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Anomalous 12 V maintenance?

Discussion in 'Technical' started by apacheguy, May 2, 2016.

  1. apacheguy

    apacheguy Sig 255, VIN 320

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    I've been keeping tabs on my car's sleeping behavior and found some interesting data today (see graph). I have sleep enabled and always connected off.

    1. Why does the DCDC converter voltage begin at 14.3 V and decline to 14.1 V? I thought the voltage should start at ~13 V for a partially discharged 12 V and rise from there, not decline.

    2. A normal wake cycle for my car lasts 2-3 hours. This one lasted 4.5 hours. Why do some wake cycles last longer than others?

    No my car did not receive an update today. And no nobody was around to disturb it - it was parked untouched in my garage during this time.

    dcdc_data.png
     
  2. dhanson865

    dhanson865 Active Member

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    The higher voltage you see from the car charging the 12v the lower the SOC of the actual 12v battery and the lower it's voltage was when the car checked it before starting it's charge routine. The more you see higher voltages or new high peaks the more likely it is you have a drain issue or a battery that is getting weak.

    13.0V is not a high enough voltage to charge a 12v battery at room tempature, at least not significantly charge it. 13.0 would barely hold it steady unless it's very hot. Here is a random batteries charge voltage requirements.

    [​IMG]
     
    • Informative x 1
  3. apacheguy

    apacheguy Sig 255, VIN 320

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    I see. IMO, it's counterintuitive to think that the charger voltage declines as a battery reaches capacity. For instance, during SpC the charger tracks the battery voltage and both rise in step. I guess 12 V is just different.

    So the higher the voltage, the lower the capacity of the 12 V. Interesting. What is a normal value range (DC/DC) for a C&D deep cycle in a new Tesla?
     
  4. dhanson865

    dhanson865 Active Member

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    The higher the voltage the lower the capacity is not true as an absolute statement. I think you maybe understood but in case someone reads this and is confused I'm going to toss some too long stuff in here.

    Let me try to restate this.

    12v batteries are very low capacity vs the HV pack in an EV. They vary in voltage greatly without load vs with load. So the same battery with no change in actual capacity or state of charge can show up as:

    Voltage 1 no load
    Voltage 2 with minor load
    Voltage 3 with more load
    Voltage 4 with even more load
    Voltage 5 with a temperature change.

    So to test health of a 12v battery you can't just look at voltage.

    Now cars computer systems only have voltage to judge the SOC by and if they see it low they try to raise it. But they do so by adding another voltage to the circuit. The difference between the voltage of the 12v battery and the voltage the inverter is supplying determines if the system is charging the 12v battery.

    Ideally to test the voltage of a 12v you would pull it out of the car and let it rest for a day, then check the voltage with no load, and then check the voltage with a known load.

    In reality we have the battery tied into various loads and not resting ever, it's pretty much always being drained or charged non stop. 99% of the time the loads will be imbalanced enough to cause it to go up or down barring the computer trying to compensate. If you are watching the voltage of the inverter or the voltage of the bus you are watching multiple variable equations in real time. The number spied is almost never going to match what you would get if you tested the inverter without a battery or the battery without the inverter.

    So yeah I'm implying that the inverter output will go up at the computer request to try and charge the battery if the battery needs it and the more you see that the worse the battery is likely to be. But it's also possible that "vampire loads" have changed and the inverter is just keeping the battery healthy. You can't be sure if the charging activity is proactive or reactive.

    It's designed to be proactive. But if it were successfully proactive on the grand scale people wouldn't be changing 12v batteries in 12-24 months.

    I have no idea what normal charging is on the Tesla. I'm used to watching it on the Prius and Leaf which both tend to undercharge the 12v in normal driving. I get the impression Tesla is doing a better job with managing the 12v but is also putting way more demand on it and unfortunately churning through 12v batteries quicker than a Toyota or Nissan would.
     
  5. richardw0000

    richardw0000 Member

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    If you think of the 12v battery as a resistor, then the inverse voltage relation makes sense. Initially, you want to charge the 12v battery quickly, which requires a higher voltage in order to increase the current into the battery. As the battery condition reaches fully charged, you want the current to decrease, so you lower the voltage accordingly. This is, of course, a simplification of what is going on during the process.

     
  6. apacheguy

    apacheguy Sig 255, VIN 320

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    OK, now explain this one where the voltage actually rose during charging. Also, note that this cycle lasted 3 hours compared to the 4.5 hours it took before.

    dcdc_data.png
     
  7. richardw0000

    richardw0000 Member

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    There are several factors that could affect the voltage, and without knowing more details of the electrical system, it is hard to say what specifically caused it. Based on the rising voltage and declining power, I would suspect that the 12v may have been abnormally low at the start of the cycle. Later, a combination of heat and state of charge of the 12v could cause increased resistance in the battery, and the voltage was increased to try and counteract. It may also have been that the car itself was demanding more power initially. This is all speculation, as I don't know anything about the algorithms Tesla is using.
     

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