When the 12 volt battery dies . . . . .

Discussion in 'Gen 4 Prius Technical Discussion' started by tzx4, Mar 19, 2020.

  1. tzx4

    tzx4 Active Member

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    A few days ago the OEM 12 volt battery died exactly four years after I bought the 2016 Prius new.
    Sort of expected. What I did not expect was that the car ran fine one day, and then two days later the warning message on the dash showed up, and I could not get the car to operate. Fortunately it in was in my garage rather than some less convenient location.
    1. Is sudden dead-in-the-water mode the way a worn out battery is supposed to happen?
    2. Is there any way to anticipate or test for this before it strands me somewhere the next time?
    3. I am assuming that jumping the 12 volt battery is not relevant here because it gets its charge from the traction battery. (?)
     
  2. booke02

    booke02 Active Member

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    1. That's how it works.Either it can provide a minimum voltage for the electronics, or it can't.
    2. Not really. In most cars you get a 'warning' because the starter motor struggles and the car is hard to start
    3. You can jump the 12V battery to get your car started, but you have a problem if it so dead/damaged that it can't get some charge from the traction battery
     
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  3. jerrymildred

    jerrymildred Senior Member

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    That would be typical. The 12V battery doesn't have much load on it. The early warning sign always used to be slow engine cranking. But the Prius 12V doesn't have that chore so little if any warning.

    I have an LED volt meter/USB charger that plugs into the 12V outlet. If you watch it when you first boot up the car, you'll see it go to a low voltage as your battery absorbs the brief inrush current pulled by the various relays and computers. Then for about a half second, it'll stop at it's true voltage (12.x) before the inverter's DC/DC converter comes on line to charge the battery back up (14.x). That middle voltage is close to the car's resting voltage.

    To be more thorough, open the passenger door in the morning and reach across to pop the hood and use a multimeter to check the voltage without causing the brake pump to run by opening the driver's door. Or, you could pop the hood the night before if the car is in a place where no one will mess with it.

    Four years is a pretty typical lifespan, but some last way longer. Mine is right at four years, so I'm starting to keep an eye on it.

    Not sure what you mean. If the 12V battery is dead, jump the car like any other car. Just make for certain-sure you have the polarity right. I carry a lithium-ion jump pack that checks polarity before putting power on the cables. I've only used it once and that was on a friend's parent's car -- another Prius, by the way.
     
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  4. Mendel Leisk

    Mendel Leisk Senior Member

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    Digital battery tester. Maybe google Solar BA9.

    And if you have frequent downtimes, often don't use the car for multiple days, and/or do mostly short trips, look into smart chargers, max 5 amps.
     
  5. alanclarkeau

    alanclarkeau Senior Member

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    Yes and no. I've had 12v batteries in other cars which died a sudden death, others died slowly. Depends on their mood, I guess.

    The 12volt battery isn't like on a normal car, as it isn't charged like with a normal alternator. Normal testing procedures could lead you to replace a perfectly good battery.

    This YouTube by John Kelly describes it very well.

     
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  6. bisco

    bisco cookie crumbler

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    i had one where the sks stopped working, but the car started fine, giving me plenty of time to replace it. and one that was ditw after driving 4 miles and shutting it off for 45 minutes.
     
  7. sam spade 2

    sam spade 2 Senior Member

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    Those two statements are grossly misleading.

    The 12 V battery in a Prius is EXACTLY like ones in most other cars.

    Charging voltage is charging voltage, no matter where it comes from.

    "Normal" testing procedures often prove to be inadequate, not matter what car the battery is in.
     
  8. kithmo

    kithmo Couch Potato

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    Apparently the electric windows get slow when the battery is low, not sure how you check them though, AFAIK the car has to already be in READY mode to operate them.
     
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  9. jerrymildred

    jerrymildred Senior Member

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    Now that you mention it, that might be a fairly good indicator except that the change might be too gradual to notice until it gets really slow. It doesn't require READY, though. ACC will also run the windows.
     
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  10. alanclarkeau

    alanclarkeau Senior Member

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    Possibly - but as the John Kelly video indicates, the 12v voltage varies at times - so it could be naturally lower or higher, depending on the SOC when shut-down. Maybe.
     
