HV battery drops quickly despite good gas mileage

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by fomenter, Jan 22, 2020.

  1. fomenter

    fomenter Junior Member

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    I bought a prius about 1.5 months ago and it has been good so far. I think i am having problems with HV battery though despite a decent gas mileage (48 MPG).

    When I drive the car in town, the battery sometimes drops from 6 bars to 3 bars quickly (in about a minute or two) then it picks up like normal. Again, the car gets 48 mpg and it has no dashlights, so no terrible symptoms there. Also, It does not lose charge overnight even when it sits for 3-4 days.

    The only thing that i need to mention is the 12v battery. When I measured it last time, the voltage was between 11.7 to 12.1 with ON mode. However, i am not sure if the drop in battery is related to the 12v battery.

    Any ideas? The car is a 08 with 125k. Thanks... The car has the original HV battery and I know that it can go bad anytime soon ( lnow to 3 years)
     
  2. davecook89t

    davecook89t Senior Member

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    I don't think this is unusual, if you are making frequent stops at traffic lights. It also depends on what accessories you have running, air conditioning especially draws down the HV battery quickly. Once you are on the highway, though, the HV State of Charge should stabilize between 4 and 6 bars, if everything is working properly.

    If you don't have a load tester, the best way to measure the strength of the battery is to put a multimeter on it and test its voltage after it has been sitting overnight with the hood unlatched*. (You want to be able to measure the voltage without disturbing it by approaching the car with your smart key or by opening any of the doors). If it shows something like 12.6 V under these conditions, it is probably alright. To do a load test, you can see what voltage you get with the car in Accessory mode (2 presses of the Power button without your foot on the brake) when you turn the headlights on. You should expect the voltage to drop somewhere below 12 V, but after leaving the lights on for a minute or 2 and then turning them off, you should see it recover close to its previous value.

    A weak 12V will negatively affect your MPG, but I would not expect to see it change the behavior of the SOC display, since each bar represents HV battery voltage that is probably an order of magnitude (i.e. 10 X) more than the difference between a strong and weak 12V battery.

    *I'm talking about measuring the voltage of the 12V at the under hood jump points, of course.
     
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  3. fotomoto

    fotomoto Senior Member

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    It's not an absolute but that is an early sign of a weak battery.

    Remove it and get it load tested (for free) at any major auto parts store that offers this service.

    GOOD LUCK!
     
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  4. fomenter

    fomenter Junior Member

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    Thanks for the reply. The weather just got colder this is where the weird battery drop started. The car behave fines on highways, freeways, always 5 (5%) to 6 blue (95%) bars

     
    davecook89t likes this.
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