Prius 2021 LE Parasitic Draw / dead 12V battery

Discussion in 'Gen 4 Prius Audio and Electronics' started by satawberry5, Oct 14, 2024.

  1. satawberry5

    satawberry5 New Member

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    Hello there ! I have a problem with my Prius 2021 LE. Bought it used a year ago. Mileage 64k.

    A month ago, the aux (12v) battery died twice in a month. I was able to start it with a jump starter. This month, the temperature dropped from 80 to 59 and the car began to ask me for a jump start almost daily. I looked at the battery. It is an "FB" brand (an unknown brand for me) 20HR 45Ah CCA battery. I measured the voltage on the battery a couple of times 12.3-12.6V (once it was even 9V). I wanted to measure the parasitic drain. But here something interesting happened. After having studied dozens of videos about how people measure this, I armed myself with a multimeter in the 10A position, plugging the wires into COM and 10A. I disconnected the negative terminal and connected the multimeter to the negative terminal and the negative pole of the battery. Aaand…nothing… On the multimeter 0.01 And it feels like it doesn't measure at all. In all the videos i watched there was always a quick surge in current at the beginning. After this, people blocked the cars and waited for the car to fall asleep.


    And here is the problem. I don't understand why, but when I connect the wires to the battery through a multimeter, the car does not wake up. There is no reaction. It refuses to be powered. I can't use the keys. I can't open or close the car via remote. It is still dead, although it is connected through a multimeter. This is the second multimeter and the third set of wires. I tried copper and aluminum. Copper/alluminum clamps. All the same. No result.


    I don't know what to do. Of course I can buy a new battery. But what drives me crazy is the fact that the car refuses to be powered through a multimeter. And where is the guarantee that the problem is in dead battery? It will be very sad if the car kills the new battery.


    Need your help guys. I will be glad to receive any information. Thank you!
     

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    #1 satawberry5, Oct 14, 2024
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  2. davecook89t

    davecook89t Senior Member

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    .01 amps that your multimeter shows is 100 milliamps. I think you need a higher level of precision to properly analyze your problem. .01 amps could be the rounded value of as little as 51 milliamps or as much as 149 milliamps. A parasitic draw of 51 milliamps would be nothing concerning, but a draw of 149 milliamps would be about 3 times as high as it should be (if none of the car's start up systems were powered on when you were taking the measurement). Now that you know the draw is less than an amp, it should be safe to plug your positive probe into the milliamp terminal on your multimeter. That will give you a truer picture of whether you have a problem or not.

    I would guess that your car will not power on with the multimeter between the battery and the cable end either because it does not have a good enough connection to draw the power it needs or else the battery is dead. The 12.3 Volts that you measured at one point is low enough that there is no guarantee it can fire up the computers on the car. If the car is not driven on a daily basis, it is not surprising that the battery is discharged. A battery maintainer would be a good investment if the car is not driven regularly.
     
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  3. BiomedO1

    BiomedO1 Senior Member

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    That looks correct, though clamps would work better. You should be able to turn on a dome light or something to see if it's working/reading.
    IF YOU EXCEED 10A, you'll pop the the meter fuse. So if the car's self diagnostic fires the ABS motor or fuel pump, you'll likely pop the fuse.
    Did you check the electrolyte levels in the battery? That one looks like the original battery. My OEM battery is a GS Yuasa. It's that time of year to check the electrolytes levels again, now that it's cooling back down. FYI; I had to top mine off last year - plates showing.

    Why not just pull the battery and take it to an auto parts store to check it out?

    Sorry, forgot to mention: on a Fluke meter, there's an independent amperage fuse within. If you popped that internal fuse; you'll get nothing - The meter will appear to work, but the amperage settings won't. The open circuit is your meter. You can get a clamp-on amp meter that'll read down to 25mA. That's about where it should be when it's asleep. Still pulsing for your key fob.
     
