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12v question

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by widrace, Oct 10, 2019.

  1. widrace

    widrace Junior Member

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    I have a 2012 with 190,000 miles and the original 12v battery in it. On Sunday we went to church and everything was fine but when we came out it was dead like a rock. No lights no nothing. We jumped it and started right up. We have been using it since then with no problems until last evening we came home and parked the car but when I hit the power button to turn it off it died instantly. The dash lights didnt stay on and the side door lights barely lit. A yellow triangle flashed for a few seconds. This morning it was dead so I had to unlock the door with the key. I get in put my foot on the brake pedal and it is firm and the power button is orange. I push the power button and you could hear the Pruis "sound" for a minute or so but nothing happens other than a dim car/lock symbol on the dash. So I get out and place my multimeter on the jump spot in the fuse panel and then ground to a bolt. The reading is 12.16 volts. I go to the trunk and check the battery and it reads 12.16 also. All connections look like new and nothing is loose including the ground cable. Funny thing was after I checked the jump point and while walking to the rear of the car I could hear the car making noise. So after I check the battery in the rear I get into the car and all seems well. I push the pedal it goes down and the light turns green and it starts up. So it seems like after I touched the Jump point it did something to make the car be able to start??? Anyway I leave the car run until the engine kicks on and then back off again and then I turn it off. I check the voltage and now the jump point is 12.6 and the battery is 12.6 also. So it seems to work again...for now. My question is why would just checking the jump point with a multimeter trigger something to make the car start? And I assume I need a new 12v battery?
     
  2. Pluggo

    Pluggo Senior Member

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    Exactly. Lead-acid batteries are not supposed to last 8 years in a car.
     
    Mendel Leisk likes this.
  3. bisco

    bisco cookie crumbler

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    the battery is probably on the edge, and enough finally landed to start it. you're only booting computers and closing relays, but a little below necessary, and the cpu can get confused.

    that being said, check the cables for loos connections at the terminals, and neg to ground.

    fwiw, the same thing happened to mine a couple months ago, but when aaa got there, it only measured 4.5 volts. a jump got me started, and i bought a new battery at the dealer. $200.
     
  4. dig4dirt

    dig4dirt MoonGlow

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    If you have a voltmeter, try popping the hood and then let the car rest without disturbing for 12 hours or so.
    Then go back and dont open doors or anything, just test the batt at the jump points.

    More than likely from sitting the "resting" voltage will be low.

    Im sure time to replace.
    A light left on or a door ajar could have been the culprit in the past, but still it is time to change it.
     
    jerrymildred likes this.
  5. jerrymildred

    jerrymildred Senior Member

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    Get it load tested. At that age, it's probably not taking or holding a charge very well and will soon leave you stranded. When you tried starting, then tested, then it started, it was likely just a voltage bounce from briefly putting a load on it. Old trick with a weak battery in the north with regular car in cold weather was when it cranked too slow to start, let it rest maybe a minute and try again. When you pull down the voltage on a battery, the voltage will tend to bounce back up when you remove the load.
     
  6. Mendel Leisk

    Mendel Leisk Senior Member

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    Yeah, at that age I would just go straight to replacement.
     
  7. 2012 Prius v wagon 3

    2012 Prius v wagon 3 Active Member

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    On an old worn out battery, it may still charge to 12.x volts, but the problem is that there is not much charge behind that, so it would very quickly become depleted.

    A 7+ year old battery wearing out would not be a surprise. But on these cars, the requirements of the 12V battery are very different from a regular car, where the main thing is to start the engine. Here, there is no such need for supplying a lot of current (which a "load test" would check for). You're more interested in capacity, measured in Ah, or Amp-hours.

    In your case, I would double check the battery post-clamp connections - clean and tight. Then I would connect your voltmeter right to the posts / clamps and without turning car power ON (which would engage the traction battery, leading to the 12V battery being charged), do some things that would draw power, such as turning on lights (brake, parking, and interior lights should be live at all times) and see how the measured voltage responds. If it holds steady that's a good sign for the battery. If it drops quickly, that's not. Even better would be to apply an external load to the battery, such as an old headlight.

    Another thing you may observe if running a test like that - as I did right before replacing a battery on a different car of mine - is if the measured voltage jumps up or down in ~2V increments. That indicates an intermittently shorted cell is shorting or un-shorting as the current drain is applied.

    The test is easier on a normal car because the only way the battery charges is if the engine is running and the alternator is doing the charging (vs. with the Prius where it is charged with a DC-DC converter off the traction battery, even if the engine is stopped). So you can just turn the car on, turn on any accessories, headlights, etc. you want, measuring voltage at the battery, and if you have a clamp meter, current at the battery as well.

    In that case, for me recently, I had a battery that usually seemed good, but sometimes bad. That test, seeing the +/- 2V jump coming and going convinced me. Yes, I could have just replaced it without testing, but I felt a lot better doing it having observed the failure.
     
    Robert Holt likes this.
  8. widrace

    widrace Junior Member

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    Thanks for the replies. I will test again tomorrow with the headlights on to draw it down and see what happens. I will also order a new battery. The AC Delco is $161 from Amazon and at Toyota the replacement is $239. Any opinion on the AC Delco?
     
  9. bisco

    bisco cookie crumbler

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    they've been fine, iirc.
     
  10. sam spade 2

    sam spade 2 Senior Member

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    That does not necessarily mean that those connections are really "good".
    Corrosion can hide where you can't see it.

    But I agree that it's probably the battery itself.
    Not at all unusual for them to get flaky before dying completely.