12v battery maintenance when Prius is not driven for several days

Discussion in 'Gen 4 Prius Care, Maintenance and Troubleshooting' started by n4in, Nov 15, 2018.

  1. n4in

    n4in Junior Member

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    I was at my dealer for the recent gen 4 recall on HV wiring harness, spent a few minutes talk with one of the service coordinators who is a Prius owner/driver for several generations and years. He recommended that if I am not driving my Prius for more than two days consider putting a trickle charger on the 12v battery to maintain its charge as that battery's charge can affect the longevity of the HV battery. Is this true? I have a Tacoma I drive infrequently now that I have the Prius but with winter weather storms I will be driving the Tacoma more in poor weather conditions. Thanks!
     
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  2. alanclarkeau

    alanclarkeau Senior Member

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    There was a 12v battery thread recently - 12v battery voltage? | PriusChat

    Might give you some idea of other's thoughts.

    There's also Weber Automotive's informative YouTube -
    worth listening to.
     
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  3. The Professor

    The Professor Senior Member

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    I doubt it.

    There's no way for the 12V battery to charge the HV, so nothing happens from that perspective, but the HV does charge the 12V when the car is in Ready mode. While this will discharge the HV battery a bit more quickly (once), which you may think will reduce its life (rechargeable batteries have a limited number of charge/discharge cycles), in reality the ICE kicks in when the HV gets down to around 2 bars and then the MG takes the electrical strain. If the 12V didn't need charging, that same HV energy would be used to power the motor a few seconds longer. So really, all that happens is you use a bit more fuel a little bit earlier, and the HV gets the same use.

    Low 12V voltage can cause all sorts of issues indirectly though. Error codes are a common one.
     
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  4. bat4255

    bat4255 2017 Prius v #2 and 2008 Gen II #2

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    Our '08 with it's original OEM battery went 3 weeks in summer when we were on vac.

    NO problems.
     
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  5. bisco

    bisco cookie crumbler

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    a battery maintainer is a good idea if you only drive a couple days a week. for prolonged periods of downtime, turn off sks on your fob.
    reducing hv battery life is nonsense
     
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  6. n4in

    n4in Junior Member

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    The conversation I had was focused on the 12v battery being used to maintain some ECU and other functions in the Prius while the car sits for several days, this was his warning that the drop in voltage from the 12v battery would cause extra drain from the HV battery bank if the car sits for too long. This is a Toyota wide issue for hybrids that sit for too long, he said the hybrids that sit for long periods on the lot offer require a charge if not driven or started for over 10 days. As the hybrid HV battery gets older and the cycle count goes up the battery can become more fragile leading to dead cells, leading to HV battery replacement prematurely. This possibly occurring way down the road but he described a trickle charge could improve battery stability/longevity. I have a battery tender trickle charger that I use on my tractor I will likely use for periods over 3 days.
     
  7. bisco

    bisco cookie crumbler

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    he's half right, and half clueless. that's more dangerous than completely wrong

    never a bad idea to use a maintainer. make sure the one from your tractor is appropriate for prius battery, it is special. and fragile
     
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  8. n4in

    n4in Junior Member

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    Oh yeah, thanks for the feedback! I enjoy the forum very much, learning a lot here.
     
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  9. n4in

    n4in Junior Member

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    The Weber Automotive video is very good, it does not address my concern. I will put a current clamp on the battery with vechicle off and see if any current drain is present.
     
  10. bisco

    bisco cookie crumbler

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    of course there is drain, electronics are on all the time. you need the spec to make sure the drain isn't above that
     
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  11. bisco

    bisco cookie crumbler

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    same here. 6-8 weeks every winter for the last 6 years
     
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  12. n4in

    n4in Junior Member

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    Wow OK that answers that! Thanks everyone.
     
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  13. sam spade 2

    sam spade 2 Senior Member

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    Absolute total BS.
    The HV battery does NOTHING while the car is OFF; thus it doesn't care what condition the 12 V is in.

    A cheap trickle charger is a BAD idea. A good automatic battery maintainer is a good idea but is only necessary if the car sits for a LONG time, like two weeks or more. If the 12 V won't hold up for 2 weeks, it is going bad already.

    That guy is a total moron. Most of what he says is just made up.
    I hope the whole shop isn't like that.
     
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  14. VFerdman

    VFerdman Senior Member

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    12V and hybrid batteries are completely separate. When the car is off, the hybrid battery is disconnected electrically by a set of heavy duty contacters. It basically does nothing when the car is off and is separted from any circuitry by air gaps (contacters). It is therefore impossible for the 12V battery to effect the hybrid battery when the car is off. To suggest otherwise is utter nonsense. There is no high voltage from the hybrid battery anywhere in the car when it's off. Period. You can probably help your 12V battery a bit by using an appropriate charger/maintainer, but it will do absolutely nothing for your hybrid battery.
     
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  15. n4in

    n4in Junior Member

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    I think that is a little harsh, he is a service coordinator not one of the technicians, so there's that. Also he is an owner of several gen2 and gen3 models with his experience. I agree with the advice here but he was pointing out his experience with a higher 12v discharge rate in hybrids, I know there is some degree of higher discharge in these cars than a conventional ICE like my Tacoma but just wasn't sure how high that discharge would be.
     
  16. n4in

    n4in Junior Member

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    Thanks!
     
  17. sam spade 2

    sam spade 2 Senior Member

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    There is no reason to think that the electronics that needs to be "kept alive" while the car is OFF in a Prius is much, or any, different than a conventional car. The electronics needed to RUN it might be more but given all of the electronic controls in "conventional" cars now even that is suspect.

    The main reason that the Prius appears to discharge the battery quicker in storage is: The battery is much smaller than a vehicle with a conventional 12 V starter.

    EVERYTHING he said are conclusions arrived at by not understanding anything about the evidence that he has seen. He is trying to impress you with his knowledge.......and is doing exactly to opposite.
     
  18. jerrymildred

    jerrymildred Senior Member

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    It was rather harsh, but it was also correct. This service manager doesn't know what he's talking about. As you've already learned, the car can sit for weeks and weeks without being driven and have no problems unless that 12V is already approaching the end of its service life. We've left my wife's '07 for weeks at a time while on trips in my car. Come back and it starts right up.

    And the HV battery is so much bigger than the 12V that the extra load of charging a low 12V is nothing. Compare that to the traction battery starting the engine by spinning it up to 1,000 rpm until it makes oil pressure before the ignition and fuel come on. And it does that every time you stop or even go really slowly. Drop in the bucket.

    Once again, the Prius mantra applies. "Just drive it."
     
  19. CooCooCaChoo

    CooCooCaChoo Active Member

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    When my Gen 2 was in an accident, it sat in the summer sun under 110 degree heat daily for 2 weeks or so. Started up no problem. But then again, I had an Optima in it.
     
  20. Mendel Leisk

    Mendel Leisk Senior Member

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    @n4in We are low usage, ours (3rd gen) sits idle often for 2~4 days. Whenever the car will not be used a particular day, I hook up a CTEK 4.3, let it run through it's charging cycle, and leave it connected, until the next time the car is used. Might be overkill, but it's working for me.

    I've measure battery drain on ours: it's roughly 18 miliamperes constantly, with spikes to around 40 every 5~10 seconds.
     
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