If in READY, is 12v being charged?

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Nickod, Feb 26, 2021.

  1. Nickod

    Nickod Junior Member

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    I have a 9yr-old battery in my 2013 Prius. Worked fine yesterday. Tried to start this a.m., nothing. No lights. Tester showed no voltage across battery. Put a charger on it, and it seems to begin taking a charge (dial on charger drops from max to 6). I detach charger and hit POWER button. Car starts. I leave it run a few minutes while I have a cuppa. I go back to look. Car is dead again, no lights, etc.
    Shouldn't the READY position make the running car recharge the battery?
     
  2. rjparker

    rjparker Senior Member

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    A discharged 9 year old 12v battery may never hold enough charge to restart the car after a short period. Just change it. Normal cars are lucky to get four years on a 12v battery.
     
  3. 2k1Toaster

    2k1Toaster Brand New Prius Batteries

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    Yes if READY the car is charging the 12v. But the exception is if there is too high of a draw, the inverter shuts that power rail down. A 9V shorted battery is probably overcurrenting and shutting down the inverter rail. You need a new 12v battery.
     
  4. ChapmanF

    ChapmanF Senior Member

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    You can easily see if that's happening with a simple voltmeter (even the one in the MFD, if your trim has one). If it's in the high thirteens to fourteens, it is charging the battery. But even if the battery is serviceable and can be charged, "a few minutes" is nowhere near enough to charge it. It's about a 45 amp hour battery. If your charger dial drops quickly from max to 6, you'll need it at 6 for close to eight hours plus an allowance for conversion loss; twelve is more like it.

    The car in READY can charge it faster than that, but still not recharged-in-minutes fast. (It's better than a Gen 2; they would charge not much faster than your charger does, so it would have to be READY for a dozen hours or more.)
     
  5. Emmanuel Isaac

    Emmanuel Isaac Junior Member

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    I just replaced the battery on my 2012 and was surprised it still had the original battery. It was still outputting 12.4v cold and rested at 12.65v after hooking up to a charger. However I was seeing unusually high voltage of 14.97v at Ready and mileage was starting to take a hit.

    I replaced it last week with a new one from NAPA as it had similar warranty to the TrueStart from the dealer (18/75 months vs 24/84 months from Toyota) since the dealers around me charge $240+labor and insisted it isn't covered under warranty unless they install it.

    The new battery rests at 12.9v after charging (the Bosch in my Gen 2 used to hold a charge up to 13.2v), but the voltage still hovers around 14.7 at Ready. My old Gen 2 used to hover at 13.8v when Ready and only went over 14v when it needed to charge the aux battery. Am beginning to see an increase in MPG but am not sure if its just since I made longer trips this weekend.

    Questions:
    1. Is the Bosch battery better since it is capable of a resting voltage of 13.2v fully charged? (I regret not swapping it out from my Gen 2 before selling it as it was replaced only about 2 years ago and it was still under warranty)
    2. Does anyone have experience with the NAPA battery? (I believe it is the same as the Deka, Duracell and ones from AutoZone, Advance Auto and O'Reilly Autoparts)
    3. What is the Ready voltage on the Gen 3 when it is maintaining the aux battery at full charge? I'm afraid the new battery might not be as good and the DC-DC system is constantly trying to charge it.

    I'd appreciate your input @Patrick Wong @bisco @ChapmanF
     
  6. burebista

    burebista Member

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    In Ready mode my old Yuasa and my new Banner shows the same 14.4V (min. 14.3 max. 14.5).
     
  7. ChapmanF

    ChapmanF Senior Member

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    My Gen 3 DC/DC converter output is usually around 14.7 when I have first started the car. It may later back off to 14.4 or 14.5.

    If I have made a sequence of short trips, I sometimes find in the later trips that it will back off as far as 13.6 or so when the transmission is in D, and return back to 14.4 or so whenever shifted back into Park. I can make it repeat that by shifting between D and Park again.
     
  8. bisco

    bisco cookie crumbler

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    i am unqualified compared to the other two you tagged, but here's my best shot:

    1) no

    2) no

    3) i think you are overthinking things. keep track of you mpg long term by the tank full, for any meaningful analysis.
     
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  9. royrose

    royrose Senior Member

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    I've had the replacement 12v "Super Start" battery from O'Reilly for 7 years and it has continued to perform well. A couple of years ago, I had it tested at a AAA event and they said it had plenty of life left.
     
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  10. rjparker

    rjparker Senior Member

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    An agm is perfectly fine when charged at 14.97v.
     
  11. Emmanuel Isaac

    Emmanuel Isaac Junior Member

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    Thank you for that! I believe that is what is happening here. While in park it goes up to to 14.8v but once I shift it to drive it drops down to 14v and even 13.6v in a few minutes. That makes me feel much better especially now that my gas mileage is back up! Thanks again guys for your input!
     
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