Winter is coming -- 12V question

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by cyberpriusII, Sep 19, 2020.

  1. Salamander_King

    Salamander_King Senior Member

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    FWIW, I monitor my Prime's 12v battery using a BlueTooth battery monitor. There is no need to pop up the hood, and it records the battery voltage 24/7. Yes, it may use a little bit of current to send Bluetooth signals, but checking once a day has negligible draw compared to opening the driver side door.

    Here is the one such data for over 24 hours. The car has sat parked for three days on my driveway, and the resting voltage was dawn to 12.45v in the morning. Then I drove my car for very short ~10 miles errands. This replenished the battery voltage to 12.68v. I had to open the driver's side door to retrieve something inside a few hours later. It was very quick open the door and grab a paper from the console and close the door. This caused the dip in voltage close to 0.2v.

    upload_2020-9-24_10-47-11.png
     
    #41 Salamander_King, Sep 24, 2020
    Last edited: Sep 24, 2020
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  2. ChapmanF

    ChapmanF Senior Member

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    What I would see as the change in SoC there would be the difference between the trace just before opening the door (by eyeball, maybe 12.6-ish? Down some from the labeled 12.68 earlier) and the just slightly lower value it returns to after the dip shown while the pump ran. It looks like a visible but small change, maybe not quite 0.1 V? Again by eyeball.

    The downward feature in the graph while the pump runs isn't a difference in SoC, just the voltage drop across the battery's internal resistance while the pump current is flowing through it.
     
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  3. Salamander_King

    Salamander_King Senior Member

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    Correct. I edited my comment. But the point is, if someone opens the driver's side door to pop open the hood to take a resting voltage of the 12v battery, it could be lower than what it was before opening the door. That's all.

    BTW, the voltage change was 12.66v max to 12.48v min for the short blip. So, if someone takes the battery reading at the lowest point, it could be 0.18v lower than it was without opening the door.

    upload_2020-9-24_12-29-39.png
     
    #43 Salamander_King, Sep 24, 2020
    Last edited: Sep 24, 2020
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  4. edthefox5

    edthefox5 Senior Member

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    Easiest test is with car off measure voltage at the front jump point. Then with car still off turn on headlights on bright for 5 minutes.
    Turn headlights off after 5 minutes. Wait a minute and then take voltage reading at front jump point. My 3 year old yellowtop does not drop one volt with this benign test.

    If yours does its getting tired. Its level of tiredness is how much voltage it lost after the test. A very tired battery will quickly reveal itself.
     
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  5. edthefox5

    edthefox5 Senior Member

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    Easiest test is with car off measure voltage at the front jump point. Then with car still off turn on headlights on bright for 5 minutes.
    Turn headlights off after 5 minutes. Wait a minute and then take voltage reading at front jump point. My 3 year old yellowtop does not drop one volt with this benign test.

    If yours does its getting tired. Its level of tiredness is how much voltage it lost after the test. A very tired battery will quickly reveal itself.
     
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  6. edthefox5

    edthefox5 Senior Member

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    this site is acting very weird today...
     
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  7. Tim Jones

    Tim Jones Active Member

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    Probably the occupants..............................................
     
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  8. Salamander_King

    Salamander_King Senior Member

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    Do you really mean 1.0v? That means, if the battery is "tired" and it was 12.6v initially, then after the test it reads 11.6v or lower. That is a huge drop indeed. I would not expect that much drop from a healthy battery, but does "tired" battery really drop that much?
     
    #48 Salamander_King, Sep 24, 2020
    Last edited: Sep 24, 2020
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  9. Mendel Leisk

    Mendel Leisk Senior Member

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    Couple of thoughts:

    1. There's a diff between opening driver and passenger door.

    2. Voltage measure alone is a poor test of battery viability. A tester that electronically measures the cranking amps gives a better overall assessment.
     
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  10. valde3

    valde3 Senior Member

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    Even better if you also measure the voltage during the load test then you can calculate international resistance of the battery and approximate cranking amps (cold cranking amps if test is done in correct temperature).
     
  11. Mendel Leisk

    Mendel Leisk Senior Member

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    Yeah I think the electronic load testers consider voltage as well as CCA. They do display voltage at outset and end, and show CCA, and give a "verdict".
     
  12. valde3

    valde3 Senior Member

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    They just calculate cranking amp based on voltage drop and current.
     
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  13. edthefox5

    edthefox5 Senior Member

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    Very Good point. Should not move more than 1/2 a volt or .5.

    Should not move at all really. My 3 year old battery did not move at all. Took a minute or 2 for it to get back up to where it was though. It still lives at 12.6 volts.
    When my previous yellow top started to fail it failed quite quickly. Out of the blue. I forget what gave me a hint but i checked it later in the day and it tanked on this test.
    It was at 2 years 11 months and 22 days on its 3 year warranty.
    Autozone gave me a brand new one free. I'm on that one.

    Seen a few that move more than a volt on this site. It will reveal a soft battery quite quickly. From what i see on this site these little AGM's are not tough at all one good 100% dead on a car light bulb and there done especialy if sitting dead for a few days 100% discharged.

    I bought my 07 brand new and its original battery lasted 6 years and 6 months before it started to scared me never was able to make it full on the charger then and i saw the red triangle flash once. During that time Prius Chat was seeing non stop horror story's about 12 volt battery's so I replaced it.
     
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  14. bisco

    bisco cookie crumbler

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    this should be a sticky across all generations
     
  15. fuzzy1

    fuzzy1 Senior Member

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    If you another digit or two on your DMM, you would see it move.
     
  16. Tim Jones

    Tim Jones Active Member

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  17. cyberpriusII

    cyberpriusII Prodigyplace says I'm Super Kris

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    Thanks to all of you for the advice and kind words about dreaming....I think.

    Nice to know the origin of the "don't open the car door before checking voltage" advice. Hope to check the voltage on Saturday and Sunday. And, while it may not be the be=all, end=all, it is the easy way for me at this moment.

    Seriously, I have been feeling as if I am running in wet cement (is there any other kind) lately. And, I suppose voltage alone does not a decision make, but it is what is easiest for me, so be it.

    We had a fairly major storm roll through in the past couple of days, which was good for dampening down the fires, but not so good for our driveway. All the ditches were still clogged with debris from summer, so the rain decided to make it's own course. And, there are the six "smallish" trees that decided to take a decided lean into the roadway. So, it is going to be ditch digging and tree dropping this weekend.

    This will be the first weekend since I was injured in a car accident that I will attempt to start the chainsaw. That thing has compression like...well...we will see if I can start it....no one has since the accident.

    And, my credit card starts a new cycle on Oct. 13, so, I think the 17th sounds like a great weekend for a new battery!

    Thanks again,
    kris
     
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  18. edthefox5

    edthefox5 Senior Member

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    Listen nothing matters really except don't hurt yourself. You have put on 25 years of life with that car accident.
    Take a really hot shower and do some extensive stretching before you attempt lifting anything heavy. You went to physical therapy right?
    Do those exercises.

    And keep dreaming........
     
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