech Junior Member

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    Placebo affect concept means little if the change in behavior occurs immediately following an input change. As an auto tech for years I can say troubleshooting boils down to cause and effect, I'm sure you know this. So, battery meter behavior stabilizes and acts way differently and the only variable is the 12V battery. And of course knowledge of theory of operation is helpful so knowing the HV/ICE synergy drive is constantly keeping the 12V battery full will tell you it will work harder to keep a bad battery full than a good one. Correcto Senor? Of course I'm open to stand corrected if need be. Please, your thoughts...
     
  4. JC91006

    JC91006 Senior Member

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    Come back here in a few days after the Placebo effect wears off and we can discuss a little more. There are many factors other than the 12v that may have contributed to what you're seeing.

    12v battery doesn't really do much in a Prius, that's why it's this little tiny small battery. In fact you can disconnect the 12v battery after your car has started and you shouldn't notice any change in your car's performance. Many people here put a big emphasis on the 12v's magical powers, it really doesn't do that much.
     
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  5. f18cmech

    f18cmech Junior Member

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    Sounds fair. I’ll give a consistency report on battery behavior in a Week. I think a vast amount of people report a mpg increase after a 12v change, but, we shall see. Perhaps you can lay down the cynicism and accept that the 12v indeed plays a major role in the integrity of our cars electronics? Lol
     
  6. sam spade 2

    sam spade 2 Senior Member

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    Thank you for the report.
    That same experience......and advice......is present on here in probably a hundred different threads.
    Now 101 !!
    P.S. Bad connections can cause similar problems.

    Also.....if you are watching a long term gas mileage number, that really is pretty useless......because it changes very slowly and takes into account things that happened years ago.
    I reset mine every time a get gas.......or watch the one that resets automatically for every trip.
     
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  7. sam spade 2

    sam spade 2 Senior Member

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    Some maybe but mostly NOT.
    What if the weak battery causes the "electronics" not to start properly ??

    You have been involved in discussions like this on here long enough that you SHOULD know that your statements above are incomplete, misleading and often just plain wrong.
     
  8. JC91006

    JC91006 Senior Member

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    If the car starts and goes ready, what electronic didn't or could have not started?
     
  9. JC91006

    JC91006 Senior Member

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    An actual failing 12v that tested bad should be replaced but I don't want readers to be mislead by all the possible benefits by just replacing your 12v battery.

    These batteries are $200+ in some cases and in "last ditch efforts", people will read these posts and say "what the hell, "since I am experiencing issues, I'll just throw in a new 12v to see if it'll fix my issues". This is done over and over again because of posts stating the magical powers of the 12v battery. It is NOT A MAGICAL DEVICE. It starts the car's electronics......that's about it. After the car is in READY mode, the HV battery runs the show. It will not fix a weak hv battery, it will not increase your MPG (unless the 12v is actually bad and has a failed cell), it will not make your SKS suddenly work if it didn't work before, none of that.
     
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  10. bisco

    bisco cookie crumbler

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    when the sks on my 2004 stopped unlocking the drivers door while in my pocket, the buttons still worked and the car started and ran fine.
    a new 12v fixed that one problem
     
  11. sam spade 2

    sam spade 2 Senior Member

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    What we have here is a failure to communicate.

    I did NOT say that anything did not "start".
    I tried to say that low input voltage may cause it to start incorrectly......with internal errors or cause it to report false errors.
     
  12. sam spade 2

    sam spade 2 Senior Member

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    It is not misleading to state the possible benefits of replacing an OLD or worn out battery.

    It is even absolutely true that sometimes the first "test" will even show one good when it really isn't.

    What is irresponsible is to lead people into spending time and money changing out parts costing MANY times as much when the battery really IS the problem.

    I don't remember ANYBODY on here EVER suggesting that there is any magic involved.

    I think you either need to read more carefully or work on your reading comprehension.
     
  13. sam spade 2

    sam spade 2 Senior Member

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    GASP. :eek:
    Some things DO rely on the 12 V battery before the car "starts".
    Imagine that.
     
  14. bisco

    bisco cookie crumbler

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    there is also a very old well known thread here regarding strange electronic glitches and checking the 12v
     
  15. f18cmech

    f18cmech Junior Member

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    Thanks guys! I can report that the SKS is back to not working. Battery meter is more stable, but I think I need to fully charge this battery for 8 hours before the sks will as reliable as death and taxes. Your thoughts?
     
  16. bisco

    bisco cookie crumbler

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    i would check the voltage
     
  17. jzchen

    jzchen Senior Member

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    Well, my neighbor JC91006 may be onto something.

    Unless you've done something like listen to the radio for an hour while waiting outside of Church like my dad did, basically something obvious to kill the battery, these AGMs are pretty robust.

    New battery dying again is my suspicion, but cause may be the alternator, or dc-dc converter, whatever charges/provides 12 V power...

    moto g(7) power ?
     
  18. Skibob

    Skibob Senior Member

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    AGM’s might be robust but a Prius battery is as big as a kiddie car battery. Don’t confuse an AGM that turns a starter with the clown car Prius battery.
     
  19. jzchen

    jzchen Senior Member

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    You’ve disconnected the battery, resetting the computers? Or did you keep the electronics powered, while replacing the battery? I got lazy when I replaced the battery on my parents’ v, so I connected a battery charger to the jump points under the hood while I swapped their weakened 12V with the Chinese LiFePO4, the reason I asked. So I did not have to do all the reinitialization because my parents car has the fancy parking aid, the one you can release the steering wheel while it helps back you into a spot.

    Placebo effect could be the computers starting with blank sheet of data, starting to reinterpret an unhealthy HV battery.

    How did you verify/determine the 12 V was bad? My parents kept reading just under the healthy voltage, in the range that the service info says to recharge and retest. But it was only 0.0x V low, I think 0.04 V to be precise. It kept reading 12.46 V, and healthy is 12.5 V and up if I recall correctly....
     
  20. jzchen

    jzchen Senior Member

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    True, but small battery also means easier to charge. We are talking about a battery limited to 4.2 A maximum charge rate. Even with 6A charge rate it would seem to overload the battery. Charger would charge to full, but if I quickly disconnect and reconnect charger would start charging again. (Battery would not truly fill). How much MPG are we talking about losing with 4.2 A 14 V of power, 58.8 W of juice being sapped by a bad 12 V?
     
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