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What charges the 12 volt battery?

Discussion in 'Generation 1 Prius Discussion' started by Q*bertZ, May 17, 2022.

  1. Q*bertZ

    Q*bertZ Member

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    I have a bunch of gen 1 prius on the road. The 12 volt system usually runs about 13.4 volts when the car is in the "ready" state. One of my 2002 Prius only puts out 11.4 to 11.9 volt. It is constantly discharging my 12 volt. What do I do?
     
    #1 Q*bertZ, May 17, 2022
  2. Q*bertZ

    Q*bertZ Member

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    The 12 volt battery is new. I'm only reading 12.2 volts on all my fuses. And it decreses as the 12 volt battery discharges from not being charged. I checked all my small fuses they are all good. Does MG1 or MG2 charge the 12 volt battery? Is there a dc/dc invertor relay?
     
    #2 Q*bertZ, May 17, 2022
  3. ChapmanF

    ChapmanF Senior Member

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    The battery is charged by the output of the DC/DC converter, which is part of the inverter/converter assembly and I don't think was available as a subassembly replaceable part in Gen 1.

    I'm used to seeing 13.8ish as the output of the converter in my old Gen 1. If you can't see that voltage at any point in the front fuse boxes while the car is READY, that would sound like a converter problem. If you do see 13.8ish in some places but not at the battery, again with the car in READY, something else may be going on.

    It would be a good idea to use Techstream and see if any trouble codes about the converter are present, or look in the live data list for clues about its operating status.
     
    #3 ChapmanF, May 18, 2022
  4. Q*bertZ

    Q*bertZ Member

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    I just replaced the mg2 and the invertor with ones I don't know for sure they were good. The old mg2 was burned and the t-iv fluid smelled burned. The car now drives but discharges the 12 volt battery. By the way, it was discharging the 12 volt battery before and would not go into gear.
     
    #4 Q*bertZ, May 18, 2022
  5. ammdb

    ammdb Active Member

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    After car is starts, will it still run if you disconnect the negative terminal on the 12V battery? If so, what's the voltage on the battery cables after the 12V battery is disconnected? If no voltage check the big 100A fuse in the engine compartment fuse box.

    Sounds like an issue with the DC/DC converter, but can also be something else loading down the 12V circuit when the car is in Ready mode.
     
    #5 ammdb, May 18, 2022
  6. ChapmanF

    ChapmanF Senior Member

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    With the inverter/converter assembly just recently having been replaced, it wouldn't be a bad idea to carefully recheck all its connections.
     
    #6 ChapmanF, May 18, 2022
  7. JahT

    JahT Member

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    I just changed my battery to OEM Toyota, when I use the MFD check detailed here on PC for the 12V battery it shows it at just above 12V (12.1-12.2), so not especially high at rest with a brand new battery. It says to turn on to accessory, when I do it just stays above 12V. I think I'll check it with a multimeter to gauge the accuracy of the MFD reading, but I'd expect a new battery to be higher, maybe 12.4 at rest.
     
    #7 JahT, May 18, 2022
  8. ChapmanF

    ChapmanF Senior Member

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    In every Prius where I've checked it, the MFD "signal check" voltage reading has been a solid fraction of a volt lower than readings elsewhere in the car at the same time. (I don't think that means the MFD uses an inaccurate voltmeter, but just that it's measuring its own supply voltage, at some internal point, and the voltage is just a bit dropped there.)
     
    #8 ChapmanF, May 18, 2022
    JahT likes this.
  9. Q*bertZ

    Q*bertZ Member

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    I figured it out. I had a donor invertor and replaced all the upper components. MG1 and MG2s components. Not knowing the the dc convertor is on the bottom of the invertor. I couldn't use the donor invertor because all the cover bolts broke off and wouldn't seal if I used the case. The connectors and gold casing by the yellow arrow is the 12 volt (13.8) supply. I'm glad it's over. 20220518_190351.jpg
     
    #9 Q*bertZ, May 18, 2022
    ammdb and bisco like this.
  10. Q*bertZ

    Q*bertZ Member

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    I'm glad it's over
     
    #10 Q*bertZ, May 18, 2022
  11. ChapmanF

    ChapmanF Senior Member

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    (y)
     
    #11 ChapmanF, May 19, 2022
  12. Q*bertZ

    Q*bertZ Member

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    I took the bottom of the bad invertor apart. 20220519_144801.jpg Bad DC convertor disassembled. Found burned component. 20220519_150617.jpg See pictures. Sure enough... it's bad.
    20220519_144955.jpg
     
    #12 Q*bertZ, May 19, 2022
  13. Q*bertZ

    Q*bertZ Member

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    I'm going to transfer my donor dc convertor components to my now spare invertor case.
     
    #13 Q*bertZ, May 19, 2022
  14. AlexY

    AlexY Member

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    Possibly you can fix it by just replace this transistor but it wouldn't hurt if you check all power components, there can be a fuse or 2 that need to be checked too.
     
    #14 AlexY, May 21, 2022
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