No-start condition. Seems like 12V, but dealer tested 'good.'

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by RyanGalliford, Mar 5, 2026.

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  1. RyanGalliford

    RyanGalliford Junior Member

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    Hello,

    I am currently having an intermittent no-start condition - well no-READY condition - when pushing the power button, and I wanted to get your thoughts.

    Just talking 12V battery in the below, not the hybrid battery.

    Long story short: car has run fine and suddenly wont go into 'ready' mode, with no other issues or faults. The same thing happened in 2024, and when I researched the issue then, I found a post recommending to check the 12V battery. In that instance, the battery turned out to be bad, and I replaced it with genuine Toyota.

    This time around I checked the the battery, and it was showing 11.7 volts, so I took it to the dealer. They tested it and said it was good - much to my surprise!

    They gave me a little ticket that states the following:

    ------------------------------------------------------
    GOOD BATTERY

    Customer Complaint: Yes
    Charging Required: Yep
    Charging Completed: Oh yea
    ------------------------------------------------------
    Reserve Health: N/A

    Cranking State of Health
    ----------------|---|-----------------
    Replace_____________Good

    (the formatting didnt come out great on this but its supposed to be a bar that shows the health state smack in-between Good and Replace)
    ------------------------------------------------------
    Voltage: 12.85 V
    Rated: 325 CCA
    Measured: 339 CCA
    Ah Replaced: 42.5 Ah
    Time to Decision: 00:51:55
    Charge Time: 01:36:12
    Temperature: 77 F
    ------------------------------------------------------

    This tells me that they had to charge it a fair bit to get it into normal range, and that after that the load test went well. Initially I was suspect that the DC converter wasnt charging the 12V battery properly, or that there was a parasitic drain.

    When I got home, I put the battery back the in car, and it started normally - but a few moments later the red triangle came on. I plugged in the OBD reader and monitored voltages and temps. Everything seemed normal, also no fault codes, so I cleared the red triangle and drove around in the parking lot while keeping an eye on everything. The car drove normally and the inverter was delivering a rock-steady 14.0 volts to the 12V battery, so I drove on city streets for an hour. Everything seemed normal and the red triangle did not return. Hybrid battery was charging normally as well. I went home, parked, checked battery voltage, and then disconnected the battery from the rest of the car by unplugging the two positive harnesses at the battery - I wanted to check for parasitic drain. The battery was at 12.6V that evening, and the next morning, it was 12.4V. That seemed normal to me - to settle a little while resting, after being charged.

    I plugged the positive battery harnesses back in, cranked it up, and it drove normally for the day. That was yesterday.

    Well today I went to the grocery store, and on trying to leave, the 'no-start' condition came back. Checked voltage and the battery was showing 11.8V. Plugged in the jump starter and the car cranked. Drove home and monitored the inverter voltage while driving - it stayed at 14.0V the whole way. Parked and checked battery voltage again immediately - 12.0V resting, and when I try to put the car into 'Ready' mode, voltage drops to 11.3V!

    My sense is that this is a 12V battery issue, but the dealer test is throwing me off.

    What might I be missing?

    - Ryan
     
    #1 RyanGalliford, Mar 5, 2026
    Last edited: Mar 5, 2026
  2. dolj

    dolj Senior Member

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    Have you confirmed that both battery terminal clamps are pushed down and sitting on the step at the base of each post?

    You may have to loosen them and spread the clamps apart with a large flat-blade screwdriver to allow that to happen. While the clamp is removed make sure the clamp and the post are clean and free of any corrosion. You will need a wire brush to clean them. Then tighten making sure the clamp does not ride up the post. Tighten enough so that you cannot twist the clamps by hand. Do this for both the positive and negative terminals/clamps. Check also that the body end of the negative cable is also clean and tight.
     
    #2 dolj, Mar 5, 2026
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  3. Tombukt2

    Tombukt2 Senior Member

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    Battery issue it looks to me .kids at dealership . Yep and yeah and oh yeah ..more interested in who is on radio.
     
