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Any previous issues with 12v battery failing, being replaced, followed by quick HV batt failure?

Discussion in 'Gen 2 Prius Technical Discussion' started by landspeed, Jun 7, 2019.

  1. landspeed

    landspeed Active Member

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    Hi!

    I was just wondering if this has every happened to anyone? I noticed some strange behaviour (as I monitor quite a few parameters on my car); my 12v battery is near to death, and it seems to mess with the battery ECU in some unusual ways. I was wondering if anyone has had 12v battery 'die', get replaced, then the HV battery died very soon after? Some will have simply due to statistics, but I'm wondering if there is any 'pattern' at all...
     
    #1 landspeed, Jun 7, 2019
  2. bisco

    bisco cookie crumbler

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    not that i recall reading
     
    #2 bisco, Jun 7, 2019
  3. TMR-JWAP

    TMR-JWAP Senior Member

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    No, but we do see a lot of HV batteries failing, which leads to people buying new 12v batteries in a last ditch effort, grasping at straws while hoping and praying a new 12v will clear up the HV battery faults....................
     
    #3 TMR-JWAP, Jun 7, 2019
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  4. sam spade 2

    sam spade 2 Senior Member

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    NO.
    Well yes, sort of.
    The pattern is: As a vehicle gets older, things will fail at an increasing rate........with those things most prone to wearing out going first.
    The batteries are right at the top of the list.
     
    #4 sam spade 2, Jun 8, 2019
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  5. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Another take: the car's computers are all powered by the 12v system. If the 12v battery is low, the computers may not be correctly evaluating performance.

    Then you renew the 12v battery one day. Computer boots up sane for the first time in weeks and announces "Hey buddy, your hybrid battery is toast. I would have told you 3 weeks ago but there were too many 12v power fluctuations to be sure."

    Of course, it can't actually communicate that last sentiment, all you ever get is the big red triangle.

    Just because the battery in your smoke detector is dead doesn't mean the house isn't on fire.
     
    #5 Leadfoot J. McCoalroller, Jun 8, 2019
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  6. landspeed

    landspeed Active Member

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    The reason I asked, is that my 12v battery in my 2008 Gen 2 is actually the original still, 276000km, and has been drained down about twice. It drops down to 10.x volts quickly and stays there, and can easily go to high 8s, low 9 volt range. I can only boot up my car if I have the fans and all lights off - otherwise it doesn't boot up the computers.

    It has, in the last few days, started getting a bit worse. Specifically, the MFD sometimes reboots (boots up for maybe 1 second, then reboots) during initial power-up (no signs of failing MFD, but it just starts up due to the CPU brownout protection in all the Prius computers).

    What I have noticed is that the battery ECU has also been 'rebooting' during these times (the odometer trip meters haven't been reset though). When the battery ECU reboots, it resets the SoC to 60% (well, 59.98%).

    The second time this happened was after arriving at work, having driven down a long hill. I had a SoC of 81%, and sat for a few mins to stabilise temperature etc, before power-down. SoC was about 79%. When I restarted it rebooted to 59.98% and then 'charged' to 78%, then put the HV battery fan on '4', and kept powering up the ICE with no fuel to discharge the battery, even at around 75% SoC. I used EV mode to discharge down, but it was still 'stuck', not recognising the fact the battery was trying to overcharge, and it wouldn't allow EV mode despite 70% SoC, normal temperature, warm engine etc. I fixed the issue by draining the battery down to about 45% SoC, then unplugging the 12v battery, so it 'reset' to 60%, and after this it has worked fine (with some initial 'adjustments' to the reported SoC occurring - maybe 5% correction at the limit of the range).

    The first time this happened was after I had arrived at home on a damp, rainy day, so had been going up a very steep hill with air-con and lights. The SoC was about 45% on power-down. On power-up the next morning, it was 59.98%. I recognised what had happened, but the car drained the battery down so that the cell capacities were into the 13v range, even at low load. In the end I coasted for a good while, until the engine warmed up enough to charge, then did my down-hill commute. I charged it more 'gently' and used friction brakes, and when I got to about 68% SoC, it 'zoomed' up to 80% SoC even though I put it in neutral - so it corrected the issue.

    In summary, due to 'resets' of the battery ECU, my car tried to severely discharge my battery the first time, and the second time, tried to severely overcharge it. My battery is worn enough as it is. The resets are due to a terribly worn 12v battery. I could have killed my battery (definitely on the first time by over-discharging it) if I hadn't had live data running at the time. If I didn't have live monitoring, the two events above could have been enough to tip my battery over the 'edge' if I hadn't intervened each time. In the short term, I am planning my drivers and have a 58-61% SoC whenever I power down! But I am finally ordering that new 12v tomorrow...
     
    #6 landspeed, Jun 8, 2019
  7. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    If a power-starved ECU makes incorrect evaluations of the hybrid battery, it certainly follows that it would make inappropriate adaptations and corrective steps- but I don't think I've seen that documented as well as what you just posted. (y)
     
    #7 Leadfoot J. McCoalroller, Jun 8, 2019
  8. sam spade 2

    sam spade 2 Senior Member

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    Well you asked the wrong question.

    You should have said: If I KNOW that my 12 V battery is unquestionably past it's useful life but I continue to try and use it anyway........could that cause the HV battery some additional strain so that IT also fails sooner ??

    The answer would be: Yes, probably.

    My question is: Why on earth did you DO that ??
    Most people would NOT do that and would get a new 12 V battery at the first sign that the old one was going bad.
     
    #8 sam spade 2, Jun 8, 2019
  9. TMR-JWAP

    TMR-JWAP Senior Member

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    What he describes happening is exactly what happens when you disconnect the 12v battery. This area of repair is full of hard learned lessons. Losing 12v power causes many items in the ECU to reset to default values. This includes HV SOC. It also includes throttle body and other "learned" items that continuously adjust when the engine is running. Disconnecting the 12v can result in the engine not starting if the default value is too far from the previously learned value.

    This trick can also be used to get one or two last ditch attempts start the engine. If the battery has been run to it's lower limit, but you're confident you've solved an "engine not starting" problem, you can disconnect the 12v battery to make the SOC reset to 60%. You'll have probably only one shot at it starting, but you may get lucky.

    What would worry me about the low voltage condition is that it would seem to endanger the integrity of the electronics. Multiple instances of partial boots, low voltage supply, etc etc can't be good for the circuits.
     
    #9 TMR-JWAP, Jun 8, 2019
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  10. Coolride17

    Coolride17 Junior Member

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    I found that leaving the dome light switch in auto mode, and you have a bad dome bulb somewhere , the system will draw 1.5 amps worrying about it.lol This causes your 12v to be dead in about 3days.
     
    #10 Coolride17, Jun 9, 2019
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  11. Skibob

    Skibob Senior Member

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    Is this true? I have never heard of this before. It could explain why some people’s batteries go dead for no reason around here.
     
    #11 Skibob, Jun 10, 2019
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