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    DonnyC Junior Member

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    I have a 2005 Prius with 137,000. I bought it new. For the past couple of weeks, I have noticed that in the morning by battery drops from 60% to 20% ( once to 10%) in the first 0.5 miles. It then recovers to 60-70% after 1 mile. Sometimes goes to 90% after another 1 mile, then returns to a normal 50-60% for the rest of the 18 miles. Today when I left work, it went to 100% while I was in the parking garage. Again after a couple of miles it returns to 60%. I checked the 12v battery tonight when I got home since I saw a post that indicated that if this 12V battery is bad it forces the HV battery to divert power to the 12V battery. I have not gotten any error messages yet. Voltage is normal both with ready light on (13.8 volts), and with accessory mode with lights, fan, defrost all on, voltage stay 10.9 volts.

    This is not normal for this car. I drive every day for the last 8 years. Is this the start of the HV battery going to shit? Is there a charging problem with the drive system? Anyone else have any of the same symptoms?


    Donny
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    cwerdna Senior Member

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    Your 12 volt likely needs replacement. That's not very good. If it's like that after being left overnight after being charged in READY mode for 8 hours or when charged by a slow (below 4 amp or so charger), it needs replacement. If you're on your original 12 volt, it needs replacement now. Forget messing around w/charging it.

    As for your other symptoms, yes, that is a symptom of a possibly dying HV battery. I'd replace the 12 volt first before mucking w/the HV battery.

    What state are you in?
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    nh7o Off grid since 1980

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    I agree, those symptoms do look like the HV battery going, not so unusual given the mileage. Have the MPG's changed? Do you notice that the ICE is running more often? If the MPG is still OK, and no codes are set, then there is not much to do but wait.
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    uart Senior Member

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    Hi DonnyC. How are you measuring the percentage state of charge? Is this as read from a scan-gauge, or are you just estimating this from the number of bars displayed on the SOC graphic.
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    DonnyC Junior Member

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    I live in Indiana. I am watching the display for percent charge. This morning I had 90 % most of the way to work. My mileage is consisted at 45-50. Can Toyota load my hv battery and determine the state of decay? What's the best way of determining if I have some bad cells?
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    uart Senior Member

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    Ok, but the display doesn't show percentage. It shows 8 bars which correspond only roughly to the actual percentage charge. So you are definitely going to confuse people if you arbitrary refer to the displayed SOC bars as percentage SOC.

    The first bar (red) corresponds to roughly 40% to 44% SOC and the 8th bar (green) corresponds to roughly 75% to 80% SOC. However the correspondence between displayed battery bars and the actual SOC is very much non-uniform.
    Last edited by uart, May 15, 2013
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    DonnyC Junior Member

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    Fair enough. What I defined as 10% is 1 bar. 20% is 2 bars. 60% is 1 bar above half way or 5 bars. What ever the true percentage, there is a change in storage characteristic. Any estimate of time I have before HV battery gives me errors?
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    uart Senior Member

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    Ok, so at least we now have some idea of what your variation in SOC bars is like. It's pretty common to have variations from 4 to 7 bars during normal driving. If you can mostly keep within this range, at least after the first few minutes when the engine has warmed up, then things mightn't be too bad.

    Also under some conditions people get a very noticeable drop in SOC in the first few minutes of driving. This is because when the Prius is first turned on there can be a larger than normal drain on the HV battery. Air conditioner load can be high, and if your 12V battery is weak then the DC-DC converter will put a significant drain on the HV system trying to rapidly get some lost charge back into it (the 12V battery).

    The biggest factor however is that when the engine is cold the car tries to use mostly electric propulsion so as to limit the demand on the engine until the cats are warmed up. You can greatly reduce the initial drain you see in the first few minutes if you leave the Prius in "P" and let the engine start and idle for about 45 to 60 seconds before you drive (btw, just tap the accelerator to force the engine to start if it doesn't do so by itself). Definitely try this out, it really works!

    Your HV battery may have lost some capacity, but it's not necessarily dead yet. The SOC bar display is very non-linear, so that once it goes below 5 bars it seems to suddenly drop very rapidly, but this is really an artifact of the non-linearity. So your battery mightn't necessarily have degraded as much as it appears. Also if your 12V battery has recently gotten much weaker then this could have made it look worse too.

    Test your 12V battery after it's been sitting overnight (or at least off for several hours), and test it in "ignition-on" mode before you make it ready (so that it doesn't get any charge immediately before the test). Test it with all the fans and demisters etc all off, just the cars systems displays and ECU's will take over 10A from the 12 battery. Your battery should hold over 12.0 volts in this state. Report back with your results.

    BTW. To get to "ignition on" mode, press the power button (without having your foot on the brake) once to get into "accessories" mode, and then a second time (again without foot on brake) to get to "ignition on" mode. Report the voltages in both "acc" mode and "ig on" mode.
    Last edited by uart, May 15, 2013
    jlimgarfield likes this.
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    DonnyC Junior Member

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    Hv battery died last Thursday. Putting rebuilt one in this week. I am driving with check engine lights on for 100 miles to have battery replaced by autobeyours com in Scottsburg, IN. Anyone have any bad experiences with them. $895 pluqs $200 to have it installed. 6 month warranty and 6-12 month prorate warranty.
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    boppo Active Member

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    From what I have read on this site they are very honest and very good in what they do.
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    uart Senior Member

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    Thanks for the update Danny. Let us know how the rebuilt one turns out.

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