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How to make Prius "forget" the battery's SOC?

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Troy Heagy, Jun 2, 2014.

  1. Troy Heagy

    Troy Heagy Member

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    On the Honda hybrids it was as simple as disconnecting the 12 volt battery (which resets the state-of-charge meter to empty). Does the same technique work on my Prius?
     
  2. drysider

    drysider Active Member

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    I will hate myself for asking, but why do you want to do this?
     
  3. David Beale

    David Beale Senior Member

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    It may work on the Prius but it quickly recovers and recalculates the current state when you hook it back up and put the car in "Ready".
    No, it's not as accurate as it was, but no, it's never all that accurate. It's very difficult to -know- the state of charge of any battery.
     
  4. BZzap!

    BZzap! Senior Member

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    The only true way to accurately monitor the SOC is with a ScanGauge. The car monitor is some what of a novelty and resetting it will not let you improve it's accuracy.
     
  5. Troy Heagy

    Troy Heagy Member

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    In the Honda hybrids, if you let them sit for a month or more, the meter will say (for example) that the battery is almost full because that was its last known state. In reality the battery has self-discharged to about half (or lower). So the meter is wrong. (If the car sits long enough, the battery will eventually become unbalanced..... the cells carry different voltages.) This is common for all NiMH batteries since internal resistance slowly drains the stored energy.

    So when I take my Honda out of its 1-2 month storage period, I disconnect the 12V battery to reset the HV battery meter to empty. That initiates a charging mode to condition the battery.

    And I was curious if the same technique worked for the Prius after it has been taken out of storage.

    BTW I'm an electrical engineer. :)
     
  6. Mendel Leisk

    Mendel Leisk Senior Member

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    If you just drive your Civic around a bit, it will realize it's perception of state-of-charge is off, and initiate a recalibration, without your intervention.

    A month is pushing the limit for IMA battery health. We had ours in the shop for a month, and I suspect that was instrumental in the battery's demise:

    civic hybrid storage warning.JPG
     
    #6 Mendel Leisk, Jun 3, 2014
    Last edited by a moderator: Jul 21, 2014
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  7. David Beale

    David Beale Senior Member

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    Unlike the Honda, the Prius battery is fine for several months. It does slowly loose charge but it's not necessary to mess with it. Pearl S sits in the garage for 6 months of winter with no ill effects. I DO keep a Battery Tender on the 12V battery if it's going to sit for more than three weeks. It can have problems if left to its' own devices. ;)
     
  8. Troy Heagy

    Troy Heagy Member

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    Well the same is true of the Prius (it is also a NiMH battery that self-discharges while sitting). I notice your Honda manual is different from mine from 2003, as the older manual says "Drive the car every two months," and nothing about permanent damage.
     
  9. Former Member 68813

    Former Member 68813 Senior Member

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    Yes, but there are different flavors of NiMH battery including low self-discharge ones. Honda must have used really cheap ones or maybe even defective ones.
     
  10. Troy Heagy

    Troy Heagy Member

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    My 2002 Honda Civic used the same cylindrical cells the 1st-generation Prius used (1997-2003). ANYWAY: Rather than start the Prius and have the car immediately start draining an empty battery (and risk driving weaker cells into negative voltage), I think it is better to disconnect the 12V battery for a few seconds, reset the battery meter to zero, and put the stored Prius into a charge mode.
     
  11. Former Member 68813

    Former Member 68813 Senior Member

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    If so, why prius doesn't brick after 1 month or even 6 months of storage like some people do here. The only instances people reported completely discharged HV battery (in Prius) was when they kept driving with no fuel ($$$$ mistake).
     
  12. drysider

    drysider Active Member

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    When the Prius starts, the engine comes on and starts charging the battery. There is little or no drain during start-up.
     
  13. Troy Heagy

    Troy Heagy Member

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    But we do know Prius batteries eventually wear out, because we've seen people posting about it. What happened? Did the pack become unbalanced from sitting in a garage too long?

    I've never seen any posts where people investigated the battery to see "why did it light the warning light". People did that with the Hondas and discovered there's nothing actually wrong with the battery... some of the cells just became imbalanced from the other cells. Perhaps Priuses with bad batteries have the exact same problem?
     
  14. Former Member 68813

    Former Member 68813 Senior Member

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    The fact you never saw doesn't prove it doesn't exist. This was well studied and people use that (common) knowledge to rebuild HV batteries: http://researchcommons.waikato.ac.nz/bitstream/handle/10289/5714/PriusBatteryRebuildability-LeijenScott2011.pdf?sequence=1

    Of course it takes a lot of driving and time for that to happen (more than 10 years on average).
     
  15. Troy Heagy

    Troy Heagy Member

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    Thanks. :) It appears the Prius pack does have the same flaw as the Honda pack: Some cells (or blades) become weaker then the rest of the cells (or blades). The battery becomes unbalanced. Of course the paper never addresses WHY the battery became unbalanced. For Hondas it is lack of driving leading to self-discharge & imbalance while sitting. For Prius, who knows?

    "Many battery pack failures are caused by a single battery blade or block failing. Figure 8 shows that the capacities of the blades within the pack either decrease in a linear and very consistent manner, exemplified by packs I79637 and hyde2, or the battery blade capacities decrease unpredictably as observed in the mudgeway and ABY battery packs."
     
  16. Former Member 68813

    Former Member 68813 Senior Member

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    Of course they did address it. :rolleyes:
    They said the center cells were weaker due to higher temps.
     
  17. Troy Heagy

    Troy Heagy Member

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    No actually it says the cells in the center MAY be weaker due to temperature differences. That's a good hypothesis but I think "imbalance between cells" is a better explanation given it's a common phenomenon. It's also a reinforcing one that gets more worse over time, because weak cells get driven to lower & higher voltages than healthy cells.
     
  18. vvillovv

    vvillovv Senior Member

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    If I understand my battery hygiene 101 correctly, one leads to the other.
    ie: higher temps of the cell modules in the middle of the pack lead to imbalance of the cell modules in the pack.

    This is not the only reason cell modules become unbalanced, but it is a real good example of one of the reasons.

    And according to a trusted authority on hybrid batteries, trusted by me, the prius NiMH modules far out preform the honda NiMH sticks
     
    #18 vvillovv, Jul 21, 2014
    Last edited: Jul 21, 2014
  19. Troy Heagy

    Troy Heagy Member

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    It depends which generation of Prius..... the first generation used the same cells as Honda used (and are just as prone to imbalance).
     
  20. vvillovv

    vvillovv Senior Member

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    I drove one of those gen 1 prius for a weekend test in Oct. 200(1)? / pics archived somewhere.
    I'm sure glad toyota followed a different path then honda followed for pack cells.
    Don't get me wrong, I'm glad to have the hch and a few backup packs, as opposed to a non hybrid
    but that is about as far as I'd recommend it to anyone other than a normal non hybrid driver,
    as that is how they drive for the most part. In my biased view anyways.
    The pack issues are typically personal and have several different scenarios that play out,
    from the reports I've read. There are a lot of complaints about how honda dealt with them in the past.
    ie: changing the pack warranty at least twice as they worked on a viable system update.
    Besides honda is into space these days, not sure what that says about the cars.
    I do still like their motor scooters. I could put a set of wings on their 375 lbs 900RR model.
    And the old single swing shaft drive 800 interceptor was a marvel of engineering skill.
    It had extra valves that engaged at 4k rpm I'm told, which I think are probably close to the same thing as vtec in the cars.
    Lots of stories of riders accelerating into a corner as the extra valves opened up. :)
     
    #20 vvillovv, Jul 24, 2014
    Last edited: Jul 25, 2014