    #3 BiomedO1, Oct 14, 2024
    Last edited: Oct 14, 2024
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  4. satawberry5

    satawberry5 New Member

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    Thank you for your answer !
    1. 0.01 A = 10 mA (not 100) you can use any converter to check it ✌
    So i agree with you about 51-149 interval, but in this case that means 5-15 mA interval (it’s nothing).

    2. I’ve tried to use copper clamps instead of 2 aluminum feeler gauges from the kit — same result.

    3. I agree 12.3 V is too low, but it’s enough to boot car computer (i can turn on my engine with this voltage). But car is still dead if i use copper clamps and multimeter. Sooo idk.
     
  5. Mendel Leisk

    Mendel Leisk Senior Member

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    The CCA is 295, per the pic you posted. It is confusing, expressing the value as “CCA 295”.
     
  6. satawberry5

    satawberry5 New Member

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    it’s CCA 295A
     

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  7. Mr.Vanvandenburg

    Mr.Vanvandenburg Senior Member

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    I have a 2020 Prime and have to charge the battery frequently to keep it good. .01 amps is 10 milliamps. Once the neg cable is off your car the emissions monitors have to reset through a drive cycle. If you get a parasitic draw value, then what? It doesn’t mean anything is wrong.
     
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  8. Mendel Leisk

    Mendel Leisk Senior Member

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    Depending on the amount of parasitic draw?
     
  9. davecook89t

    davecook89t Senior Member

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    :oops:, you are correct. 10 milliamps is nothing.
     
  10. BiomedO1

    BiomedO1 Senior Member

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    Your assuming that meter reading is accurate and it isn't a rounding error. OP couldn't get the car to respond to anything - Like there was an open in the circuit. Unless I could turn on a dome light or something and watch the meter move - I'd assume that isn't an accurate reading.

    Just my two-cents....
     
  11. Mr.Vanvandenburg

    Mr.Vanvandenburg Senior Member

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    If that was for me, I wasn’t assuming anything, but you are. I was correcting the statement in the thread .01 amps is 100 ma, which it isn’t. I know about rounding errors. It’s a pet peeve actually.
     
  12. ticedoff8

    ticedoff8 Junior Member

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    This is kind of an old thread, but I'll add my 2¢.
    -The basic assumption is that the 10A fuse in the meter isn't already blown. If the fuse is blown and the scale set to 10A, a reading of .01 could just be background noise.
    -If the test lead is plugged into the 10A jack on the MM, then changing the scale will not blow a fuse. If the MM has an autoscale function, the readout will adjust to the load. Personally, I tend to disable the autoscale function when I think I know what to expect. If I'm wrong, the reading will be off the scale, but won't hurt the meter.

    A parasitic draw of around 50mA is normal (A/K/A: "Nothing", because 149 ÷ 3 = 50)?

    I've owned this 2016 Prius Touring model since it was new (12 miles on the clock). There have been several times I've had it parked at the airport for 2 or 3 weeks while traveling and never had a problem starting it when I got back. While I never checked the voltage in these cases, I just know it always started.
    Untill...
    It sat for 4 days in the driveway at home and the battery was down to 8v. I used the charger to get it back to 12.5v, and it started with no problems (except for the stupid "ICS Unavailable" warning). Now, 2 days later, I checked the voltage, and it was down to 12.2v, but it started.
    When the car is "On", it's charging at 14.3v and will regulate as needed while I'm driving.

    Given the battery's age, I'm wondering if I should just dive right in and replace it or check to see what the parasitic draw is at.

    Also...
    I'm assuming it's a good idea to hook up the battery charger / tender to the +/- leads before removing them while replacing the battery.
    I haven't had to change a battery in a Prius yet, but I've heard "bad things" can happen if the battery is disconnected completely without a backup.
     
    #12 ticedoff8, Feb 3, 2025
    Last edited: Feb 3, 2025
  13. Paul Gregory

    Paul Gregory Active Member

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    0.01 amps is 10 milliamps, not 100.
     
