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Navigation voltage readings

Discussion in 'Gen 3 Prius Technical Discussion' started by 4md, Oct 13, 2017.

  1. 4md

    4md Member

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    Does anyone know from where the navigation unit gets the voltage readings? Does it have a voltage meter in it, or it gets the data from car's ECU?

    I'm asking because when I checked my 12v battery few months ago, I remember the multimeter showed about a volt higher (when measured directly on the battery terminals) than the readings from the sat nav. I'll check it, but wondering if it's possible to have a bad connection somewhere that causes the voltage drop.
     
    #1 4md, Oct 13, 2017
  2. ChapmanF

    ChapmanF Senior Member

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    It has a voltmeter in it, and it's out at the end of a fairly low-ampacity circuit with fuses in the way and current being drawn over them, so you can expect it to read a bit low.

    As for getting voltage data "from the car's ECU", well, the car has over two dozen ECUs, and many of them have their own voltmeters. I have not yet taken the trouble to compare the nav voltage reading to my own actual meter on the battery terminals (too lazy to take rear floor apart), but yesterday evening I was extensively playing with a new OBD dongle and reading live data from all the ECUs it could talk to, and when I compared those voltage readings with what was showing on the nav, they were typically around 0.6 V higher.

    At least one ECU has two different power circuits feeding it and measures the voltage at both, and that one was showing (IIRC) a 0.1 or 0.2 V difference between those ... kind of a proxy indication of how much current gets drawn on each.

    -Chap
     
    #2 ChapmanF, Oct 13, 2017
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  3. 4md

    4md Member

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    Thanks! What you said relieves me, I was worried about the huge difference. Today I took some measurements, the difference between the nav and voltmeter on the battery terminals is between 0.5 to 0.8 volts. Can you look the readings below, do you think those voltages are normal?

    Volts CM and Ad are from Torque + wifi obd adapter. Torque says only that CM means "Control Module" and Ad is "adapter".

    Location | Accessory | Ready
    Battery | 11.77v | 14.49v
    Sat Nav | 11.0v | 14.0v
    Volts CM | 11.7v | 14.5-14.6v
    Volts Ad | 11.4v | 14.3v

    Btw, the 12v battery seems low? It's only 4-5 months old :(
     
    #3 4md, Oct 13, 2017
  4. ChapmanF

    ChapmanF Senior Member

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    What's its voltage with the car off? You've only shown ACC and READY, where current is being drawn from it or pushed into it, respectively. When's the last time it got a long relaxing charge?

    Volts Ad is probably the voltmeter built into your OBD adapter itself, measuring the +12V line that comes out at the OBD connector, and Volts CM is probably Torque querying the data from some one of the many ECUs in the car that contain voltmeters.

    I just walked out to my car and saw a "Volts Ad" around 12.2 to 12.4 with the car off, about 12.0 in ACC, 11.7 in ON. (I trust you're consistently using ACC to mean the first button push with no brake, where the second button push with no brake is considered ON?) The auto headlights happened to come on when I switched ON, partly explaining the larger drop.

    My battery is older than yours, but I wouldn't over-stress about those numbers.

    The downside risk of a frail battery is the car maybe not going READY some time unexpectedly, but that's easily mitigated by getting one of the handy Li-ion tiny jump packs available these days and tossing it in the glove box.

    I don't think I've needed mine to start my own car yet, but I did use it to start another couple's big decrepit van in a parking lot once. I think they were surprised it could. It's one of the lowest-capacity ones out there, but absolutely plenty to boot up a Prius.

    -Chap
     
    #4 ChapmanF, Oct 13, 2017
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  5. 4md

    4md Member

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    Ouch, you caught me. My bad - I meant ON, not ACC above - two pushes without the break. So the 11.77 volts, measured on the battery were while in ON.

    The adapter's volt measurements have deviation (0.2-0.5v) from the real values - I tested it on another car with the multimeter plugged in the obd port so I can't trust it for correct measurement.

    Every month I use it for 150 miles trip in each direction on the highway, it was two weeks ago.

    Today I'll check the battery voltage before getting in and will post it.

    Thank you, your post was very helpful!
     
    #5 4md, Oct 14, 2017
  6. bisco

    bisco cookie crumbler

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    best way is to pop the hood in the evening, and test it at the jump points the next morning. that way, nothing has been used recently, except the normal draw.
     
    #6 bisco, Oct 14, 2017
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  7. ChapmanF

    ChapmanF Senior Member

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    Your voltages sound a lot like mine then, and I'm not over-stressed about mine. :)

    -Chap
     
    #7 ChapmanF, Oct 14, 2017
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  8. 4md

    4md Member

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    Checked the voltage today. First thing was opening the trunk without any of the doors and getting to the 12v battery, it was 12.09 volts after 20h without touching the car.

    I'll use the topic to post two pics that I promised few days ago in other thread...
    A lot of people here say putting a regular lead-acid battery inside the car was a stupid idea because of the dangerous fumes. They're absolutely right, but the Prius has ventilation hose that can (must...) be connected to the battery ventilation hole. I've seen a lot Volvos that use the same design - lead-acid battery inside the trunk and ventilation hoses. All lead-acid batteries I've seen has two holes - one in each of the narrower side and they have caps in them. This battery had smaller holes, so the "nozzle" of the hose didn't fit right in and some "carving" was needed :D
     

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    #8 4md, Oct 14, 2017
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  9. bisco

    bisco cookie crumbler

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    if it won't maintain higher than 12.1, it is time to replace it. but opening the hatch turns on the dome light, no?
     
    #9 bisco, Oct 15, 2017
  10. 4md

    4md Member

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    I'm not 100% sure, but I think if the trunk is opened, only its light comes on.
    I think to pull it out and charge it before the winter. It's still in warranty, only few months old:cautious:
     
    #10 4md, Oct 15, 2017
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