    #3 Tombukt2, Mar 6, 2026
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  4. Hayslayer

    Hayslayer Active Member

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    How did you check for parasitic losses if you had the 12v battery disconnected?

    It seems to me that a parasitic drain could still be your culprit, but you need to have the battery connected and monitor current levels as the car 'relaxes' to see what the current draw is.

    You had it disconnected overnight and the battery voltage stayed good.
    You left it connected the next night and the battery voltage was apparently nfg.

    You either have a poor connection or you have a small drain going on which is draining the battery.
     
    #4 Hayslayer, Mar 6, 2026
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  5. BiomedO1

    BiomedO1 Senior Member

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    From what you typed; it looks like the dealership followed the correct procedure in checking your battery. Did you bring in the entire car or just the battery? Dealership can't check for parasitic drain, if all they have is the battery.
    FWIW; your parasitic drain test is invalid - you need to do a proper test to rule out a parasitic drain. What are you using to get your voltages and where are you taking those voltages from? Why not use that meter to verify parasitic drain? Did you make sure the battery connectors was clean and has good contact? I've seen dirty contacts cause intermittent starts.

    Hope this helps...
     
    #5 BiomedO1, Mar 6, 2026
  6. Mendel Leisk

    Mendel Leisk MMX GEN III

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    In a nutshell: put a multimeter in series between the battery’s neg post and the car body and the ground post, so that all current flowing has to pass through the meter.

    to achieve this: disconnect neg battery cable at car body and connect it it to jumper wire. Connect a second jumper wire to car body. Run both jumper wires out of hatch and gently close hatch.

    wait about 1/2 hour with all key fobs at least tens of feet distant, then connect multimeter to the two jumper wires. If it’s not auto-ranging set meter to amperage first, then milliamperes, and see what you get. In my experience it was around 20 milliamperes, with regular spikes to around 45, perhaps due to the blinking icon in dashboard.

    @rjparker has a variation, an extra jumper wire involved, that I believe avoids the car losing memories. Basically keeping the battery constantly connected during the process. He may see this and post.
     
    #6 Mendel Leisk, Mar 8, 2026
  7. bisco

    bisco cookie crumbler

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    #7 bisco, Mar 8, 2026
  8. dolj

    dolj Senior Member

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    ??

    Maybe Roedoras/Roedores.
     
    #8 dolj, Mar 8, 2026
  9. RyanGalliford

    RyanGalliford Junior Member

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    Hello,

    Thanks for your comments! I double checked the terminals, which were clean as-is, and hit them with a wire brush just to be sure; the problem still persisted in the same manner. I ended up putting my meter on the battery while 'starting,' and recorded the drops from 12.2v down to 11.3v for the dealer to see. The substantial drops didnt happen every time I tried to start the car, hence the intermittent issue, but when they did the car wouldn't go into ready mode. I took it to a different dealer this time. They did a more thorough test and confirmed that there was a bad cell in the battery.

    Battery was replaced under warranty and now the car runs perfectly.

    If you're reading this in the future and troubleshooting a similar issue, with a good suspicion that the 12v is at fault, I recommend asking the dealer to perform an in-depth analysis of the battery, and not just whether it meets voltage and CCA criteria at a single point in time.

    Kind regards,
    Ryan
     
    #9 RyanGalliford, Mar 11, 2026
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  10. BiomedO1

    BiomedO1 Senior Member

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    That's the difference between testing using a "mystery box" battery tester and an old fashion carbon pile load tester. An old analog carbon pile load tester won't give your CCA numbers; but it will stress the battery - so you don't get a false positive test. Those "mystery box" testers uses a small load resistor, short test window and calculates a pass/fail results. As you can see here, it gave a false positive. A carbon pile tester would've registered border-line pass and drift down into fail territory, the longer you hold down the test button. IMHO; It's tougher to fool a brute force test. More than likely the way they eventually determined it was a bad battery. Some old-timer knew how to run those old fashion analog test or they had a better "mystery box" that was able to do serial/prolonged load testing with a larger load resistor.
     
    #10 BiomedO1, Mar 11, 2026
    Last edited: Mar 11, 2026
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