  14. Paul Gregory

    Paul Gregory Active Member

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    50 milliamps is what was measured on my Gen 3. I could live with that, as long as I drove the car at least once a week. After about 10 days, the battery would surely be too low to start the car.

    If I were leaving it for longer, I hooked up my battery charger, which had a maintainer feature. That is, once the battery is charged, it checks the voltage periodically, and gives it a top-up charge if needed.
     
  15. ticedoff8

    ticedoff8 Junior Member

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    There's a guy on YT that did a test on his 2016 Prius. It's pretty good.
    The title includes "Problem Solved", but it's more like "Found the problem, and I'll live with the work-around".


    Basically, he's showing how to measure parasitic draw, what (in his opinion) is bad (200mA = "bad") and how to debug where the problem is (pull one fuse at a time until there is a significant drop).
    In his case, when he pulled the fuse for "Power Outlet 2" (the one for the back seat passengers), it went from 200mA to 20mA. He didn't troubleshoot the issue, he just left the fuse out.

    I would guess that around 20mA may be the target. I'll check mine today and update this.
     
    #15 ticedoff8, Feb 3, 2025
    Last edited: Feb 3, 2025
  16. ticedoff8

    ticedoff8 Junior Member

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    Update: The last time I drove the car was day and a half ago (about 3 hours of running and 5 cycles), so I figure the battery was fully charged. And the voltage today was 12.2v.
    Then, I used the same setup as the guy did in his video did (including the hose clamp).
    Plugged into the 10A jack on my MM and set it to 40A (to start).

    When I connected the ground test lead to the battery, it spiked to 650mA and slowly settled to 150mA (not good).
    After a minute or so, it looked like 150mA was a low as it was going to go, so I started pulling fuses.
    When I pulled the "Fuel Opn" (10A) fuse, it dropped to 50mA (aha!).
    But, when I plugged it back in, it stayed at 50mA. I figured the "Fuel Opn" must be the solenoid to open the fuel door, but cycling the fuel door didn't trigger a rise any higher than 50mA (BTW: When you push the button for the fuel door, the car doesn't go back to sleep. I ended up having to disconnect the test lead from ground and reconnect it to get it go to sleep. If I didn't do that, the car was drawing >3.5A).

    But, 50mA isn't the target. So I went back to pulling fuses.
    I got to a fuse marked "D/C Cut" (25A) and it dropped from 50mA to 22mA (aha!!). And when I plugged that back in, the current spiked to 650mA and then settled back to 22mA. After I did that a couple of times, 22mA seemed to be consistent for its sleep mode.
    I don't know what "D/C Cut" is, but I'm guessing it's a relay that powers up the basics needed to get the car ready to press the <Start> button.

    So, I seem to have two smoking guns, but I have no way to know what caused them to "stay on" and draw current when the car was "off" or how to replicate the problem(s). Or exactly what to do next.

    In the meantime, I topped off all 6 cells with pure water (they were not too low to begin with, so I just added up to the "cat's eye" level) and put it back on the charger to top it off. I'll drive it around tonight and check it again tomorrow to see what the voltage is and then test the parasitic draw again.
     
  17. BiomedO1

    BiomedO1 Senior Member

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    You'll need to define "bad things". If you leave the battery disconnected for too long your historical mpg may zero out and your ECU may have to relearn engine set parameters and possibly have to pair your phone and reset your radio stations.
    I don't think anyone has bricked their car by changing the battery. A few has popped some fusible links by cross connecting jumper cables (polarity issue) - but that's about it.

    Hope this helps....

    If your going to apply power to the jump point while changing the battery - It would be a GREAT idea to wrap an insulator around your positive battery lead. So it doesn't inadvertently touch a ground while your changing the battery.
     
    #17 BiomedO1, Feb 3, 2025
    Last edited: Feb 3, 2025