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BSM: V_ess too low during precharge

Discussion in 'Roadster: Technical' started by bigbill, Jul 5, 2018.

  1. bigbill

    bigbill Member

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    Location:
    Silverton, OR+Hana,HI
    After doing a lot of very slow charging to recover my battery pack, I now get the BSM: V_ess too low during precharge error. It happens when trying to charge and when trying to drive. The battery is good, lowest brick is 3.97V highest is 4.00V. Pack voltage is 395V. But in the service screen the PEM has only 254V. That sounds like a problem! I'm thinking a resistance between the ESS and PEM?
     
  2. bigbill

    bigbill Member

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    Location:
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    It looks like v_ess is the battery voltage after the contactors, and v_bat is at the battery. So there is either a resistance in the line to the pem, or a short somewhere after the contactors. I'm suspecting the insulators on the pem igbt's, but could be a shorted capacitor too.
    I'm getting the new 3.0 battery, so the pem will be updated anyway, but that means no car until then. It also means towing the car to the other side of the island, and probably shipping it to Honolulu on a barge. Very expensive! Since it won't drive, It would have to go in a container, even more $$$.
    I'll probably fix the pem, even though it will get changed out later. grrrrrr
     
  3. petergrub

    petergrub Member

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    It's normal. The voltage stays there until the HV contactors close and then PEM sees full battery voltage. So, the reason your PEM doesn't see full battery is that the precharge test failed. The precharge circuit is used to determine if the high voltage circuit is safe to apply high voltage.

    The IGBTs shorting would cause a desaturation fault. The capacitors on the other hand could possibly be part of that precharge issue. If you find nothing awry there, then definitely check the precharge circuit in the PEM. Perhaps thefortunes could share his experience with the precharge circuit error and subsequent fix.

    I think you need to get the PEM issue resolved before you go into Tesla to get your 3.0 pack. I heard Tesla doesn't replace the PEM, but replaces a board in the PEM. Check with your service center ahead of time and let us know what they say. If you need to repair the PEM ahead of time, then we should get going on that ASAP.

    Preserve your battery in the meantime by properly shutting down the Roadster and removing the service disconnect plug on the battery.
     
  4. bigbill

    bigbill Member

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    Not normal. The precharge circuit is a relay and resistor that goes around the negative contactor. It's purpose is to slowly charge the large capacitors in the PEM. This avoids damage from high inrush currents. The precharge relay closes, and the V_ESS voltage ramps up as the capacitors are charged. When V_ESS is very close to V_BAT the negative contacor closes and the precharge relay opens.

    Since V_ESS didn't make it up to near V_BAT in the allotted time, the error was thrown and the relay opened.

    The precharge circuit is in the ESS. I plan on inspecting everything in the PEM and replacing caps and insulators since they have been going bad on others. If the problem persists, I'll be stuck with shipping it to the SC for the 3.0 battery.
     
    • Like x 1
  5. MLAUTO

    MLAUTO Member

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    Actually there are precharge circuits in both. They protect the contractors in both the PEM and the ESS.
     
    • Informative x 2
  6. petergrub

    petergrub Member

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    You're on the right train of thought. Check the resistance values of the precharge RC circuit. Since it's an RC circuit, either the resistance of the resistors, or the resistance of the capacitors is throwing off the precharge time assuming the voltage source is functioning properly.

    In thefortunes case, he found a blown IGBT on the CIC/IO board in the PEM.
     
    • Informative x 1
  7. thefortunes

    thefortunes Member

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    Sorry, I have been way too busy at the office (and a vaction thrown in there too) and have been neglecting my duties on this forum :)

    Yes, when I got that error (after replacing the insulation due to a desat fault), I found a capacitor blown as well as the IGBT on the CIC I/O board blown. That IGBT is the one surrounded by the heat sink on the board.

    If you are still having issues shoot me a PM and I may be able to help. The CIC I/O is kind of a pain to get out, if I recall correctly.
     
    • Like x 1
  8. DeedWest

    DeedWest 2010 Roadster 2.0 - VIN 523

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    Location:
    North Dallas, TX
    I had this error a couple of years ago on my 2.5 Sport at the time. It ended up being a blown 400V controller and was about $1,500 to replace. Nicely enough, it carried a one year warranty once I did.
     
  9. bigbill

    bigbill Member

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    Location:
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    Do you have a schematic or other info on the PEM? It would help.
    The other possibility is the HVAC unit. It uses a 400V compressor, so a short in that unit can also give the error.
    Since I don't have a way to fake the HVAC, I still plan on servicing the PEM, especially with many having problems with it. If that doesn't fix it, it's on to the HVAC.
    It will be a while. I am in Oregon for a few months and the car is in Hawaii with the plug pulled on the battery.

    Can anyone confirm that the existing PEM is kept with the new battery install? I've heard yes and no.
     
  10. petergrub

    petergrub Member

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    All the owners I have talked to say the service centers let them keep their PEM and only replace a PCB in the PEM before upgrading the firmware for the 3.0 battery upgrade. On the chance of coincidence, a factor that may be playing into the PEM replacement rather than replacing a PCB board in the PEM may have something to do with whether the PEM is universal or not. Check here if you want to see which PEM you have.

    I still do not have a definitive answer on which board they replace, or when they determine a PEM needs to replaced for the 3.0 battery upgrade. Please let us know if you can get a clear answer.
     
  11. ICON

    ICON Member

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    Jan 31, 2015
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    92
    Location:
    Hong Kong

    13th April 2018
    Hong Kong

    Morning Richard,

    For 3.0 Battery upgrade, the PEM unit will not replace, but a Power Electronics Controller (PEC) board will be replaced which is inside the PEM unit. A newest firmware will perform update once all hardware install back to the vehicle.

    After 3.0 Battery upgrade, we will provide 3 years / 36000 miles (~ 57936 km) warranty with the new battery unit.


    Thanks

    xxxxx xxxx | Lead Service Advisor – Hong Kong
    Tesla Motors HK Limited
     
    • Informative x 2
  12. petergrub

    petergrub Member

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    Interesting choice on Tesla's part. The PEC board is the brains of the PEM and contains the DMC (Digital Motor Controller), PM (Pedal Monitor), and DFC (Dual Fan Controller) firmware on three separate processors. All of the latter processor's firmware can be, and is usually , upgraded via the CAN bus.

    Then why replace the PEC board to upgrade the firmware? Perhaps there are hard-coded lookup tables on the PEC board that need to be readjusted to compensate for the new aerodynamics, battery weight, and rolling resistance to allow the Roadster to achieve 300-400 miles range.
     

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