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  11. Leo

    Leo Leo

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    This thread has caused me to wonder why the car needs two batteries? Why can't the traction battery be used to power the electronics? And if there is a good reason (that I don't understand) why you need two batteries, then my question would be: why can't the car be designed use the traction battery to supplement the 12-volt battery if the latter has insufficient charge to start the car? It could send a warning to the dashboard "12 volt battery needs replacement" after the car starts.
     
  12. ChapmanF

    ChapmanF Senior Member

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    For safety reasons, the high voltage battery is completely isolated from anything when the car is off. The relays that connect it do not pull in unless the aux battery has powered the car's computers to complete their checks and decide that it's safe to do so.

    I had to replace my 12 volt battery a couple months ago, after about ten years. If you're watching for things like lights being a bit dim before the car is on, or the brake pump sounding more like a wind-up toy when you open the driver's door, you can get a bit of advance warning, if not necessarily much.
     
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  13. Mendel Leisk

    Mendel Leisk Senior Member

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    Maybe that's why the hybrid battery can handle long hiatus well?
     
  14. sam spade 2

    sam spade 2 Senior Member

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    AND for practical, operational reasons too.
    Same reason that a Motor Home has 2 battery systems that are isolated when no charging is present.
    So that if you make a bone head move and discharge "the battery", it won't leave the unit totally dead in the water.
    And totally discharging the HV pack is not good for it.
    The list goes on.
     
  15. jerrymildred

    jerrymildred Senior Member

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    Because if a fireman is trying to cut you out of a mangled car and he cuts into one of those orange cables he doesn't get killed along with everyone who's touching him or the car. So that if a mouse eats through the insulation, you don't get killed opening the door to get in. And as has been mentioned, so that if you leave something on too long, you don't kill the very expensive battery.
     
  16. Michael N Bennett

    Michael N Bennett Junior Member

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    A battery maintainer is the way to go if you do not drive much. You can leave on on board permanently connected to the 12v battery and run the cord to someplace easily accessible. Doing this prevents the plates from corroding and eventually breaking which shorts the battery internally, killing it permanently. When this happens the battery will not take a charge even after a boost. I injured my leg last year and could not drive for 3 months. My 2016 sat in the driveway all this time and then would not start for this very reason. The battery had to be replaced. I was unable to attach a maintainer to my 1974 Porsche 911 at that time as well and its battery also had to be replaced.
     
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  17. Leo

    Leo Leo

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    Can't this happen now? How would removing the 12-volt battery and replacing it with a 12-volt line from the traction battery be any more dangerous? As far as "leaving something on too long will kill the very expensive battery" some people who have left their car parked while out of town for several months have returned with the traction battery completely dead. This doesn't "kill the very expensive battery".

    Which raises the corollary question, why can't the Prius software be programmed to protect the traction battery from total exhaustion? This feature would disconnect the traction battery by hardware if it reaches a certain predetermined level, The traction battery could be later be switched back into service by the driver (this would have to be non-software of course - perhaps a hardware sensor in the seat that would detect someone sitting in it and would reconnect the traction battery at that time and the driver could drive off. A few settings such as the clock would need to be reset of course. But the car could be stored almost indefinitely.)
     
  18. kithmo

    kithmo Couch Potato

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    You're having a laugh aren't you, a switch under the seat connected to the HV battery ? :eek:
    That's highly unsafe.
    There is one EV manufacturer, I forget which one it is now, that has the 12v battery under the back seat next to the HV battery and they have a "12v reset" switch on the dash that allows the 12v to be rejuvenated from the HV, but I don't know how it's powered.
     
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  19. jerrymildred

    jerrymildred Senior Member

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    No. The traction battery is physically disconnected from the rest of the car by a double pole relay INSIDE the traction battery. Shutting off the car or otherwise removing the 12V control power from this relay opens the contacts. No high voltage anywhere outside the traction battery.

    It wouldn't be more dangerous. It would just be impossible. There is no 12V source in the traction battery. The 12V bus is fed by the 12V battery, which is in turn fed by the inverter under the hood, which is in turn fed by the traction battery with power that passes through the previously mentioned relay. The 12V battery closes the relay. That runs power to the inverter from the traction battery. The DC/DC converter within the inverter mounted on the transaxle then maintains the 12V battery.

    If it self discharged like that, it was not long for this world anyway. A healthy traction battery won't do that.

    It already is disconnected by hardware every time you shut off the car.
     
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